Historical Development of the Global Water Cycle as a Science Framework
Summary and Keywords
The global water cycle concept has its roots in the ancient understanding of nature. Indeed, the Greeks and Hebrews documented some of the most some important hydrological processes. Furthermore, Africa, Sri Lanka, and China all have archaeological evidence to show the sophisticated nature of water management that took place thousands of years ago. During the 20th century, a broader perspective was taken and the hydrological cycle was used to describe the terrestrial and freshwater component of the global water cycle. Data analysis systems and modeling protocols were developed to provide the information needed to efficiently manage water resources. These advances were helpful in defining the water in the soil and the movement of water between stores of water over land surfaces. Atmospheric inputs to these balances were also monitored, but the measurements were much more reliable over countries with dense networks of precipitation gauges and radiosonde observations.
By the 1960s, early satellites began to provide images that gave a new perception of Earth processes, including a more complete realization that water cycle components and processes were continuous in space and could not be fully understood through analyses partitioned by geopolitical or topographical boundaries. In the 1970s, satellites delivered quantitative radiometric measurements that allowed for the estimation of a number of variables such as precipitation and soil moisture. In the United States, by the late 1970s, plans were made to launch the Earth System Science program, led by the National Aeronautics and Space Agency (NASA). The water component of this program integrated terrestrial and atmospheric components and provided linkages with the oceanic component so that a truly global perspective of the water cycle could be developed. At the same time, the role of regional and local hydrological processes within the integrated “global water cycle” began to be understood.
Benefits of this approach were immediate. The connections between the water and energy cycles gave rise to the Global Energy and Water Cycle Experiment (GEWEX)1 as part of the World Climate Research Programme (WCRP). This integrated approach has improved our understanding of the coupled global water/energy system, leading to improved prediction models and more accurate assessments of climate variability and change. The global water cycle has also provided incentives and a framework for further improvements in the measurement of variables such as soil moisture, evapotranspiration, and precipitation. In the past two decades, groundwater has been added to the suite of water cycle variables that can be measured from space. New studies are testing innovative space-based technologies for high-resolution surface water level measurements. While many benefits have followed from the application of the global water cycle concept, its potential is still being developed. Increasingly, the global water cycle is assisting in understanding broad linkages with other global biogeochemical cycles, such as the nitrogen and carbon cycles. Applications of this concept to emerging program priorities, including the Sustainable Development Goals (SDGs) and the Water-Energy-Food (W-E-F) Nexus, are also yielding societal benefits.
This article provides a historical perspective on the global water cycle as a concept and organizing framework for water science. It traces the development of this concept as it expanded from a measurement challenge for the satellite community and a modeling and simulation challenge for the scientific community to a framework for integrating observations, research, and applications. The review identifies a number of the decisions that have advanced the research and observational programs that support the understanding of the global water cycle and, in some cases, discusses the challenges of obtaining full value from this research. It also provides a brief look at the role of the global water cycle as a potential link between other biogeochemical cycles. The report does not provide an in-depth review of research papers on the global water cycle. Readers wishing this type of insight are invited to review the sources listed in the references.
This contribution should be of interest to anyone who wants more background on the recent history of Earth observations (EO) and science related to water, and to students and researchers who seek background on the development of the global water cycle concept. The article will also provide insight on how international science and national programs interact.
An Overview of the Global Water Cycle
The global water cycle is a scientific concept that has structured thinking about the global nature of water fluxes and water problems and has facilitated studies of local water problems by accounting for both local and remote processes. Furthermore, the concept has provided a basis for integrating the disciplines of meteorology, hydrology, and oceanography and has led to joint integrated measurements and scientific research. Among the many benefits of the concept’s application are very substantial gains in the accuracy of weather and hydrological predictions, out to two weeks and longer.
The global water cycle concept plays an important role in planning and assessing the degree to which different societies have appropriate access to water. While everyone has some appreciation for the quantity of water that is available in their immediate environment, the picture can be substantially different if we assess the global availability of water and consider how much water to which each person could expect to have access on the basis of a global average. It also informs dialogues such as the 2010 United Nations (UN) resolution recognizing access to clean water and sanitation as a human right, and the Water Sustainable Development Goal, approved in 2015. Furthermore, an understanding of this global context is important to assess how trends such as climate change will affect access to water. The global water cycle framework provides a basis for discussions that could not previously be held regarding water the human condition in a global or regional context.
A primary reason for the effectiveness of this conceptual framework is its strong roots in the natural system. The global water cycle consists of a number of reservoirs for water that include oceans, the atmosphere, land surfaces, surface water stores such as lakes and wetlands, and underground aquifers. The processes that move water between these stores include evaporation and evapotranspiration, precipitation, runoff, infiltration, and groundwater discharge. The movement of water in the Earth’s environment is essential for life and well-being, industrial activity, and the sustainable functioning of natural ecosystems. The global water cycle encompasses all of the water stored in reservoirs and moving as fluxes on land, in oceans, and in the atmosphere. These reservoirs and processes are shown in Figure 1.
The global water cycle characterizes a significant component of the total Earth system. In addition to linkages with the environment’s physical components—such as landforms and atmospheric composition—water plays a critical role in the welfare of societies and affects ecosystems and biodiversity around the world. Virtually all fauna and flora are constituted with a significant proportion of water and must maintain those proportions to remain alive. On a more general level, water is an essential input that strongly affects the productivity and success of a number of economic sectors, from agriculture to energy production. In short, the survival of every human, every region, and every society is dependent on access to a share of the world’s fresh water through global water cycle processes.
The stores of water in the global water cycle are very different in size. Approximately 97% of the world’s water is stored in oceans. Water in the ocean has a significant salt content. The salt remains in the ocean as water evaporates to the atmosphere to form clouds and precipitation. As a result, the rain over oceans and land has reduced salinity and is referred to as fresh water. Of the 3% of Earth’s water that resides as freshwater on land, two-thirds exists in ice caps, glaciers, permafrost, swamps, and deep aquifers, where it is largely inaccessible. A very small fraction of the total is held by the atmosphere, but this water vapor is radiatively active and plays a critical role in the energetics of the climate system and in the net transfer of moisture from oceans to land areas. The annual amount of precipitation that falls on land is related to the renewable part of the available freshwater resource. However, depending on the area, only a proportion of this precipitation runs off into streams and rivers and, to ensure that ecosystem and downstream needs are met, only a certain percentage of that discharge is made available for local human use. The availability of clean water is further reduced by industrial and related activities that degrade water quality so that it is no longer fit to drink.
The variability of local flows arising from anomalies in the global water cycle must be accounted for. For example, during floods, much of the water flows to oceans without opportunities for communities to retain it in reservoirs in anticipation of future water shortages. Some humid areas have abundant runoff and are not seriously affected by the regional and temporal variability in the water cycle processes, while other, more arid areas must resort to utilizing groundwater reserves during dry periods. In some areas where rapid industrial development is taking place, the rate of groundwater consumption now exceeds the rate of recharge, which puts the groundwater resource at risk. Figure 2 shows the estimated amounts of water included in these fluxes and reservoirs for the global water cycle.
The energy required to keep the global water cycle functioning is supplied by the sun’s heat, which creates the atmospheric pressure differential that, in turn, maintains the atmospheric circulation and provides the energy required by phase transitions. Water exists within the global water cycle as a solid (ice and snow), as a liquid (rain and stream flow), and as a vapor (water vapor). It absorbs or releases energy as it moves from one phase to another. Water at mean sea level pressure and 0° C, a condition that frequently occurs at the Earth’s surface, enables water to exist in equilibrium conditions in solid, liquid, and gas phases (known as the triple point). In areas where and at times when, temperatures are colder than this “triple point,” cryospheric processes (solid-gas) dominate, while in those situations that are warmer than 0° C, liquid/gas processes dominate. Phase changes from solid to liquid to gas (or vice versa) involve the absorption (or release) of latent heat. This has implications for understanding climate change because a warming climate is expected to cause a decrease in the areas and time periods where the cryospheric processes dominate.
Water is the third most abundant gas in the atmosphere. Water vapor is a very effective greenhouse gas. The global water cycle has a major influence on the distribution of energy in the climate system. For example, atmospheric water is responsible for the formation of clouds, which alter the energy budget at the Earth’s surface. The formation and fallout of precipitation results in the release of latent heat into the atmosphere and supplies water to the Earth’s surface. Water evaporates from both oceans and land surfaces into the atmosphere, where it increases the atmospheric water vapor, which in turn absorbs outgoing radiation from the Earth’s surface and maintains the mean global temperature above the values that a dry atmosphere could sustain.
Adequately characterizing the global water cycle with earth observations should enable us to close regional and global water budgets. This goal will be easier to achieve as more nations make their hydrometric data available through central global data archives. To better understand the water cycle at global and regional scales, the following water cycle variables must be determined with greater accuracy: precipitation, soil moisture, evapotranspiration, streamflow, surface water storage, and groundwater. Earth observations can contribute, particularly for areas where in situ observations are not available. Budget closure is a critical test for the completeness and accuracy of hydrological and other prediction models. The best possible integrated data products are needed to initialize and evaluate these models.
Effective water management requires models and tools that provide spatially and temporally robust information on water availability and accessibility. Without the broad framework provided by the global water cycle, we would not know as much about the total amount of freshwater that is available for humanity and for the environment, nor would we know its distribution in space and time. The critical elements of the water cycle and their variations on different space and time scales, phase changes, and interactions with the energy cycle must be fully understood to provide the models, tools, and applications needed to incorporate Earth observation data into water management decisions. Improved understanding of the integrated use of EO assets has the potential to improve our understanding of the global water cycle and the estimation of water availability for critical uses.
The global water cycle is a critical part of the climate system. The Intergovernmental Panel on Climate Change (IPCC, AR5, 2014) affirms that climate change is already taking place and that its main cause is human activity. Concerns about the impacts of climate change on freshwater resources and their implications for society are significant. Water cycle scientists are considering the implications of climate change for the water cycle by examining whether the global water cycle is accelerating or intensifying. The global water cycle framework also facilitates the assessment of the importance of other anthropogenic factors that affect the climate. These factors include population growth and urbanization, industrial water pollution, higher water demands for irrigation to support food security, and land use change, which all have feedbacks on the global water cycle and the climate system.
This article provides some of the key references on the global water cycle concept and also notes the names of projects and individuals who provided leadership for its development. Its primary focus is on research and the use of Earth observations to achieve a better understanding of the global water cycle and to facilitate its applications. In this context, Earth observations are considered to include satellite and in situ observations and, where appropriate, the outputs from models or data assimilation systems. A list of acronyms is provided in Appendix 1 for readers who may not be familiar with the programs and projects that support the global water cycle. Inevitably, we will not have been familiar with some water cycle activity that is important to a reader, or will fail to mention the name of an important scientific contributor. Though such omissions may exist, we remain confident that the people and programs we mention have made lasting contributions to the legacy of the global water cycle science.
Early Observations and Studies of the Global Water Cycle
The global water cycle concept has its roots in the earliest understanding of nature. Ancient civilizations were concentrated along major rivers because people understood that rivers provided vital water resources. Many of these ancient civilizations left archaeological evidence to show the sophisticated nature of their water management techniques. In other cases, literature contains references to the cycling of water. An estimated 4,000 years ago, the book of Job in the Bible observed that all the rivers run into the sea, but that the sea is not full. Similar written records were advanced by Greeks, particularly Anaxagoras of Clazomenae, who in 460 BCE wrote that the water cycle is a closed cycle. Further developments involving water supply included aqueducts, cisterns, filtering systems, rainfall-harvesting systems, terracotta pipes for water supply, fountains, baths, sewers, and toilets, all of which were used in Greece, Minoan palaces, and other settlements (Angelakis, Koutsoyiannis, & Tchobanoglous, 2005). Some of these systems were so sophisticated that they rivaled systems used in European and American cities in the second half of the 19th century.
