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date: 23 May 2017

Environmental Benefits and Concerns of Center-Pivot Irrigation

Summary and Keywords

Center-pivot irrigation systems started in the United States in the mid-20th century as an irrigation method which surpassed the traditional surface irrigation methods. At that time, they had the potential to bring about higher irrigation efficiencies with less water consumption although their requirements in energy were higher too. Among their benefits, it is highlighted the feasibility to control water management as well as the application of agro-chemicals dissolved in the irrigation water and thus, center-pivot irrigation systems have spread worldwide. Nevertheless, since the last decade of the 20th century, they are facing actual concerns regarding ecosystem sustainability and water and energy efficiencies. Likewise, the 21st century has brought about the cutting edge issue “precision irrigation” which has made feasible the application of water, fertilizers, and chemicals as the plant demands taking into account variables such as: sprinkler´s pressure, terrain topography, soil variability, and climatic conditions. Likewise, it could be adopted to deal with the current key issues regarding the sustainability and efficiency of the center-pivot irrigation to maintain the agro-ecosystems but still, other issues such as the organic matter incorporation are far to be understood and they will need further studies.

Keywords: Center-pivot irrigation, sustainability, efficiency, benefits

Introduction

According to the Food and Agricultural Organization (FAO), the world population will reach about 9 billion by 2050, and this will require at least a doubling of global food supplies. However, this rise will vary among regions; as an example, commodities would have to increase by 75% in Southeast Asia. What has to be done to achieve this goal? Among other possibilities, food production must be guaranteed by taking into account factors such as an increase in farmland acreage and irrigation; sustainable use of fertilizers; biotechnology developments (breeding crop varieties producing more with less water/fertilizer, less prone to pest diseases, etc.). Nevertheless, agricultural land is limited, particularly in the developing world, and opportunities for further expansion will imply deforestation, soil erosion, and loss of soil organic matter. Likewise, irrigation accounts for about 70% of world water consumption and many areas worldwide are suffering water scarcity, more so with the impact of climate change, and have shifted from using surface water to groundwater without taking into account sustainability issues. As a result, aquifer water levels have declined steadily and most coastal aquifers have been salinized. Likewise, water scarcity has favored competition for water among users, creating social conflicts.

In addition, irrigated agriculture could have an effect in water quality since agrochemicals are usually dissolved in the irrigation water, and their application rates are often higher than required for a potential crop production. This may result in pollutant contamination of wells, rivers, and streams. Within this framework in irrigated agriculture, the increase in the efficiency of water/agrochemical application within irrigation systems is a key issue to meet future agricultural water demands. Application of modern technologies (site-specific irrigation agriculture) to irrigation systems, and the development of proper criteria for their management, considering the concepts of sustainability and resilience in the agro-ecosystems, would minimize offsite water quality impacts of irrigated production and will be at the core of future solutions for global food security.

Among irrigation methods, center-pivot irrigation systems (CPIS) offer some advantages from an environmental point of view, such as automated operation and lower labor costs, feasibility for applying shallow depths, good uniformity in water application, easier application of agro-chemicals, and reliability. These systems have been steadily spread worldwide since the 1980s.

CPIS

A CPIS is a pressure irrigation method which rotates at a center point irrigating a circular area (see Figure 1), although it can also operate in half-circle configurations (wiper pivot) by reversing its movement when a lever hits a stop placed in the field. The emitters locate along the main pipe usually at variable spacing; this is different from other sprinkler methods, where the emitters’ spacing is uniform. Nevertheless, some manufacturers offer CPIS whose emitters are at uniform spacing. In the last decades of the 20th century, the required inlet pressure, evaporation losses, and wind drift were highlighted as major drawbacks compared to other irrigation methods. However, the sprinklers from the early center pivots, which operated at high pressure, were replaced with smaller rotary impact sprinklers with lower pressure requirements; more recently, low-pressure spray nozzles, which operate at pressures as low as 69 to 103 kPa, have been introduced. Thus, CPIS can be selected to operate as low, medium, and high pressure. Regarding the second issue, wind drift and evaporation losses under sprinkler irrigation have been a research topic since the advent of sprinkler irrigation. A wide range of values of applied water, varying from 0.7% to 45%, has been reported in the literature, which has created misconceptions and confusion about the amount of water lost to wind drift and evaporation among irrigation experts, irrigation industry personnel, and the general public.

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Figure 1. Aerial view of center-pivot-irrigated area in Finney County, Kansas. From ASA/METI/AIST/Japan Space Systems, and US/Japan ASTER Science Team.