While the design of many of these ancient systems was based on estimates of water cycle variables, there was a growing appreciation for the benefits of applying mathematics and physics to quantitative measurements of these variables. Data were available as early as 500 bce, when the ancient Greeks first began keeping rainfall records (see the Wikipedia entry on rain gauges). One hundred years later in India, rainfall records were used to estimate agricultural productivity and provide a basis for land taxation. Standardized rain gauges were invented and used in ca.1441 in Korea. They spread to many other countries as the engineering community began to use these data to accurately design water storage and conveyance systems. As the need for reliable freshwater supplies grew, rain gauges became more sophisticated and rain gauge networks were expanded to meet the growing need for assessments and predictions.
Weather and climate systems eventually became the subjects of rigorous study, such as George James Symons’s study of British rainfall in 1860. Meteorological networks and early telegraphic communications systems enabled reliable short-term weather forecasting, especially in the mid- and higher latitudes, where “frontal” systems possessed somewhat predictable characteristics.
There was a growing recognition that the climate possessed certain cycles of wet and dry conditions and that some parts of the globe received much more precipitation than others. Long-term records and studies of the correlations between observations at different locations provided insights into the East-West overturning “Walker circulation.” In the early 1900s, Gilbert Walker developed a set of regression equations correlating the oscillating pressure difference between India and the Pacific Ocean and the temperature and rainfall patterns across much of the Earth’s tropical regions (Walker, 1923). This phenomenon, now known as the El Niño Southern Oscillation (ENSO), consisted of recurring 2- to 7-year periodicities in the global tropical water cycle. Impacts varied from drought to floods, depending on the region. This global variability was evidence that oceans influenced the global water cycle.
Many network improvements occurred from 1880 to 1950 as individual nations began to use data and forecasts to plan their economic activities. The need for expanded networks was heightened with World War I and especially World War II, when troops were sent to all parts of the globe. This included navies that sailed the oceans and aircraft that flew in many different theatres of war. During World War II, nations realized the value and strategic advantage for their war efforts of having access to continuous weather data and reliable weather predictions on a global basis. However, gaps in data sets arose as countries withheld data or, in some cases, stopped data collection programs altogether as part of a military strategy against enemies. During and after World War II, global in situ hydrometeorological networks expanded rapidly in support of efforts to improve global meteorological predictions. The UN established the World Meteorological Organization (WMO) in 1950 to coordinate global observational services that would support better forecasting and promote more collaboration in meteorology and hydrology.
First Glimpses and Data Collection in Support of a Global Water Cycle
The year 1957 is generally accepted as the beginning of the space era, with Russia’s launch of the Sputnik satellite. By the early 1960s, satellites were producing images that provided new perceptions of the spatially continuous nature of water cycle components and their seamless interconnectedness across geopolitical and topographical boundaries. Photographs of planet Earth, as seen from space, were not cluttered with political boundaries and revealed phenomena that were interrelated and integrated across media and disciplines. Conceivably, some of the first visualizations of the global nature of weather, the climate system, and the water cycle were enabled by early satellites such as TIROS-1 (Television Infrared Observation Satellite-1), launched in April 1960 to observe the Earth’s cloud cover and weather and to demonstrate the value of relaying these data back to Earth for use in weather forecasting (see TIROS and Satellite Meteorology). These emerging perspectives were the basis for the preliminary formulation of the global water cycle as an entity. By the 1970s, satellites were also providing quantitative radiometric measurements so that variables such as precipitation, vegetation, and soil moisture could be quantified and included in equations that would form the basis for improved understanding and better predictions.
While lower-orbit satellites provided coverage of the entire globe, they did not observe specific locations with sufficient frequency to be useful for some applications. GOES-1 (Geostationary Operational Environmental Satellite-1) and its successors were launched in geostationary orbit, which allowed the satellite to continuously view the Earth from a fixed position (relative to the Earth’s surface), thus providing continuous coverage for a fixed geographical region. Since then, the National Oceanic and Atmospheric Organization (NOAA) and other meteorological agencies around the world have launched dozens of geosynchronous satellites. A constellation of geosynchronous geostationary satellites is currently jointly operated by the United States, Europe, China, and Japan.
Although there is only one global water cycle, the concept’s development proceeded more quickly on the atmospheric side than on the hydrologic side. In the 1960s and 1970s, while atmospheric scientists were modeling the global circulation of water in the atmosphere, the hydrological community pursued a more localized approach to address local water management problems. In particular, the hydrologic community continued to produce highly calibrated hydrologic models that were appropriate to specific watersheds. These so-called “lumped” models used catchment-wide averages for input variables and characterized the surface with a few values that were tuned to provide the best estimates of the output when averaged over a number of years for that basin. Due to the availability of global precipitation data and the interests of international water managers in closing surface water budgets on regional or large basin scales, efforts were directed at building hydrologic models for larger transboundary river basins. To understand local runoff generation using spatially resolved precipitation inputs, it became very attractive to use distributed hydrological models such as the Variable Infiltration Capacity (VIC) hydrologic model (Liang, Lettenmaier, Wood, & Burges, 1994), which allowed for the measurement of the surface’s heterogeneity to be directly incorporated into the models through more physical equations rather than regression or statistical equations. This new approach to hydrologic modeling created new requirements for satellite data.
As outlined in UNESCO’s report on its water programs (2015), in the early 1960s the U.S. delegate to UNESCO raised concerns about the growing demand for water for agriculture, drinking, and industry and the inability to assess the consequences because knowledge of the global freshwater resources did not exist. R. Nace, V. Korzun, and J. Rodier developed a proposal and plan. Together with WMO and a number of UN member nations and agencies with interests in water, UNESCO implemented the International Hydrological Decade (IHD) in 1965.
Lasting until 1974, IHD consisted of a number of national research programs and projects that embraced all aspects of hydrology and took into account the great diversity in the quality and quantity of hydrologic information available in various countries. The highest priority was given to acquiring accurate basic data over a sufficiently long time period to support development projects. Many important results were obtained right from the start. National coordination activities took place in 96 countries, leading them to understand the global nature of water problems. The IHD also brought into focus the fragmented sub-discipline of hydrology by assisting in the development of a global perspective on water. This period saw the launch of basin-wide projects in river basins around the world and enhanced the availability of in situ observations. Transferability of results from one area to another and regionalization, where basins with similar rainfall/runoff processes and responses were identified, formed the basis for many of IHD’s scientific outputs. UNESCO has continued its strong contributions to global hydrology by renewing and expanding its hydrologic activities through the International Hydrological Programme (IHP), which adjusts its research priorities every six years. While IHP has not addressed the global water cycle directly, many of its themes support this goal: computing water budgets over lakes and experimental basins, human influences on the hydrological cycle, hydrology of particular basins and land areas, and the role of snow and ice in the global water cycle, among others.
To support this activity, UNESCO initiated the Global Runoff Data Centre (GRDC) in 1965. A major output of IHP was the comprehensive global river discharge database, The Discharge of Selected Rivers of the World (Shiklomanov & Rodda, 2003), which was published in book form for the period 1969–1984, and was updated in 1988 and 2003. The digitized version of this data compendium served as an important data product available through the GRDC. The Centre is now located at the Federal Institute of Hydrology (Bundesanstalt für Gewässerkünde) in Koblenz, Germany. The initial database had monthly average discharge records for over 3,500 stations with varying temporal coverage that expanded to over 8,000 stations over the past two decades. Initial monthly data was partially replaced and recalculated with daily discharge records. The GRDC distributes and analyzes discharge data from around the world and makes its products freely available. Although efforts are made to keep the Centre’s data as up-to-date as possible, there are continuing challenges, such as delays in quality control of the data, and the reluctance of some governmental services to share their national data. UNESCO also maintained its hydrologic efforts through projects such as Flow Regimes from International Experimental and Network Data (FRIEND-Water) and Water and Development Information for Arid Lands (G-WADI), among others.
With the advent of global datasets obtained from satellites, the climate community developed plans to study the atmospheric component of the global water cycle by examining the global atmosphere. The Global Atmospheric Research Programme (GARP) was initiated in 1967 as a 15-year international research program led by WMO and the International Council for Scientific Unions (ICSU) (renamed the International Council for Science in 1996). GARP was an international coordinated study with the central objectives of understanding the limit of the predictability of weather systems, the physical mechanisms underlying climate fluctuations, the development and testing of climate models, and the understanding of general atmospheric circulation and its interaction with land areas and oceans.
The first major field experiment within GARP was the GARP Atlantic Tropical Experiment (GATE), which took place in the summer of 1974 in an experimental area that covered the tropical Atlantic from Africa to South America. An integrated observing system composed of satellite observing systems, research aircraft and ships, numerous ocean buoys from 20 countries, and enhanced surface observing systems was deployed. GATE’s primary research objective was to understand the tropical atmosphere and its role in the general circulation of the atmosphere. Hurricanes are an important water cycle phenomenon in the climate system that influences the distribution of water over land in the summer and fall seasons in western North America and Asia. GARP also led to the planning and implementation of the First GARP Global Experiment (FGGE), also called the Global Weather Experiment (GWE), in 1978–1979. FGGE, which involved 140 countries, was the largest international atmospheric experiment of its time. FGGE provided insights into the mechanisms involved in weather, clouds, radiation, and precipitation processes and the formation of tropical cyclones and other severe weather phenomenon.
GARP/GWE also encompassed several important regional field experiments that addressed major drivers of variability in the global water cycle, including the summer and winter Asian Monsoon Experiments and the West African Monsoon Experiment. The research objectives of these regional GWE experiments were to study the onset and mature phases of monsoon systems. Subsequently, the Alpine Experiment was launched 1982 to undertake a comparable “mountain airflow” experiment in the Alps. These field experiments contributed significantly to progress in meteorology, numerical weather prediction, and the understanding of the dynamics and thermodynamics of the global water cycle.
The success of the GARP regional and global field experiments and the critical climate questions they raised led to the formation of the WCRP, which was officially initiated in 1980 as a joint WMO/ICSU collaboration. Since 1993, the WCRP has also been co-sponsored by UNESCO’s Intergovernmental Oceanographic Commission (IOC). The main objectives of the WCRP, set at its inception, related to determining climate predictability and the effect of human activities on the climate. It soon became evident that an improved understanding of the global water cycle was needed to achieve these objectives.
In 1986–1987, the WCRP working group on satellite observing systems discussed options for using satellite data products to more effectively meet the needs of the climate community.
Oceans cover over approximately 70% of the globe; consequently, ocean-atmosphere interactions are very important for the global water cycle. In 1990, the WCRP supported the World Ocean Circulation Experiment (WOCE) as a 10-year UNESCO/IOC initiative. In addition, WCRP supported the development of the Climate and Ocean: Variability, Predictability, and Change (CLIVAR) project, which was launched in 1995 and focused on the consequences of ocean variability on the climate and climate predictability, cold regions and processes, and their sensitivity to climate change. Water vapor in the atmosphere was also addressed by the initiative for Stratosphere-troposphere Processes and their Role in Climate (SPARC), launched in 1993. WCRP also launched the Arctic Climate System Study (ACSYS) as a 10-year project in 1993. Ice and snow store substantial amounts of water, some of which melt each spring to provide water for agricultural production.
Climate Change and the Global Water Cycle
Understanding the causes of and solutions to local water problems in the context of changes in the global water cycle is a key challenge. The amount of water available in any reservoir depends on the balance of fluxes into and transports out of the reservoir to the entire system. The interactions of natural and anthropogenic factors increase the complexity in predicting changes in the water stored in a reservoir. As a result, it is not always possible to predict how the water stored in a reservoir will change during a drought even when large-scale fluxes are known.
The Intergovernmental Panel on Climate Change (IPCC) was formed in 1988 as a collaboration between WMO and the United Nations Environment Programme (UNEP). IPCC provides assessments of all aspects of climate change by reviewing the most recently published material on the topic. Water and its circulation in the atmosphere were discussed in each of the five reports IPCC released since its creation. Clouds and water vapor feedbacks continue to be a major scientific challenge for climate modelers. IPCC has also produced reports addressing the impacts of climate change on various sectors including the water sector. Among other issues, the most recent assessment (the Fifth Assessment Report), often abbreviated as IPCC-AR5) (IPCC, 2014), comments on the impacts of climate change on water cycle processes, extreme events in the water cycle, and on water resources management. According to this report, changes in climate have impacted water availability and management in recent decades, confirming the sensitivity of natural and human water systems to changing climate. Regional climate change impacts include changing precipitation patterns: changes in melting snow and ice, altered hydrological systems, and the combined effect of these changes on the quality and quantity of water resources. Furthermore, changes in the terrestrial water cycle explain the shifts of geographic ranges, seasonal activities, migration patterns, abundances, and species interactions for many terrestrial, freshwater, and marine species in response to climate change.
Extreme events in the water cycle are also changing. In some cases, these changes have been observed since ca. 1950. In particular, the increase of heavy precipitation events in different regions is associated with temperature increase and the associated increased water holding capacity of the atmosphere. Recent detection of increasing trends in extreme precipitation and discharge in some catchments, along with sea level rise, implies greater risks of flooding at regional scales, especially in coastal areas. In addition, impacts from recent climate-related extremes such as floods, droughts, cyclones, and wildfires reveal significant vulnerability and exposure of ecosystems and human systems to current climate variability (see Section 1.4 of AR5-SYR). Furthermore, changes in water availability are affected by changing precipitation patterns, and changes in surface temperatures are expected to contribute to changes in water use patterns.
The global water cycle community of scientists continues to debate the extent to which trends in water cycle components are evidence that the global water cycle is accelerating or intensifying with climate change. IPCC has found that the magnitude of projected temperature changes for a doubling of atmospheric CO2 have been quite stable from assessment to assessment. According to Morel (2007), these values are quite similar to those produced by very simple late-1970s climate models. Projections for water cycle variables show more uncertainty and are often model-dependent. For example, although clouds and cloud-radiation feedbacks and other water cycle processes are approximated in climate models, they are recognized as sources of uncertainty in climate change projections. For precipitation changes, there is confidence that annual precipitation (primarily rain) will increase at higher latitudes. However, geographical effects indicate that local stability patterns and topography can also have a significant influence on these trends. The Global Climate Observing System (GCOS) has taken an initial step to account for the connection between the global water cycle and climate change. As the custodians of the requirements for observations to monitor changes associated with climate change, GCOS has identified a number of Essential Climate Variables (ECVs), including water cycle variables that must be monitored with specific accuracy and resolution. In this process, GCOS receives guidance on terrestrial water variables from the Terrestrial Observation Panel for Climate (TOPC). Together with WMO, GCOS and TOPC formed the Global Terrestrial Network for Hydrology (GTN-H) in 2001 to coordinate existing networks and systems for integrated observations of the global water cycle.
GEWEX: A Programmatic Response to the Global Water Cycle
WCRP soon recognized that water cycle research needed to be strengthened and Earth observational systems had to be improved if our understanding of the global climate system was going to adequately address climate change. The study of climate’s predictability and water cycle variability required a multi-scale approach (Wood & Lettenmaier, 2008). Forcing factors such as ocean and land surface conditions are drivers for the global water cycle and its regional manifestations. Studies have shown that land processes can dominate changes in precipitation on daily to weekly time scales. On longer time scales, ocean processes (such as El Niño events) affect precipitation patterns on seasonal to inter-annual time scales and influence the trajectories of synoptic systems that bring moisture to the continents.
Verner Suomi, Pierre Morel, and Leonard Bengtsson developed a proposal for a “wet atmosphere” research program as a follow-on to GARP, specifically because it did not initially focus on the water cycle. Workshops were held in the late 1980s and early 1990s to develop a science plan that would allow observational systems to contribute more effectively to the understanding of the global water cycle. This plan, which became known as the Global Energy and Water Cycle Experiment (GEWEX)1, was approved by the WCRP Joint Steering Committee in 1988 and was officially launched as a core project of WCRP by WMO and ICSU in 1990. As evident from Tables 1 and 2 (in the Appendix), GEWEX1 has been pivotal in advancing the breadth of global water cycle activities on multiple time scales and its links to the global energy cycle.
NASA supported the establishment of an International GEWEX Project Office (IGPO) and has maintained that support on an uninterrupted basis for the past 25 years. Chairs and co-chairs of the GEWEX Scientific Steering Committee have included Moustafa Chahine, Soroosh Sorooshian, Thomas Akerman, Kevin Trenberth, Howard Wheater, Graeme Stephens (current), and Sonja Seneviratne (current). Directors of the IGPO have included Paul Try, Robert Schiffer, Richard Lawford, and Peter van Oevelen (current). Other professional contributors included Sam Benedict and Gilles Sommeria, who represented GEWEX1 in the WCRP office, and Dawn Erlich, a senior officer in the IGPO.
Briefly stated, the abbreviated goals for the first phase of GEWEX1 (1990–2002) were described as follows:
• Determine the Earth’s hydrologic cycle and energy fluxes using global measurements.
• Model the global hydrologic cycle and assess its impact on the Anthropocene.
• Develop the ability to predict variations in the global and regional hydrologic processes and water resources.
• Foster the development of observing techniques, data management, and assimilation systems for operational applications.
Sorooshian et al. (2005) provide an overview of the successful aspects of the first phase of GEWEX1.
Global Data Products
At its launch, GEWEX1 consolidated a number of pre-existing global data projects such as the International Satellite Cloud Climatology Project (ISCCP), Global Precipitation Climatology Project (GPCP), and International Satellite Land Surface Climatology Project (ISLSCP) and began planning a number of new initiatives, all aimed at addressing some aspect of the global water cycle. GEWEX1 is a multi-scale project that has given attention to the global-scale atmospheric circulation and its role in surface water budgets over land areas. From its beginning, GEWEX1 provided an interface for WCRP with all the national space agencies regarding the global water cycle and related research and data requirements. GEWEX1 also relies on the networks of surface observing systems coordinated by the WMO and operated by the National Meteorological (and Hydrological) Services. It makes use of data that are distributed internationally through WMO’s Global Telecommunications System, data collected through the project’s process studies made available via other means, and satellite data from the operational network of polar-orbiting and geostationary satellites coordinated by WMO.
The initial phase of the first GEWEX1 component was built on existing WCRP activities that addressed the global water cycle. One of the first projects GEWEX1 adopted was the ISCCP, led by Robert Schiffer and Bill Rossow (Schiffer, 2013), due to the relevance of clouds and radiation processes in modulating the Earth’s climate. ISCCP was initially designed to produce a 5-year global cloud climatology, taking advantage of the planned international array of operational geostationary and polar-orbiting satellites. The first global radiance dataset was released in 1984. Since then, ISCCP has provided information on global trends in cloud cover and contributed to understanding the role of clouds in the global water cycle. In ca. 2008, ISCCP transitioned from a satellite-based cloud climatology research project led by NASA to an operational real-time system in NOAA. Through ISCCP, GEWEX1 provides datasets that enable the assessment of various factors affecting the global radiation balance so that better estimates of climate warming rates can be computed.
The Global Precipitation Climatology Project (GPCP), initially led by Robert Adler and Phil Arkin, focused on the development of global precipitation products. NASA began to issue global precipitation products in 1986. GPCP was incorporated into GEWEX1 in 1990. This project, which provided precipitation rates over both land and water, was based on the success of estimating precipitation rates from cloud top temperatures. As more satellites provided relevant data, rainfall estimates were derived over land and oceans from infrared and microwave satellite observations and more complex algorithms were developed. In situ gauge data and ground-based radar were also used for some regional products. GPCP products continue to be developed and produced today.
To document the influence of land on the atmosphere over different landscapes using satellite data, ISLSCP, led by Forrest Hall, Pavel Kabat, and the late Piers Sellers, was launched in 1987. This project was initially developed in parallel with the International Geosphere Biosphere Programme’s Biospheric Aspects of the Hydrologic Cycle (BAHC) project , which undertook field campaigns to address land and hydrology issues. As one of its early activities, ISLSCP developed a geospatially consistent suite of data products, including satellite data and model outputs, which proved very popular with researchers developing land surface schemes. WCRP and subsequently GEWEX1 also collaborated with BAHC to launch intensive field projects under ISLSCP to assess the atmosphere-land interactions in areas with homogeneous vegetation types. Most notable were the First ISLSCP Field Experiment (FIFE) on Kansas grasslands in 1987 and the Boreal Ecosystem-Atmosphere Study (BOREAS) experiment in the Boreal forest in northern Saskatchewan and Manitoba, Canada in 1993 and 1994. The FIFE project provided extensive measurements of evapotranspiration processes in a semi-arid environment over a significant portion of the annual cycle (Sellers & Hall, 1992). The BOREAS project determined how coniferous forests impact the climate and how they might respond to future climate change. Background information for modeling the forest’s interaction with the atmosphere was developed by assessing the exchange of heat, energy, water, and gases such as carbon dioxide with the atmosphere (Sellers et al., 1995). Data from these projects on land/atmosphere interactions have proven invaluable for model development, process understanding, and the retrieval of satellite data.
In 1989, the Global Precipitation Climatology Centre (GPCC) was established under the auspices of WMO. Since then, it has assembled, quality-assured, and analyzed rain gauge data gathered from land areas all over the world. The resulting database exceeds 200 years in temporal coverage and contains data from more than 85,000 stations worldwide. The German Weather Service (Deutscher Wetterdienst) operates GPCC. It provides global precipitation analyses for monitoring and researching the Earth’s climate to meet the needs of the WCRP (including GEWEX1), GCOS, and many other users.
The hydrometeorological community also saw the value of strengthening links between surface water and atmospheric water cycling by developing large-scale distributed hydrologic models that would realize improvements from the better characterization of physical processes rather than intensive calibration. The GEWEX1 project provided a natural home for the pursuit of macro-scale hydrology as well as leading research related to understanding the global water cycle, closing the water budget on multiple scales, and understanding the processes underlying climate variability, and their applications to water resource management. Changes in the structure of global water cycle activities over time are given in Table 2, in the Appendix, and details of the GEWEX initiative are listed in a table in Table 3, also in the Appendix. More broadly, these process studies address the range of processes needed to understand the interactions between humans and the global water cycle.
Some notable examples of process interactions include:
• Assessing the significance of clouds’ effects and their changes over time. While the direct effects of aerosols on clouds’ radiation budget is well known, the influence of aerosols of different sizes and composition on clouds is more complex. GEWEX1 facilitated research to test the hypothesis that more aerosols increase the number of cloud condensation nuclei, reduce the average number of cloud droplets, decrease the cloud albedo, and augment the cooling effects of clouds (Twoney, 1974).
• Assessing the memory in the land system that contributes to seasonal prediction. Studies have concluded that most of the hydrologic seasonal prediction skill (especially for the first month) comes from initial hydrologic conditions (Wood & Lettenmaier, 2008). This suggests that better land surface data assimilation is needed to strengthen seasonal prediction.
• Improving high-latitude winter temperature predictions by including accurate albedos for the larch forests over Eurasia in numerical weather prediction models.
The focus on the global water cycle led to a number of data products aimed at describing different components of the cycle. These efforts were consolidated in a single project component, which was later known as the GEWEX Radiation Panel (GRP) and which included the development and validation of global products with direct ties to Earth satellite products. In addition to ISCCP and GPCP, which transitioned from WCRP to GEWEX, GRP activities included the Baseline Surface Radiation Network (BSRN), led by the late Ellsworth Dutton: the Surface Radiation Budget Project (SRB1), led by Paul Stackhouse: the Global Aerosol Climatology Project (GACP), led by Joyce Penner: the Global Water Vapor Project (GVap), led by Thomas vonder Haar: Land Surface Fluxes (LANDFLUX), led by Eric Wood: and Sea Surface Fluxes (SEAFLUX), led by Carolyn Clayson.
GVaP developed global water vapor products using satellite data. It also supported special data product research using global positioning data and water vapor sensors on commercial aircraft.
Projects involving the comparisons of radiation codes for analyzing three-dimensional radiation fields and radiation codes in climate models emerged later. Working groups have been established to address more general topics for which specific projects may yet be developed or that provide guidance to projects in other panels and programs. These topics include cloud and aerosol profiling, data management and analysis, and precipitation radar networks. Many of these developments took place in the 1995–2005 period, under the watchful eye of Bill Rossow and the GEWEX Radiation Panel.