Nowadays, the market offers not only pivot center configurations but also linear-move systems which cover square or rectangular fields. Many of the observations for CPIS apply to linear-move systems. Manufactures can develop irrigation designs integrating CPIS and linear-move systems, adapting to the geometry of the agricultural field perimeter (see Figure 2). Growers may choose a specific center-pivot layout, taking into account crop water requirements, available water, and soil properties.

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Figure 2. Agricultural area irrigated by center-pivot and linear-move systems.

Source: Valley Irrigation, http://www.valleyirrigation.com/.

Elements and Design of CPIS

The CPIS are composed of one pipeline supported by several mobile A-frame towers which are suspended between 2 and 4 m above the soil (see Figures 3, 4b and 5), and are manufactured of aluminum or galvanized steel. At each tower, pipe sections are connected with flexible joints, which let the pipe move within a limited range (fixed by a given angle between two adjacent sections) without twisting or breaking. This flexibility allows vertical bending and enables CPIS climb moderately hilly slopes (see Figure 3). The first tower is anchored to a small concrete base at a fixed water supply point (hydrant) at the center of the field and houses the pivot mechanism and the main control panel (see Figure 4a). The towers move on wheels slowly (up to 6 m/min in the outermost span) around the pivot point, driven by an electric motor; thus, the irrigated area is circular. The last tower moves first and stops when the angle, corresponding to the joint between this tower and the preceding one, reaches a certain value. Then, the next tower starts moving and the process continues towards the pivot center point. The CPIS driven by electric motors have a discontinuous movement, while the new electric CPIS models have a continuous tower movement. In the hydraulic CPIS models, the movement is continuous and driven by the water pressure.

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Figure 3. CPIS adjusted to field topography.

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Figure 4. A CPIS. (a) detail of the first tower and (b) micro-sprinklers connected to the goose pipe.

Since the irrigated area enlarges as water moves along the pipe, the sprinkler nozzles increase gradually outward to maintain the same irrigation depth elsewhere. Likewise, a sprinkler end gun is usually inserted at the further pipe, whose discharge is the highest, irrigating the largest area located at the field boundaries. Figure 5 A picture of a rotator and an end gun can be seen in Figures 6.

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Figure 5. Field evaluation of a CPIS with an end gun sprinkler.

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Figure 6. Emitters in CPIS: (a) rotator and (b) end gun.

Source: http://www.nelsonirrigation.com/.

The length of a CPIS can vary from about 100 up to 1000 m, covering a average circular field of between 0.5 and 80 ha on average. The required inlet pressure in these systems will depend on variables such as the sprinkler model, center pivot length, and field topography, although common values are within the range of 202 to 506 kPa.

The sprinkler emitters can be connected directly to the pipeline and/or to the end of a drop tube inserted in the pipe, whose length depends on the maximum crop height. The ones connected to the irrigation line are called sprinklers and they spread the water far above the crop; the emitters suspended in the drop tube are called micro-sprinklers, and the water spreads just right above the crop canopy. Most modern CPIS feature drops hanging from a U-shaped pipe (see Figures 3 and 4).

In older systems, the sprinklers were connected to the pipe and operated at relatively high pressure (405–506 kPa), providing wide water spray patterns. However, in newer designs, the sprinklers usually locate on the drop tubes and are designed to discharge the same flow at lower pressures (101–301 kPa). Likewise, irrigators frequently install pressure regulators upstream of each sprinkler nozzle, which maintains the selected design pressure and improves irrigation efficiency.

Although there is no definite boundary between low- and high-pressure systems, it is commonly accepted that low-pressure systems have pressures at the pivot inlet less than 200 kPa, whereas high-pressure systems are those with pivot pressures higher than 350 kPa. In the last decades, the trend has shifted from manufacturing high-pressure sprinklers to low-pressure models. Moreover, adoption of low-pressure systems has been reinforced by the increase in energy prices worldwide. Likewise, current advances in sprinkler technology focus on the location of spray heads and low-pressure rotating sprinklers and nozzles. Also, advances in remote control of sprinklers and individual nozzle control are featured by precision agriculture.

The configuration of CPIS models allows emitters to be inserted at various locations along the main line. As the irrigated area increases towards the further end, sprinklers or spray nozzles can be located along the pipe at variable spacing or at fixed spacing but with variable sprinkler nozzle size. The CPIS are provided with a given configuration chart, recommended by the manufacturer, for the spacing and sprinkler nozzles, which would achieve the target uniformity for water application (see Table 1). Nevertheless, this is usually customized by irrigators according to field observations.