Starting in 2005, GEWEX1 undertook a major data reprocessing effort to ensure that its historic global data sets had temporal continuity. Before this reanalysis these data sets were thought to be unreliable for time series analysis because some satellites providing the measurements had been replaced and the sensors on board other satellites had slowly degraded. This ambitious reprocessing effort involved the review of observational systems, the application of techniques for assessing the time series needed to identify significant trends and changes, and the development and application of correction methodologies to bring them to a common standard. This effort put GEWEX1 in a prime position to provide reliable advice and tools for climate change studies and IPCC assessments.
GEWEX and the Terrestrial Water Cycle
The role of oceans in the global water cycle has been explored through both GEWEX evaporation studies and the CLIVAR program. GEWEX addresses terrestrial or “fast” climate processes, while CLIVAR addresses processes over oceans, often referred to as “slow” climate processes. CLIVAR was launched in 1995 to build on the successful Tropical Ocean—Global Atmosphere (TOGA) Project and WOCE. CLIVAR provided relevant information for the global water cycle through its studies of the seasonal and inter-annual variability and predictability of monsoon systems, weather and climate extremes, and ENSO in a changing climate. CLIVAR and GEWEX jointly address issues such as monsoons, droughts, and the role of oceans and land in forcing these phenomena.
A number of factors helped shape the overall approach GEWEX1 took in addressing terrestrial water cycle budgets. There was an evident need to understand the global water cycle’s role in the global climate system and the impacts of its variability for societies around the world. A GEWEX working group collaborating with the International Association of Hydrological Sciences (IAHS) proposed to bring together the meteorological and hydrological communities to study the water balance study in a large river basin. John Schaake and Eugene Rasmusson advocated carrying out this study in the Mississippi River Basin, widely recognized as the best-instrumented continental-scale basin in the world. Other experts recommended large basins in other climate regimes where diverse processes could be studied and multiple nations would contribute financially. Based on these discussions, plans for the GEWEX1 Continental-scale International Project (GCIP) in the Mississippi River Basin and a number of other large basin studies were conceived.
The general philosophy for this approach was supported by a National Academy of Sciences study led by Peter Eagleson. The resultant report, Opportunities in the Hydrological Sciences (NAS, 1995), drew attention to the importance of the global circulation of water in regulating climate, causing hydrometeorological hazards, and sustaining biogeophysical systems. The report also noted that the hydrological sciences were data-limited and identified a number of priority research areas related to climate, disasters, and biodiversity. It also called for the hydrological community to advance hydrological sciences as a science alongside oceanography, meteorology, and the solid Earth sciences by taking a more global and scientific approach. The report called for more evidence-based research to support a better understanding of the chemical and biological aspects of the hydrologic cycle, the scaling of dynamic hydrological processes, land-atmosphere interactions, variability of reservoirs, fluxes of water and energy, and the hydrologic impacts of human activities.
GCIP studied interactions between the atmosphere and the Mississippi River Basin’s underlying hydrosphere to understand how atmospheric conditions affect the regional hydrology and inform local water resources management, and how basin processes influence the atmosphere. GCIP helped bridge the gap between global-scale atmospheric controls arising from oceanic forcing of the atmosphere and the widely varying land surface processes occurring within continental-scale basins. It made extensive use of satellite data, including precipitation, cloud, and shortwave radiation products from operational satellites, and new products related to precipitation, vegetation, and snow cover derived from research satellites. These studies showed the importance of adapting observational systems to the phenomena being observed. For example, diurnal fluxes can be significant and hard to capture when few or infrequent observations are taken (e.g., 12-hourly radiosondes.) Fluxes into the basin were underestimated because they reached their maximum values at times when there were no satellite overpasses or when no radiosonde measurements were available. Virtual soundings produced by NOAA’s operational regional model were used to provide improved analyses and better representations of the boundary fluxes. On longer time scales, the effects of storage terms such as water in soil layers, deep aquifers, and snow packs were important for accurate land-atmosphere water budgets.
Concern over these and other water budget issues helped define GCIP’s science plan: “A Continental Scale Experiment in the Mississippi River Basin”, led by John Schaake, Michael Coughlin, and John Leese. Approved in 1992, the plan was implemented (primarily with NOAA and some NASA funding) by Richard Lawford and John Leese from 1994 until 2001, when it transitioned to the GEWEX Americas Prediction Project (GAPP). The Mississippi River Basin was an ideal study area because of extensive observational networks, access to the expertise of U.S. universities, strong support from the operational meteorological and hydrologic services, and links with water resources management stakeholders. GCIP executed data collection activities and sequential studies throughout the Mississippi River Basin in four regions, starting with soil moisture and arid conditions in the southwest, cold regions processes in the north central, summer process in the north central and humid southeast, and the basin-wide water budget (Lawford, 1999).
Four other continental-scale experiments were initiated in the early to mid-1990s: projects in the Mackenzie River Basin (Ronald Stewart, John Pomeroy, and Terry Krauss), the Amazon River Basin (Carlos Nobre and Pavel Kabat), four large areas in eastern Asia (Tetsuzo Yasunari and Toshio Koike), and the Baltic Sea drainage basin (Erhard Raschke and Han-Jörg Isemer). Other regional hydroclimate projects (RHPs) that emerged in the late 1990s are discussed under the next phase of GEWEX1.
The Mackenzie GEWEX1 Study (MAGS) included a range of large-scale hydrological, atmospheric, and land-atmosphere studies aimed at understanding and modeling processes in the Mackenzie River Basin and assessing the effects of the river outflow into the Arctic Ocean’s coastal areas. Building on early support from Environment Canada and funding from the Natural Sciences and Engineering Research Council of Canada, field campaigns were carried out in the Basin, starting with the Beaufort and Arctic Storms Experiment (BASE) in 1994–1995 and followed by other field projects over the 1997–1999 time frame. MAGS provided new insights on water budgets in a high latitude river basin and the hydrologic characteristics of north-flowing rivers into the Arctic Ocean (Stewart et al., 1998; Szeto, Stewart, Yau, & Gyakum, 2007).
The GEWEX1 Asian Monsoon Experiment (GAME) featured four critical regions in Asia that either influence or are influenced by the Asian monsoon system. GAME combined monitoring activities to assess changes across the region, process studies to understand and characterize various hydrometeorological processes in the area, and modeling studies to ensure that the understanding was applied to prediction systems. Extensive field campaigns were carried out in the humid tropics in the Chao Phraya River Basin, Malaysia, and Sri Lanka: the humid Huahe River Basin: the Tibetan Plateau (Yang, Koike, & Yang, 2003): and the Siberian tundra in the Lena River Basin. Intensive observations were obtained through phased field campaigns in the 1996–1999 time period and data were incorporated into the GAME regional modeling system (Yasunari, 1996).
The Baltic Sea Experiment (BALTEX), which was developed under the leadership of Erhard Raschke, explored and modeled the space and time variability of the water and energy cycles in the Baltic Sea catchment area and in the Baltic Sea itself (Raschke & Isemer, 1996). It addressed the influence of runoff and sea ice on the circulation and properties of the Baltic Sea. In addition to data collection activities and modeling studies, a number of field experiments and process studies were undertaken to support the calibration, validation, and improvement of models and remote sensing data.
The Large-Scale Biosphere-Atmosphere Experiment in Amazonia (LBA) assessed the degree to which the Amazon functions as a regional entity and the effects of changes in land use and climate on the biological, chemical, and physical functions of the Amazon, including sustainable development in the region and the influence of development on the global climate (Avissar et al., 2002; Nobre & Kabat, 1996). Through the efforts of Carlos Nobre and Pavel Kabat support was garnered for this project from Brazil, the United States, and Europe. The principal data collection period spanned 1998 to 2006. Deforestation was (and is) a critical cause of changes in the Amazon Basin. Landsat data were very valuable in assessing the extent of land surface changes due to deforestation and they contributed to the evaluation of these changes to the hydroclimate and biogeochemical cycles over the entire basin.
As the GEWEX1 project matured, it developed a wide range of data products, measurement opportunities, process studies, and model improvements for better predictions. The integration of global data sets and regional models and applications posed scientific challenges. The regional nature of hydrological issues provided opportunities for agencies like NOAA, whose mandates are primarily national, to contribute to projects that addressed national- and global-scale issues. Other challenges have involved the development of more robust collaborations among scientists from hydrology, atmospheric sciences, and oceanography.
The GEWEX1 Hydrometeorology Panel (GHP) was established in 1994 to encourage convergence among the Continental-scale Experiment (CSEs) (later referred to as RHPs) and to strengthen connections with global perspectives (Lawford et al., 2004). GHP’s overall stated objective was to demonstrate skill in predicting changes in water resources and soil moisture on seasonal and annual time scales as an integral part of the climate system. GHP coordinated studies to assess regional water cycle processes and placed them in the context of the global water cycle. Modeling efforts led by the late John Roads provided the basis for these inter-comparisons (Roads, Kanimitsu, & Stewart, 2002). GHP also oversaw studies of extremes in the different basins and their effects on water resources. Through the efforts of Toshio Koike and Ronald Stewart, GHP developed plans for the Coordinated Enhanced Observing Period (CEOP1). The University of Tokyo took the lead in developing the infrastructure for CEOP, including in situ flux tower measurements, satellite data, and model outputs combined in a comprehensive archival and analysis framework (Lawford et al., 2006). During the early stages of GHP, leadership was provided by the CSE chairs, and in the later stages of Phase 1 by the late John Roads and Ronald Stewart. Ronald Stewart was also responsible for launching the WISE initiative.
ISLSCP supported GEWEX1 (and all global land surface modeling efforts) by developing a comprehensive collection of data sets and interpolating them to a common grid and data protocols (to the extent possible). These data products were generated from satellite data in some cases and from models in others. ISLSCP was a major contributor to studies in the RHPs; in return, data collected in basin studies allowed for the validation of ISLSCP data products. The popularity and success of ISLSCP’s data collection helped GEWEX1become a source and standard for global data sets. In following years, as the number of land-related data sets multiplied, the role of GEWEX1 shifted from gathering data sets to assessing of the products provided by others and the development of high-quality products for specific applications. GEWEX1 provided a focal point for integrating water cycle data products and, thanks to its policy of free and open data exchange, the data sets could be made available for the community to evaluate and cross-validate. However, the issue of data exchange was a challenge with some of the CSEs/RHPs, particularly for those in nations that did not have a policy of open data exchange. In these RHPs, procedures were established for the release of data after a specified period of time and/or co-authorship with the data provider was arranged.
Modeling the System
A third component of the GEWEX project focused on model development and applications. These activities involved process studies aimed at addressing model deficiencies in models, particularly where clouds were involved. They also included a Global Soil Wetness Project (GSWP); the GEWEX Atmospheric Boundary Layer Study (GABLS), which studied boundary layer processes; the Global Land/Atmosphere Systems Study (GLASS) (Van den Hurk, 2011); the development of Land Data Assimilation Systems (Rodell et al., 2004); and the Project for the Intercomparison of Land-surface Parameterization Schemes (PILPS). PILPS was an intercomparison of land surface schemes used primarily in climate models using local research-quality data sets for evaluation purposes (Henderson-Sellers, Pitman, Love, Irannejad, & Chen, 1995).
The Global Cloud System Study (GCSS), led by Keith Browning, David Randall, and Christian Jakob, involved a number of field campaigns to observe clouds (primarily over oceans) to improve the parameterization of cloud processes in cloud-resolving models and global climate models. Studies have focused on boundary layer clouds, cirrus cloud systems, precipitating convective clouds and polar clouds (Randall et al., 2000). As the focus if these studies began to include more studies of clouds over land discussion increased on the relative role of cloud microphysics in cloud development as compared to cloud dynamics. The Global Soil Wetness Project (GSWP) relied on the development of global hydrologic models, which could provide preliminary model soil wetness outputs that could represent soil moisture distribution (Dirmeyer, 2011). To make these models sufficiently sophisticated and yet applicable everywhere, scientists needed hydrological parameterizations that used variables that could be measured throughout the world. One of the first steps in this process was the development of a global river routing map based on the world’s rivers and watershed boundaries (Oki, Kanae, & Musiake, 1996). As higher resolution topographic data have become available, more precise river network maps have been produced, facilitating the more accurate modeling of river and stream discharges. In the late 1990s, several global and regional distributed models and digital channel networks were introduced. These distributed models could track runoff, soil moisture, and water infiltration into the groundwater reserve using parameters measured from satellites. They also interfaced easily with atmospheric models. In more recent years, this work has been directed at building land data assimilation systems and has been complemented by the Hydrologic Ensemble Prediction Experiment (HEPEX); a project initiated by John Schaake and others to demonstrate the value of hydrological ensemble forecasts to support water resource decision-making.
Space Agency Contributions to Global Water Cycle Understanding
Space agencies in a number of developed countries with a mandate to consider global observations and technologies to observe the Earth system for the benefit of civil society have greatly facilitated the development of global water cycle understanding. During the 1970s and 1980s, systems were developed for the routine monitoring of the Earth, its weather patterns, and land cover. Serious investments were made in the applications of Earth satellite systems in the 1990s. For example, in 1995 the European Space Agency (ESA) launched two earth satellites known as the Earth Remote Sensing Satellites (ERS-1 in 1991 and ERS-2 in 1995). Both satellites carried synthetic aperture radars and provided high-quality data for land and ocean surfaces. The breadth of interest in Earth satellites led to the formation of the Committee on Earth Observing Satellites (CEOS).
The NASA Earth Science Enterprise (ESE) Earth Observing System (EOS) Program was established in 1991 as a U.S. Presidential initiative to provide in-depth scientific understanding of the Earth as a system. This knowledge would become the foundation for understanding natural and human-induced variations in the Earth’s climate system and for providing sound information for environmental policy decision-making. During the first phase of the EOS program, NASA funded the development and launch of a number of Earth observing satellites and a comprehensive data and information system to support data archiving, distribution, and analysis.
With its morning equatorial crossing, Terra (or EOS-AM) began collecting data, in December 1999, on changes in the land and ocean surface and interactions with the atmosphere through the exchanges of energy, carbon, and water. EOS Aqua (or EOS-PM) was launched in May 2001 with an afternoon equatorial crossing. Both satellites carry Moderate Resolution Infrared Spectroradiometers (MODIS) that measure atmospheric, land, and ocean processes (including both land and ocean surface temperatures), ocean color, global vegetation, cloud characteristics, temperature and moisture profiles, and snow cover. They view the entire surface (land, ocean, clouds, aerosols, etc.) of the earth every 1 to 2 days, at a moderate resolution of 250 meters to 1 kilometer (see Aqua and Terra, for details).
The Earth observing system (NASA, 1996; Greenstone & King, 1999) serves as the observational component of NASA’s global change research program. It contributes to a better understanding of the relationships between the Earth system’s components. NASA’s Earth Observations Data and Information System provides free and open access to products through the Distributed Active Archive Centers (DAAC). In addition to satellite data products, these data centers provide access to data from field campaigns and a wide range of physical, geophysical, biochemical, and other parameters. Through the DAAC, users can download small data sets directly and download larger data sets to the cloud or through selected media.
In 1999, the National Research Council Committee on Hydrological Science, chaired by Dara Entekhabi, published a report entitled The Hydrological Science Priorities for the U.S. Global Change Research Program (USGCRP)—An Initial Assessment (NRC, 1999). The Committee established a water cycle study group, chaired by George Hornberger, to prepare a plan for a new USGCRP science initiative on priorities for global water cycle research (Hornberger et al., 2001). The plan advocated an enhanced scientific research effort on the global water cycle based on three central questions involving water cycle variability and trends, prediction, and links with ecosystems. In this context, the global water cycle continued to be a unifying framework for bridging the spatial scale gap between atmospheric and hydrological sciences.
The USGCRP formed an interagency program to address the global water cycle. Its primary goal was to develop greater understanding of the seasonal, annual, and inter-annual mean state and variability of the water and energy cycles at global scales and all other space and time scales, and their interactions among the terrestrial, atmospheric, and oceanic components in the Earth’s climate system (see the USGCRP Water Cycle Study Group). This federal program was an important mechanism for coordinating federal water cycle research activities as they related to climate change.
The period 1995 to 2016 marked the advance of global Earth observing systems, which were first coordinated under the Integrated Global Observing Strategy-Partnership (IGOS-P) and, subsequently, by the Group on Earth Observations (GEO): the Committee on Earth Observation Satellites (CEOS) played an important implementation role in both cases. A water theme report (IGOS-P, 2004) was developed through IGOS-P, and different groups, including CEOS, WCRP, and WMO, initiated a number of relevant activities to support the directions outlined in the report. The implementation of these actions fell to the Integrated Global Water Cycle Observations (IGWCO) Community of Practice (CoP), which was formed primarily for this purpose.
Many of these activities became part of the GEO Water Cycle Societal Benefit Area (SBA) when GEO was initiated in 2004. GEO provides excellent outreach for these activities because it now includes more than 100 member nations and over 90 participating organizations all working together to build a Global Earth Observation System of Systems (GEOSS). The IGWCO CoP continues to connect the interests of the global observing system with global and regional water cycle science and applications. For example, it promotes water applications in Asia and Africa through the Asian Water Cycle Initiative (AWCI) and the African Water Cycle Coordination Initiative (AfWCCI). The convergence of activities for AWCI and AfWCCI integration are expected to be facilitated through the use of the Water Cycle Integrator (WCI) and through the guidance of the University of Tokyo and Toshio Koike who led the development of these Asian and African initiatives. Other water-related studies have been supported by ESA through the TIGER initiative in Africa. The IGWCO CoP developed the GEOSS Water Strategy (GEO, 2014), which defines a number of gaps in observational systems and technical programs that need to be addressed to fully meet the needs of the user community. The IGWCO CoP also serves as a model for how communities of practice can be used to coordinate activities in other areas (Mohtar & Lawford, 2016). Most recently a GEO initiative has been developed for Global Water Sustainability (GEOGLOWS) which seeks to consolidate observations, research and applications which support sustainability.
Global water cycle research has been limited by the difficulty of acquiring data from different countries. GEO promotes open access and the distribution of data at minimum cost and coordinates the development of tools and infrastructure to facilitate this goal. Within the limits of a volunteer “best efforts” program, GEO has helped coordinate earth observation capabilities to support water cycle research and to address water management needs and opportunities. As part of its gap assessments, GEO undertook a study of user needs that identified the priorities for water cycle observations by different user groups (Unninayar et al., 2010). The study reinforced the importance of the water cycle data across all GEO Societal Benefits Areas (SBAs). In this review precipitation was the most frequently requested variable, and soil moisture was the second most frequently requested.
With the multiplication of satellite missions, an increasing number of products are becoming available, and the issues of data integration and product assessment are becoming more challenging and urgent. GEWEX found it necessary to upgrade these analyses to take advantage of the information coming from high-resolution systems such as EOS AM-1 (Terra) and PM-1 (Aqua). GEO also promotes the merger of data products and the integration of satellite and in situ data to create products that build on geospatially consistent, high spatial-resolution satellite data and the high time resolution for in situ monitoring stations.
The global water cycle is an observational priority for all of the major space agencies, including NASA, ESA, the Japan Aerospace Exploration Agency (JAXA), and a number of other space agencies, including the Chinese Space Administration and the Chinese Meteorological Agency. As noted earlier, specific missions supporting global water cycle research go back at least to the early 1990s, when ESA launched ERS-1 and ERS-2, two satellites that provided observations of the land surface and data that was relevant for monitoring the global water cycle. Many other satellites also contributed over the next 25 years (see Appendix 4).
The NASA CloudSat mission, launched in 2006, created many research opportunities for the GEWEX Community. Graeme Stephens and other GEWEX scientists were major contributors through the development of radiation transfer models and the applications of the observations to vertical cloud droplet distributions, cloud microphysics and their effects on cloud albedo and precipitation processes (Stephens et al., 2008).
The demand for an understanding of the processes that govern the global water cycle emerged, increasing the need for supportive data. JAXA has recently launched the initial satellite in its GCOM-W satellite series, which is dedicated to measuring water variables. ESA is continuing to launch missions in its Sentinel series, which includes several satellites that will provide moderate-resolution data in wavelengths relevant to the analysis of water variables. Copernicus will provide open access for anyone who wishes to use these data. ESA and Copernicus are making these data sets fully open and compliant with GEO’s data policy. The operational satellite agencies are also welcome contributors to global water cycle studies and meteorological prediction systems. At the same time, the global community became interested in understanding and modeling the cycling of water through the global environment. The global cycle of other variables such as nitrogen and carbon soon drew attention. Satellite systems are now being designed and, in the case of carbon, launched in support of these global cycles.
The global water cycle has also accelerated innovation in observational techniques. The need to measure precipitation with higher accuracy and resolution has advanced the development and application of space-borne radars. The Tropical Rainfall Measuring Mission (TRMM) used an onboard radar to map three-dimensional precipitation intensity and provide high-resolution precipitation products. TRMM was de-commissioned approximately 18 years after its June 1997 launch. It provided strong evidence of the benefits and relevance of satellite radar precipitation observations. The NASA/JAXA Global Precipitation Measurement (GPM) mission, launched in February 2014, is the core of a precipitation constellation that forms an international network of satellites that provide next-generation global observations of rain and snow.
In November 2009, ESA launched the Soil Moisture and Ocean Salinity (SMOS) mission to obtain globally consistent measurements of soil moisture and ocean salinity from space. The satellite, which carries a microwave imaging radiometer using aperture synthesis, has opened the door to many applications of soil moisture data. Data from this system also have led to many new applications of soil moisture information in planning agriculture operations, flood forecasting, among many other areas. More recently, in January 2015, NASA launched the Soil Moisture Active Passive (SMAP) satellite, dedicated to accurately measure soil moisture from space. SMAP carries two instruments, a 1.2 GHz L-band synthetic aperture radar (active) and a 1.41 GHz L-band radiometer (passive), that together measure global-scale land surface soil moisture and freeze/thaw state. The L-band frequency enables observations of soil moisture through moderate vegetation cover, independent of cloud cover and time of day.
Another innovation has been the Gravity Recovery and Climate Experiment (GRACE) satellite, which measures anomalies in the gravitational field, which are in turn used to infer variations in groundwater and surface water storage. This U.S.-German satellite estimates groundwater changes on a monthly basis and has been used to detect aquifer depletion in California, northeastern India, and Brazil, among other regions. GRACE also monitors drought and water stress. A follow-on GRACE mission is being planned to replace this popular but aging system.
Other systems are under development. Most noteworthy is the Surface Water and Ocean Topography (SWOT) mission, a U.S.-France collaboration that explores options for measuring water levels to estimate lake and reservoir volumes and flows for larger rivers. Important developments are also anticipated in missions planned for evapotranspiration and soil moisture.
CEOS is developing a response to the GEOSS Water Strategy’s recommendations that deal with satellites (GEO, 2014). Through its Water Strategy Implementation Study Team (WSIST), CEOS is currently reviewing these systems that monitor the individual variables to determine how more synergies can be achieved through joint planning of satellite missions.
In addition to supporting many of the above initiatives through the provision of data and models, NASA initiated an integrated energy and water cycle study (NEWS) in 2003. This effort emphasized observationally based advancements related to the global water cycle in process understanding, modeling, prediction, and applications. In particular, NEWS drew upon NASA’s Earth science programs to address changes in the global water cycle and its components, including global precipitation, evaporation, and the cycling of water in response to a changing climate (Schiffer, Houser, Belvedere, & Entin, 2015a; Schiffer, Houser, Belvedere, & Entin, 2015b). NEWS documented the global water cycle using satellite-based energy and water cycle climate data records, including continental and oceanic averages of the earth’s radiation balance as well as precipitation, evaporation, and water vapor (Schlosser & Houser, 2007; Rodell et al., 2015; L’Ecuyer et al., 2015). The assessments include uncertainty estimates, which give users a higher level of confidence in the products and help NASA managers understand where new measurement capabilities are most urgently needed. Through the use of MODIS and Advanced Microwave Scanning Radiometer (AMSR)-E data, NEWS has assessed the effects of climate change on snow and snowmelt in northern latitudes, as well as drought and its impacts. Among its many other developments, NEWS also estimated movements of water through the Earth system, the acceleration of the global water cycle under climate warming, improved estimates of evapotranspiration, and new representations of energy and water cycle processes in models (Bosilovich, 2013).