Table 1 Emitters’ Configuration Chart for a CPIS Given by the Manufacturer

Spacing Number

Distance From Center Point (m)

Emitter Model

Nozzle (#)

Diameter (mm)

3

7.1

R 3000 Plug Green # 10 Beige

10

1.98

5

12.7

R 3000 Plug Green # 10 Beige

10

1.98

7

18.3

R 3000 Plug Green # 10 Beige

10

1.98

9

24

R 3000 Plug Green # 11 Beige w/Gold

11

2.18

11

29.6

R 3000 Plug Green # 12 Gold

12

2.38

13

35.3

R 3000 Plug Green # 13 Gold w/lime

13

2.58

15

40.9

R 3000 Plug Green # 14 Lime

14

2.78

17

46.6

R 3000 Plug Green # 15 Lime w/lav

15

2.97

19

52.2

R 3000 Plug Green # 15 Lime w/lav

15

2.97

21

57.9

R 3000 Plug Green # 17 Lavender w/gra

17

3.37

23

63.5

R 3000 Plug Green # 17 lavender w/gra

17

3.37

25

69.2

R 3000 Plug Green# 18 Gray

18

3.57

27

74.8

R 3000 Plug Green # 19 Gray w/trqu

19

3.77

29

80.5

R 3000 Plug Green # 20 Turquoise

20

3.97

31

86.1

R 3000 Plug Green # 20 Turquoise

20

3.97

33

91.8

R 3000 Plug Green # 21 Trqu w/yllw

21

4.16

35

97.4

R 3000 Plug Green # 21 Trqu w/yllw

21

4.16

37

103

R 3000 Plug Green # 23 Yllw w/red

23

4.56

39

108.7

R 3000 Plug Green # 22 Yellow

22

4.36

41

114.3

R 3000 Plug Green # 24 Red

24

4.76

43

120

R 3000 Plug Green # 24 Red

24

4.76

45

125.6

R 3000 Plug Green # 24 Red

24

4.76

47

131.3

R 3000 Plug Green # 25 Red w/white

25

4.96

49

136.9

R 3000 Plug Green # 26 White

26

5.16

51

142.5

R 3000 Plug Green # 26 White

26

5.16

53

148.2

R 3000 Plug Green # 26 White

26

5.16

55

153.8

R 3000 Plug Green # 27 White w/blue

27

5.35

57

159.5

R 3000 Plug Green # 28 Blue

28

5.55

59

165.1

R 3000 Plug Green # 28 Blue

28

5.55

61

170.7

R 3000 Plug Green # 29 Blue w/brn

29

5.75

63

176.9

R 3000 Plug Green # 31 Brwn w/orng

31

6.15

65

182.9

R 3000 Plug Green # 30 Dark Brown

30

5.95

67

188.9

R 3000 Plug Green # 31 Brwn w/orng

31

6.15

69

194.9

R 3000 Plug Green # 30 Dark Brown

30

5.95

71

199.6

R 3000 Plug Green # 26 White

26

5.16

Further end (end gan)

199.6

P85ASV

10

The control panels for most CPIS models include a knob which adjusts the pivot speed, which in turn will set the amount of water applied over a given area (see Figure 7). This process is called variable-speed irrigation (VSI), also known as a percentage time setting (PTS), and farmers set the knob towards the position corresponding to the required irrigation depth. For a given inlet pressure, Table 2 shows the manufacturer’s recommendations to farmers for handling the knob. In addition to VSI, some CPIS models include also another option allowing variable-rate irrigation (VRI). The latter option is made possible by switching on and off groups of sprinklers during irrigation by means of additional feed valves.

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Figure 7. Elements of control panels in CPIS: (a) old model; (b) digital control in modern models.

Table 2 Relation Between Knob Position and Irrigation Depths of a CPIS

Knob Position

Irrigation Depth (mm)

1.00

2.81

0.95

2.96

0.90

3.13

0.85

3.31

0.80

3.52

0.75

3.75

0.70

4.02

0.65

4.33

0.60

4.69

0.55

5.11

0.50

5.63

0.45

6.25

0.40

7.03

0.35

8.04

0.30

9.38

CPIS with a Christiansen Uniformity Coefficient for Water Application (CUC) = 91%.

Water can be delivered to the pivot point from a river, a well, or a nearby reservoir. Modern CPIS have been manufactured to reduce both water application losses and energy requirements such as the low-energy precision application (LEPA) or low-elevation spray application (LESA). In both of these, drops with drag hoses allocate the water directly onto the soil surface between crop rows (LEPA) or with spray-type sprinklers located less than 0.6 m above the soil (LESA). The closer the water application to the soil, the lesser the evaporation losses and wind drift. In consequence, the uniformity for water application would increase.