The Water Cycle Multimission Observation Strategy project was launched by ESA in conjunction with GEWEX1 to develop novel products for use in global water cycle science and to explore and assess methods for developing long-term data sets to assess variations and trends in the global water cycle. In particular, the initiative was intended to help scientists in ESA member countries develop and validate multi-mission products to maximize the use of ESA data, to address the coupling between the terrestrial and atmospheric branches of the water cycle, and to assess its influences on climate variability and predictability. Four thematic areas were addressed: evapotranspiration, soil moisture, clouds, and water vapor. The project held is final workshop in the spring of 2011.
The Water and Global Change (WATCH) project was undertaken to assess the ability to close the water budget on different space scales (Harding, Dolman, Gerten, Haddeland, Prudhomme, & van Oevelen, 2014). The European Union funded the project to co-ordinate an intercomparison of hydrological models (Haddeland et al., 2011), using a new global meteorological data set (Weedon et al., 2011). Eleven models were included in the intercomparison, including global hydrological models and standalone versions of the land surface models commonly used in climate models. The initial analysis was carried out for “naturalized” conditions (Haddeland et al., 2011) without human influences such as land cover changes, damming, water abstraction, and irrigation. The models showed a significant spread of the partitioning of precipitation into evaporation and runoff. Although there was no single cause for the spread in model outputs, the different treatments of snow was a significant factor.
Bridging the Gap Between the Global Water Cycle and the Global Water System
Recognizing the need to make information about the global water cycle available to decision makers and those concerned about trends in water, member countries of the United Nations Commission for Sustainable Development initiated a series of World Water Development reports in 2000. Through a Secretariat initially collocated with the UNESCO IHP, this program undertook an ambitious set of studies. Its first report in 2003 (UN-WWAP, 2003) led to the UN Water for Life program. Issues addressed in the reports included, among others, water scarcity, access to drinking water, sanitation, hygiene, and disaster risk reduction. Africa was recognized as a region with greatest need. After more than a decade, many of those early priorities remain.
The beginning of the 21st century was marked by a new focus on the interpretation of global science in support of emerging societal objectives such as the Millennium Development Goals. In 2001, the Amsterdam Declaration of the Open Science Conference “Challenges of a Changing Earth” concluded that “the earth system behaves as a single self-regulating system comprised of physical, chemical, biological and human components.” The conference identified the central role of water in transmitting the energy and materials required to maintain these links. It also drew attention to the influences of human activity, including climate change and infrastructure development, on freshwater over the world’s land areas and underlined the need for more research on these topics.
Within the programmatic framework of the Earth System Science Partnership, four crosscutting interdisciplinary projects were implemented in 2004, including the Global Water System Project (GWSP). As shown in Figure 3, the global water system extended beyond the global water cycle to include biotic and anthropogenic factors. Its central tenet was that human-induced changes to the water system are now global in scope, that there is a need to understand how the water system works and responds to disturbances, and how society can adapt to rapidly evolving system states. Led by Joseph Alcamo, Charles Vörösmarty, and Claudia Pahl-Wostl, GWSP addressed these problems by looking at the magnitude of anthropogenic and environmental changes and their mechanisms for causing changes in the global water system. It also looked at the global water system’s resilience and adaptability to change and identified ways to implement sustainable management strategies. The project launched initiatives in three areas: global-scale water resource assessments, basin-scale water management responses to large-scale environmental stresses and to anthropogenic factors such as development, and governance studies that considered the role of institutions and laws in the management of water. Many of the highlights of these studies are described in the December 2013 special issue of Current Opinion in Environmental Sustainability, which focused on aquatic and marine systems. For example, Vörösmarty, Pahl-Wostl, Bunn, and Lawford (2013) show the many ways in which GWSP addressed the influence of the anthropogenic effects of demographics, climate change to water cycle, and its ability to meet the needs of humans and environment.
Some threats to the global water system can be assessed on a global basis. In most cases, however, solutions focus on assessing local or regional water pollution, land cover and land use change, local infrastructure development, regulations of rivers and water works, terrestrial biodiversity, overall changes in the water supply, urbanization, and effects on human health. The effects of scale can be very important when assessing the interactions between these factors. It is important to understand how regional effects and developments accumulate to cause global impacts. Linkages involve different time scales, including diurnal and annual cycles and short times associated with some extreme events.
Resilience in the global water cycle depends on the water system’s ability to respond to major hydrologic extremes, including floods and regional water scarcity. Management strategies must enhance during short-term extreme events as well as longer-term changes in the global water cycle. (As described by Milly et al. (2008), the climate and, in turn, the global water cycle is no longer stationary.)
In 2002, GEWEX1 adopted a science plan for Phase 2 (2003–2012) of global water cycle studies that recognized this imperative. The plan focused GEWEX expertise and resources on a number of change-related science questions, including:
• Are the Earth’s energy budget and water cycle changing?
• How do water cycle processes feed back into the climate system and explain natural variability?
• To what extent can we predict changes in the global water cycle on seasonal to inter-annual time scales?
• What are the impacts of global water cycle changes on land and water resources management?
New initiatives led to an expansion of the links between data and modeling, with a greater focus on data assimilation. During this period, the Eta model used at the NOAA National Centers for Environmental Prediction (NCEP) was modified to assimilate precipitation data. One consequence of this development was the production of a regional 50-year reanalysis for North America, which was completed in 2004 (Mesinger et al., 2006). Land data assimilation capabilities allow for the combination of slowly changing land surface properties and more rapidly changing hydrological state variables to produce the best estimate of water cycle fields. Following the development of the North American Land Data Assimilation System (NLDAS), Europe and Japan also developed their own data assimilation capabilities. A global version of the NLDAS system was developed for application primarily with satellite data. Further developments have incorporated measurements of both land surface characteristics and real-time data to produce high-resolution (less than 1 kilometer) products.
A number of new or revised RHPs allowed GEWEX to address some of the Phase 2 questions. The new RHPs included:
The African Multidisciplinary Analysis Project (AMMA) led by Thierry Lebel. This French-led project in West Africa relied on land and ocean measurements to examine the processes leading to the formation, promulgation, and termination of the African monsoon.
The GEWEX Americas Prediction Project (GAPP), led by a science committee (and Richard Lawford and later Jin Huang as program managers) used existing observational and modeling systems developed by GCIP to address water cycle prediction issues over the contiguous United States. It also included the North American Monsoon Experiment (NAME), led by Wayne Higgins and Dave Gochis, a collaborative project with the Pan-American Climate Studies program developed to address the southwest U.S. monsoon system. Both GCIP and GAPP maintained a core project with NOAA operational services, which provided an effective conduit for research to help upgrade operational models used at NCEP and by the Office of Hydrology. This program later expanded to become the Climate Prediction Program for the Americas (CPPA) through more intensive collaboration with the US CLIVAR program.
The Northern Eurasia Earth System Science Partnership Initiative (NEESPI), led by Pavel Groisman, focused on the effects of climate, governance, and land use change in northern Eurasia and their effects on the regional water cycle and extreme events. NEESPI also was recognized as a regional Earth System Science Partnership activity. Efforts have been underway since its conclusion in 2015, to develop a follow-on project to continue its NEESPI’s work in Eurasia.
The Murray Darling Basin (MDB) RHP, led by Alan Seed and Jason Evans, studied the MDB basin located in a semi-arid environment in southern Australia. The project provided insights into the effects of a multi-year drought on agriculture and water resources.
The La Plata Basin (LPB) project, coordinated by Vincente Barros, Hugo Berbery, and Roberto Mechoso, focused on the La Plata River Basin’s regional climate and hydrology. It demonstrated how changes in the basin’s water cycle arising from climate variability, in turn, affected water resources management, agriculture, and ecosystems.
The Hydrological Cycle in Mediterranean Experiment (HYMEX), launched by the French hydrometeorological community, studies the Mediterranean Sea and its drainage basin. It seeks to improve our understanding of the area’s regional water cycle, emphasizing its seasonal, annual, and decadal variability and extreme events. Its purpose is to evaluate societal and economic vulnerability and the capacity to adapt to extreme weather and climate conditions.
RHPs were coordinated under the GHP. However, in 2008, the Coordinated Energy and Water Cycle Observational Period (CEOP2) consolidated CEOP1 and GHP activities until 2010, when GHP was separated out again, and the global collection of in situ and satellite data and NWP model outputs was left for GEO’s considerations. Figure 4 shows the global distribution of CSEs and RHPs in the 1992–2012 period.
These studies were supported by a water resource applications activity (referred to as the Water Resources Assessment Project) under Larry Martz and, later, Eric Wood, who made recommendations regarding priority user needs associated with the variability and change in the global water cycle. One specific plan by Lettenmaier and Wood (2009) recommended that GEWEX address non-stationarity in information available for water resources models.
Issues of hydrologic prediction on different time scales are addressed by governments and by non-profit organizations. IAHS, founded in Europe in 1922, has become a central body where hydrological scientists exchange their ideas on the latest issues and developments in hydrology. They have launched a number of studies and promoted hydrological observations and models and influenced the GHP activities. Through the Prediction in Ungauged Basins (PUB), a project initiated in 2003, IAHS scientists led an assessment of the current capability to provide runoff predictions and estimates in basins with no or inadequate observations (Wagener, Freer, Zehe, Bevan, Gupta, & Baradossy, 2006).
WCRP has stimulated initiatives to address broad water cycle issues. Two critical long-term issues, extremes and monsoons are closely tied to the global water cycle. The normal functioning of the global water cycle is often accompanied by extremes that lead to floods and droughts. Within the RHPs, flood and drought studies have made important contributions to the understanding of regional water cycling. It is also important to understand these variations in the context of the global water cycle, since there is some evidence of teleconnections between different regions of the world associated with anomalies in global precipitation patterns. Droughts are anomalies that often affect large areas for months to years. Floods are smaller in spatial scale and shorter in temporal scale but impacts can be sudden and devastating. These anomalies are driven by planetary and local scale forcing factors. GEWEX and CLIVAR are collaborating to assess the factors that cause, sustain, and terminate drought-producing anomalies. This collaboration, which began in 2006, also extends to joint efforts to develop a better understanding of the Asian and African Monsoon systems. In 2003, WCRP increased its capability to assess the role of polar regions in monitoring climate change and influencing the global circulation by launching the Climate and Cryosphere (CliC) project. This project dealt with the Antarctic, the Arctic, and other cold region/cold season areas, ensuring that the cryospheric aspects of the global water cycle were dealt with more uniformly. In the 2011–2012 period, cold season scientists began to study the “third pole,” or areas such as the Tibetan Plateau and other high elevation areas that influence the global atmospheric circulation through their impacts on the global energy and water cycle.
The Future of the Global Water Cycle
The global water cycle framework is positioned to continue its influence on program development in the coming decade. It is still an effective tool for promoting and coordinating research aimed at improving our understanding of water cycle processes and ensuring the public receives the maximum benefits from its investment in Earth observations. A future area of development will involve the integration of our understanding of the global water cycle with other biogeochemical cycles, including carbon, phosphorus, sulfur, micronutrients, and microorganisms. These biogeochemical processes, which are so important to life on Earth, are facilitated and mediated by global, regional, and local water cycles. The flows of water through the atmosphere, and on and beneath the land surface, control the transport, transformation, and interaction of the mineral and biological elements that sustain complex physical and chemical processes as well as environments for organisms and life forms. For example, runoff transports erode sediment, nitrogen, and phosphorus from land to water bodies where phytoplankton blooms and eutrophication occur as a result of these inputs. The dead zone at the outlet of the Mississippi River arises because nitrates from fertilizer applications are carried off agricultural fields and funneled down the river system to the Gulf of Mexico. Streamflow also carries salt and organic carbon to oceans in the form of eroded soils and salt.
GEWEX2 adapted to the scientific needs for a broader project when it implemented its third phase (2012 to 2020). Along with a name change to the Global Energy and Water Exchanges (GEWEX2) project, the new GEWEX2 mission emphasizes the use of improved observational, diagnostic, and modeling capabilities for measuring and predicting global and regional energy and water variations, trends, and extremes such as heat waves, floods, and droughts, and provides the scientific underpinning for support of water research management and climate services. It is important to quantify uncertainty in observations so that the signals associated with change (such as climate change) can be can be separated from natural variability. GEWEX2 will continue its role of facilitating, enabling, and coordinating international climate and related research activities with an emphasis on land-atmosphere processes and interactions. As a result, GEWEX focuses on data issues, analysis, modeling, and applications in its tradition of assessing the global water cycle, its linkages with energy budgets and other cycles, and interactions with society through many local water cycle processes.
The GEWEX2 project organized itself into four panels including the GEWEX Data and Assessment Panel (GDAP), the Global Atmosphere Surface Studies Panel (GASS), the Global Land/Atmosphere System Study (GLASS), and GHP. New RHP studies (current or planned under GHP) now include the Saskatchewan River Basin (SRB2), the Great Plains and Central Valley Project, the Latin American project, the Changing Cold Regions Network (CCRN), the planned North American Water RHP, the Southeast Asia RHP, the Southeast Asia Rice and Wheat regions, OzEWEX in Australia, the hydrology of Lake Victoria Basin (HYVIC), the Baltic Earth, the Pannonian Basin, and the continuation of HYMEX (see GEWEX for more details). It has also very actively engaged in the WCRP Coordinated Regional Climate Downscaling Experiment (CORDEX), an intercomparison of regional climate models.
Water often drives the processes that transport nutrients and supply water for plants and animals. Improved prediction of changes in the water cycle will allow us to anticipate the changes likely to occur in other biogeochemical cycles. Changes in the water cycle are a key mechanism for transmitting the effects of climate change to natural and human ecosystems. In the future, when the connections between the water cycle and other biogeochemical cycles are better understood, movements and predicted changes in water will contribute to an understanding of controlling factors and an ability to predict the living and abiotic factors in the Earth system. As described in the GEOSS Water Strategy (GEO, 2014), more attention will need to be given to essential water variables (EWVs) and to applications of our monitoring and prediction capabilities.
To address future global water cycle information needs, the scientific community has begun restructuring its activities to address the increasing effects of anthropogenic factors on the distribution of water globally. There is an urgent need to apply our knowledge of the global water cycle to resource management problems on all scales. The role of water in various nexus combinations is increasingly being understood. For example, water plays a central role in the water-energy-food nexus and, through its links with wetlands, the water-ecosystems-biodiversity nexus provides a useful method for monitoring biodiversity habitat losses.
Many of these water issues are being addressed by the Sustainable Water Future Project (SWFP) and Water Future, its contribution to Future Earth. Water Future addresses water problems using data, models, diagnostics, and the development and evaluation of options for problem solutions. SWFP was approved in 2015 and implemented in 2016. It examines the effects of climate variability, water infrastructure, and land use changes on water use and water quality to provide guidance on how data, information, and models can be used to achieve the SDGs and other objectives related to the sustainability of clean water for humans everywhere. SWFP also considers the application of technology, better management, and improved and more stable governance to solve problems. Two specific themes will guide these developments: Global Water System Assessments, which use knowledge synthesis of water research and incorporates it into sustainable solutions for water problems, and a Water Solutions Lab Network that facilitates the production and application of scientific and practical knowledge to water issues at local and regional levels.
The importance of water (including water quality) to ecosystems, biodiversity, and human health has been recognized. Many of these linkages are embedded in the SDGs, which the UN approved in September 2015. Not only are water monitoring capabilities important for tracking progress on these goals and their targets, but they also contribute to achieving these targets because they provide inputs for better water resources management and more timely decisions. The global water cycle activity seeks to more effectively use this knowledge for the benefit of society.
Integrated observation and assimilation systems are needed to provide data for the above applications. In 2015, CEOS formed a Water Strategy Implementation Study Team (WSIST) to assess the feasibility of developing or more effectively planning a satellite constellation to form the space segment of an observation system that would capture all fluxes and stores of the water cycle. The Study Team has explored the potential for developing synergies among measuring systems for precipitation, soil moisture, evapotranspiration, runoff, groundwater, and water storage. Its report was accepted by the CEOS Plenary in 2016. The Global Terrestrial Network in Hydrology (GTN-H) and the GEOGLOWS initiative continue to follow up on the GEOSS Water Strategy report. The linkages between observational and modeling capabilities are also being addressed as part of this assessment.
In summary, the global water cycle concept and its associated scientific framework have provided an incentive for understanding water cycle processes, spawned a great deal of research, and promoted discipline and product integration. A central component for this success has come from the emergence of technologies for observing the Earth from space and the commitment of space agencies to facilitate the use of these data for the benefit of society. While not all physical processes and interactions are fully understood, progress is being made thanks to improved access to comprehensive observations, significantly better process models, and an improved understanding and appreciation of water cycle processes and their interactions with other biogeochemical cycles. The global water cycle framework has provided a domain of scientific work that has allowed policy-makers to appreciate the comprehensiveness, scope, and urgency of water issues and the role of observations and research in addressing them. For program managers, the global water cycle framework also provides a basis for comparing research needs and on-going activities to identify gaps in programs and services and duplications in programs and to adjust priorities and investments accordingly. It has also led to innovations related to Earth satellites and tested methodologies that can be appropriately used to detect and assess water on other planets.
The authors would like to thank their home institution, Morgan State University, and NASA’s GESTAR program for their on-going support. They would also like to acknowledge the assistance of Andrée-Anne Boisvert in providing technical editing support and Robert Schiffer, Andras Szollosi-Nagy, and Dawn Erlich for providing background information of various global water cycle activities.
Appendix 1 List of Acronyms
(includes acronyms used only in the appendices)
Arctic Climate System Study
African Water Cycle Coordination Initiative
African Monsoon Multidisciplinary Analysis
Advanced Microwave Scanning Radiometer—Earth Observing System
Advanced Microwave Scanning Unit
Advanced Technology Microwave Sounder
Along Track Scanning Radiometer
Advanced Very High Resolution Radiometer
Asian Water Cycle Initiative
Biospheric Aspects of the Hydrologic Cycle project
Baltic Sea Experiment
Beaufort and Arctic Storms Experiment
Boreal Ecosystem-Atmosphere Study
Baseline Shortwave Radiation Network
Changing Cold Regions Network
Coordinated Enhanced Observing Period
Coordinated Energy and Water Cycle Observational Period
Committee of Earth Observing Satellites
Clouds and Radiation Energy Sensor
Climate and Cryosphere
Climate and Ocean: Variability, Predictability and Change project
Community of Practice
Coordinated Regional Climate Downscaling Experiment
Climate Prediction Program for the Americas
Cross Track Infrared Sounder
Canadian Space Agency
Continental Scale Experiment
Distributed Active Archive Centers
Dual Frequency Precipitation Radar
Essential Climate Variable
El Niño Southern Oscillation
Earth Observing Satellite or Earth Observing System
Earth Research Satellite
European Space Agency
Earth Science Enterprise
Essential Water Variable
First GARP Global Experiment
First ISLSCP Field Experiment
Flow Regimes from International Experimental and Network Data
GEWEX Atmospheric Boundary Layer Study
GEWEX Aerosol Climate Project
GEWEX Asian Monsoon Experiment
GEWEX Americas Prediction Project
Global Atmospheric Research Program
Global Atmosphere/Surface Studies Panel
GARP Atlantic Tropical Experiment
GEWEX Continental-scale International Project
Global Change Observation Mission—Water
Global Climate Observing System
Global Change Research Program
Global Cloud System Study
GEWEX Data and Assessment Panel
Group on Earth Observations
GEO Global Water Sustainability
Global Earth Observations System of Systems
Global Energy and Water Cycle Experiment
Global Energy and Water Exchanges
GEWEX Hydrometeorology Panel
Global Land/Atmosphere Systems Study
GPM Microwave Imager
Geostationary Operational Environmental Satellite-1
Global Ozone Monitoring Experiment
Global Precipitation Climatology Centre
Global Precipitation Climatology Project
Global Precipitation Measurement
Gravity Recovery and Climate Experiment
Global Runoff Data Centre
GEWEX Radiation Panel
Global Soil Wetness Project
Global Terrestrial Network for Hydrology
Global Water Vapor
(Global) Water and Development Information for Arid Lands
Global Weather Experiment
Global Water System Project
Hydrologic Applications Project
Hydrologic Ensemble Prediction Experiment
Hydrologic Cycle in the Mediterranean Experiment
Hydrology of the Lake Victoria Basin
International Association of Hydrological Sciences
International Council for Science (formerly International Council of Scientific Unions
International Geosphere Biosphere Programme
Integrated Global Observing Strategy Partnership
International GEWEX Project Office
Integrated Global Water Cycle Observations
International Hydrologic Decade
International Hydrologic Programme
International Land Ecosystem-Atmosphere Processes Study
Intergovernmental Oceanographic Commission
Intergovernmental Panel on Climate Change
International Satellite Cloud Climatology Project
International Satellite Land Surface Climatology project
Japan Aerospace Exploration Agency
Land Surface Fluxes
Large-scale Biosphere-Atmosphere Experiment
Lightning Imaging Sensor
La Plata Basin (RHP)
Mackenzie GEWEX Study
Monsoon Asian Hydro-Atmosphere Scientific Research Initiative
Murray Darling Basin
Microwave Humidity Sensor
Microwave Imaging Radiometer with Aperture Synthesis
Moderate Resolution Imaging Spectroradiometer
North American Monsoon Experiment
National Academy of Sciences
National Aeronautics and Space Agency
North American Water Project
National Centers for Environmental Prediction
Northern Eurasia Earth System Science Partnership Initiative
NASA Energy and Water Cycle Program
North American Land Data Assimilation System
National Oceanic and Atmospheric Organization
Natural Primary Productivity
Ozone Mapping and Polar Suite
Project for the Intercomparison of Land-surface Parameterization Schemes
Polar Orbiting Environmental Suite
Predictions in Ungauged Basins
Regional Hydroclimate Projects
Synthetic Aperture Radar
Societal Benefit Area
Sustainable Development Goals
Sea Surface Fluxes
Soil Moisture Active Passive (Mission)
Soil Moisture and Ocean Salinity (Mission)
Stratosphere-troposphere Processes And their Role in Climate
Surface Radiation Budget
Saskatchewan River Basin
Sustainable Water Future Project
Surface Water and Ocean Topography mission
Terrestrial Initiative in Global Environmental Research
Television Infrared Observation Satellite-1
TRMM Microwave Imager
Tropical Ocean—Global Atmosphere
Terrestrial Observation Panel for Climate
Tropical Rainfall Measuring Mission
United Nations Environment Programme
United Nations Educational, Scientific and Cultural Organization
US Global Change Research Program
Variable Infiltration Capacity (Model)
Visible and Infrared Imaging Radiometer
Visible Infrared Scanner
Water and Global Change
Water Cycle Integrator
World Climate Research Programme
Worldwide Integrated Study of Extremes
World Meteorological Organization
World Ocean Circulation Experiment
Water Resources Applications Project
Water Strategy Implementation Study Team
World Water Assessment Program
Appendix 2 Timeline for the development of global water cycle and related activities. (Refer to Appendix 1 for the definition of acronyms)
World Meteorological Organizations (WMO) formed
Provides international structure for national meteorological and hydrologic services
Launches Sputnik (and the space race)
TIROS-1 satellite relays TV images of the earth’s atmosphere
UNESCO and WMO launched the IHD
Measurement systems and studies were undertaken in basins around the world
WMO and ICSU launched GARP
Study of the global atmosphere
Global Runoff Data Centre (GRDC) formed
Provision of data services for streamflow data
USA Civilian Agency (NOAA)
First in a continuing series of geostationary satellites launched
GARP launched the GATE field program
GARP launches a study focused in the tropics
GARP launched the FGGE field campaign
The first global experiment for GARP
WMO and ICSU initiated the World Climate Research Programme (WCRP)
WCRP established to study climate processes on multiple scales
Global cloud products were derived from satellite data
CEOS was established based on the recommendation of an expert panel under the aegis of the G7
Coordination of space-based systems that remotely sense the earth’s environment.
Global precipitation data products were produced routinely from satellite data
ISLSCP carried out FIFE
Measurement of land surface fluxes in Kansas prairie
Formation of IPCC by WCRP and UNESCO
Assessments of the change in climate based on review of scientific literature
First phase of GEWEX began
Study of the global water and energy cycle
WCRP undertook the World Ocean Circulation Experiment (WOCE)
Study of the factors that drive the ocean circulation
GEWEX Continental International Project (GCIP) initiated
Study of water and energy budgets in the Mississippi River Basin
ISLSCP carried out BOREAS
Study of Boreal forests in northern Saskatchewan and Manitoba (Canada)
ACSYS and SPARC formed under WCRP
Study of the climate of the Arctic
WCRP initiated the CLIVAR project
Study of the role of oceans in the atmospheric circulation and predictability
CEOS and Space Agencies launch IGOS-P
Provision of data coordination at level of users and policy makers
Second phase of GEWEX launched
Continuation of study of water and energy budgets with greater role for models and data assimilation
CliC was formed by WCRP
Study of the interactions between the cryosphere and the climate
IAHS initiated the PUB initiative
Study of prediction capabilities and limitations in basins with no or limited measurements
IGWCO CoP formed and operated under IGOS-P and then GEO
Coordination of GEO-related water activities
The Group on Earth Observations (GEO) was formed
Coordination of the use of space assets in addressing societal issues including water resources
GWSP was formed as part of Earth System Science Partnership
Studies were undertaken of water resource assessments, basin water characteristics and water governance
Launch of CloudSat
Satellite provided data for expanded studies in cloud processes
The WATCH project was initiated
Intercomparison of hydrologic models
CEOP1 ended as a GEWEX initiative
Data systems continued to support AWCI
Launch of the Global Institute of Water Security (Saskatoon, Canada)
GIWS provided leadership for environmental water programs including SRB2 and CCRN
Third phase of GEWEX2 was initiated
Continued study of water and energy studies with focus on product assessment, models and data assimilation
CEOS launched WSIST to respond the GEOSS Water Strategy
Feasibility study of a global water cycle constellation
The GWSP project concluded
Projects under GWSP were transferred to SWFP or concluded
The GEO Global Water Sustainability Program (GEOGLOWS) was initiated
This project seeks to coordinate a broad range of water issues under GEO
The UN approved the SDGs and their targets
The water SDG creates opportunity for water science and monitoring
Future Earth was officially launched
New interdisciplinary research framework to explore human interaction with the environment
Launch of the SWFP under Future Earth
Links physical and social science to address water problems
Appendix 3 Projects and Activities that have been developed under GEWEX.
Pre-GEWEX (Projects started before 1990 and brought into GEWEX)
Geographical Focus (if any)
Gridded land surface data products, specialized regional data sets, land surface parameterizations
Precipitation data products
Cloud data products
GEWEX1 (1990–1994) (Included ISLSCP, GPCP, ISCCP)
Mississippi River Basin, U.S.
Specialized regional data sets, process studies, modeling studies, model development
Mackenzie River Basin, Canada
Specialized regional data sets, process studies, modeling studies
Baltic Sea Drainage Basin
Specialized regional data sets, process studies, modeling studies, model development
Four areas of eastern Asia (Tibet, Southeast Asia, Huai River Basin, Northeast Russia)
Specialized regional data sets, process studies, modeling studies
GEWEX Radiation Panel (GRP)
(Including GPCP, ISCCP)
Selected sites around the world
Surface radiation budget measurements
Water vapor data products
Aerosol data products
Radiation data products
GEWEX Hydrometeorology Panel (Including GCIP, MAGS, BALTEX, GAME, ISLSCP)
Amazon River Basin
Specialized regional data sets, process studies, modeling studies
Russia and surrounding nations
Specialized regional data sets, process studies, modeling studies
La Plata River Basin
Specialized regional data sets, process studies, modeling studies
Specialized regional data sets, modeling studies, model output data sets
GEWEX Modeling and Prediction Panel
Global soil wetness model data sets, model development
Land surface model development
Regional, global applications
Specialized regional cloud data sets, process studies, model development
Specific field study sites
Specialized regional data sets, process studies
GEWEX Radiation Panel
(Including GPCP, ISCCP, BSRN, GVaP, GACP, SRB)
Global data products, analytical studies
GEWEX Modeling and Prediction Panel (Including GSWP, PILPS, GCSS, GABLS)
Specialized regional cloud data sets, process studies, model development, model data products
GHP (including LBA, LPB, NEESPI, GAME, MAGS, CEOP)
Specialized regional data sets, process and diagnostic studies, modeling studies, model development
Specialized regional data sets, process studies, modeling studies, model development
Baltic Sea Drainage Area
Specialized regional data sets, process studies, modeling studies, model development
Process studies, modeling studies, model development
Specialized regional data sets, process studies, modeling studies
Mediterranean Sea Drainage Basin
Specialized regional data sets, process studies, modeling studies
Specialized regional data sets, process studies, modeling studies, model development
Regional applications in RHPs
Applications of information to water resource management issues and hydrologic problems
Diagnostic studies of extremes (floods and droughts)
Saskatchewan River Basin, Canada
Specialized regional data sets, process studies, modeling studies
Specialized regional data sets, process studies, modeling studies
Modeling and process studies for land-atmosphere interaction
Global water cycle data products
Modeling and process studies
GHP (Including HYMEX, SRB2, CCRN)
Lake Victoria Drainage Basin
Specialized regional data sets, process studies, modeling studies
Western U.S. and Canada
Specialized regional data sets, process studies, modeling studies
Southeast Asia rice and wheat regions
(*) Projects that are still in the planning stage.
Appendix 4 Selected Satellite Observing Systems referred to in the article—listed in chronological order.
Launch and/or begin data for data
End or decommissioning date
Instrument relevant to GWC
GWC Relevant Data/Products
NOAA-TIROS-1 (Television Infrared Observational Satellite)
April 1, 1960
June 15, 1960
TIROS (Two television cameras, one wide-angle, one telephoto)
Earth cloud cover and weather. First satellite to demonstrate that satellites could be useful to study the Earth.
NOAA-GOES-1 through 15 (Geostationary Operational Environmental Satellite system)
GOES-1: October 16, 1975 through GOES-15: March 4, 2010.
GOES-1 through 11: Decommissioned; GOES-12 through 15: Operational [USA maintains GOES-East at 75W, and GOES-West at 135W]
Multichannel Imager (IR, VIS) and Sounder.
Full disc view of earth; Frequent cloud imaging; Earth’s surface temperature, water vapor fields; Vertical temperature and moisture structure; Real-time coverage of severe storms and tropical cyclones.
ESA-ERS-1 (European Remote Sensing Satellite)
July 17, 1991
March 10, 2000
RA (Radar Altimeter); ATSR-1 (Along-Track Scanning Radiometer—4 channel scanning radiometer and microwave sounder); SAR (Synthetic Aperture radar); WS (Wind Scatterometer) MWR (Microwave Radiometer)
Single frequency radar altimeter data from RA; sea surface temperature and temperature at top of clouds (ATSR-1); changes in surface heights with sub-millimeter precision (SAR); Surface wind speed and direction (WS); Atmospheric water and correction for altimeter (MWR)
ESA-ERS-2 (European Remote Sensing Satellite)—successor to ERS-1
April 21, 1995
September 5, 2011
ERS-1 instrument suite plus GOMW (Global Ozone Monitoring Experiment), ATSR-2 (included 3 visible spectrum bands)
Similar to ERS-1 plus chlorophyll and vegetation from ATSR-2
NASA/JAXA TRMM (Tropical Rainfall Measuring Mission)
November 27, 1997
Decommissioned April 8, 2015
Precipitation Radar (PR), TRMM Microwave Imager (TMI), Visible and Infrared Scanner (VIRS), Clouds and Radiant Energy Sensor (CERES), Lightening Imaging Sensor (LIS)
Tropical precipitation, 3-dimensional structure (PR), horizontal patterns and rainfall intensities (TMI), clouds and radiant energy fields (CERES)
NASA-Terra (EOS-AM): Polar sun-synchronous; 10.30 a.m. equatorial crossing.
December 18, 1999. Data collection began: February 2000.
MODIS (Moderate Resolution Imaging Spectroradiometer):
Precipitable water, Clouds and cloud properties, land surface temperature, land cover, vegetation, evapotranspiration, albedo, snow cover
NASA-Aqua (EOS-PM): Polar sun-synchronous, 1.30 p.m. equatorial crossing.
May 4, 2002
MODIS (Moderate Resolution Imaging Spectroradiometer):
Clouds and cloud properties, water vapor, precipitable water, land surface temperature, land cover, vegetation, evapotranspiration, albedo, snow cover
NASA-Germany-GRACE (Gravity Recovery and Climate Experiment)
Two identical spacecraft 220 km apart in polar orbit. K-band ranging system, plus others.
Earth’s gravity anomaly field; Changes in ground water, soil moisture, surface water, etc.
NASA and CSA
Cloud profiling 94GHz nadir looking cloud profiling radar
Cloud optical depth, water content, cloud classification, column integrated precipitation, cloud ice microphysics
ESA-SMOS (Soil Moisture- Ocean Salinity)
2D Microwave Imaging radiometer with Aperture Synthesis, MIRAS; band (1.4 GHz)
Global soil moisture and ocean salinity
NOAA-Polar Orbiting Environmental Satellites. (N-POES). Also known as Advanced TIROS (TIROS-N or ATN.)
Most recent NOAA-19, launched: February 7, 2009
Operational. [Two satellite system, sun synchronous, with equatorial crossings at 7.30 a.m., and 1.40 p.m. local time]
AVHRR (Advanced Very High Resolution Radiometer); AMSU-A (Advanced Microwave Sounding Unit); MHS (Microwave Humidity Sounder)
Atmospheric variables, atmospheric data and cloud images; Visible and IR radiometric data used for imaging purposes and temperature and moisture profiles
NOAA-Soumi-NPP (National Polar-orbiting Operational Environmental Satellite system Preparatory Project)
October 28, 2011
ATMS (Advanced Technology Microwave Sounder); CrIS (Cross-track Infrared Sounder); VIIRS (Visible and Infrared Imaging Radiometer—22 band radiometer); CERES (Clouds and Earth’s Radiant Energy System); OMPS (Ozone Mapping and Polar Suite)
Global temperature and moisture (ATMS); Moisture and Pressure (CrIS);
Observations of weather, climate, oceans from 22-band radiometer in infrared and visible (VIIRS); Thermal radiation and reflected solar radiation (CERES)
JAXA-GCOM (Global Change Observation Mission)
GCOM-W (water): May 18, 2012
AMSR2 (Advanced Microwave Scanning radiometer 2);
Water circulation changes; precipitation, water vapor amounts; wind velocity above ocean, seawater temperature, water levels on land areas, snow depth.
NASA/JAXA-GPM (Global Precipitation Mission)
February 27, 2014
GPM Microwave Imager (GMI); Dual-frequency Precipitation Radar (DPR)
3-dimensional structure of precipitating particles (DPR); precipitation intensities and horizontal patterns (GMI). GPM provides global precipitation data sets.
ESA-Sentinel-1A, Sentinel 1B
Sentinel 1A: April 3, 2014; Sentinel 1B: April 25, 2016
All-weather, day-and-night radar imaging mission: C-Band synthetic aperture radar at 5.405 GHz
Radar imaging: monitoring sea ice, oil spills, marine winds, waves and currents, land-use change, land deformation and others such as floods and earthquakes.
NASA-SMAP (soil moisture active passive)
January 2015. Radar non-functional in July 2015.
Synthetic Aperture Radar, 1.2 GHz L-Band; 1.41 GHz L-Band Radiometer
Global soil moisture
June 23, 2015
Multispectral high resolution imaging for land monitoring
Imaging of vegetation, soil and water cover, inland waterways, coastal areas
NASA-France-SWOT (Surface Water and Ocean Topography)
Planned for 2020
Ka-band radar interferometry radar. Two radar antenna at either end of 10 meter mast
Earth’s land surface water heights
Sea surface heights
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