Water Application Uniformity

The main factors affecting water distribution uniformity on CPIS are as follows: evaporation and wind drift, nozzle pressure, wind velocity, speed rotation, and sprinkler design. Inadequate pressure will result in a non-uniform water distribution and a reduction in the irrigated area; high pressures produce smaller drops (more prone to evaporation), high application rates near the sprinklers, and a smaller coverage. The wind will carry the water to further areas, affecting water application uniformity. Likewise, evaporation and wind drift often decrease the water application efficiency of the center pivot.

Water distribution patterns are affected by the sprinkler features: spray pattern, the nozzle type, number of nozzles, nozzle angle, and rotation speed. Other variables such as sprinkler wetting and inlet pressure must also be considered when designing CPIS. Thus, a proper design must be able to achieve the target water application uniformity across the field, avoiding under-irrigated areas (prone to plant stress) and over-irrigated areas (prone to runoff).

The Christiansen uniformity coefficient (CUC) is typically considered to determine water application uniformity. It is calculated through data gathered during the field evaluation following the experimental procedures available in standards such as ASAE S436.1 (1998). Basically, all of them consist of locating catch cans uniformly spaced (from 1 to 5 m depending on pivot radius) over one or two radii of the field (as shown in Figs. 3, and 4); then the CPIS operates under the farmer's conditions of inlet pressure and irrigation time, and water is collected in the containers during the irrigation. The CUC is later calculated as

CUC=100(1|ΔV¯|V¯)

where |ΔV¯| is the absolute standard deviation of the water volume collected andV¯ is the mean water collected. CUC values of ≥80 are considered adequate.

Field application efficiencies for properly designed and operated systems may vary worldwide from 50% to 95%. Figure 8 presents the values obtained in a field evaluation of the CPIS in a modern irrigation district in northern Spain in their second year of operation. Complementary to this, Figure 9 presents the simulations, considering the manufacturer configuration chart, for the CPIS, which shows that the end gun sprinkler lowers the CUC values. Likewise, end guns are not recommended since they usually create an uneven wetting pattern, particularly under windy conditions (even light winds).

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Figure 8. Values of the Christiansen uniformity coefficient from field evaluation of CPIS (PC); lateral move systems (RAF) and solid sprinkler sets (CT) from a modernized irrigation district in northern Spain.

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Figure 9. Simulated values of the Christiansen uniformity coefficient showing the effects of the end gun that were calculated considering the manufacturer configuration chart of CPIS (PC) and lateral move systems (RAF) from a modernized irrigation district in northern Spain.

Evaporation and wind drift increase at high pressures, small nozzle diameters, high wind speeds, and high vapor pressure deficits. In general, the losses produced by evaporation and wind drift account for 3% to 8% of the sprinkler's discharge. A high relative humidity environment around the crop has been observed in spray sprinklers discharging near the plant canopy, which reduces evapotranspiration. Likewise, evaporative losses increase in bare soils.

Precision Irrigation

Agricultural fields show variable yields in relation to differences in topographic heights, soil properties, tillage and soil compaction, fertility differences, localized pest distributions, intra-irrigation meteorological variability, and water application by the irrigation system. During the 20th century, and continuing into the 21st, the concept of “precision irrigation” has emerged from the integration into CPIS of technology by which specific on-site water application may take soil and crop type and crop growth stage into account.

Recent advances in communications and microprocessors have enabled the implementation of site-specific water applications by self-propelled center-pivot and linear-move sprinkler irrigation systems. CPIS have adopted the VRI, which defines various management zones considering variables such as soil physical and chemical properties, terrain elevation, and farming practices. Solenoid valves and electronic elements control and adjust water application rate in the management zone where VRI allocates the proper water depth for each zone. Thus, water mismatching is avoided in low- and high-elevation areas, reducing the spatial crop yield variability. In addition, leaching of agrochemicals towards groundwater may also reduce, providing a better area control of salinity and saturation.

In precision irrigation, the system automates and logs the readings of soil water content and crop canopy sensors to monitor water stress in plants. Sensors (located in the irrigation system and in the field) log the required data to assist technicians and practitioners in their decisions for irrigation management on a real-time basis. The approaches will require the integration of various sensor systems (on the machine and in the field), hardware, GPS, controllers, and computing power. These data are input into climate and crop growth models to estimate irrigation requirements “in situ” and “online.” Communication systems such as cell phones, satellite radios, and Internet-based systems broadcast the data to the main control panel or computer directly or through wireless networks. Then, these data are input into software algorithms to estimate irrigation criteria such as water doses and frequency in each management zone (see Figure 10). Irrigators can use the decision support system screen from any location at any time.

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Figure 10. Management zones in precision irrigation of a CPIS. (Note: green, light blue, and gray correspond to areas receiving 100%, 140%, and 160% of the required water depth, respectively; purple and pink highlight areas receiving between 60% and 70% of the required water).

In general, broad-based and easily modified software for managing these decision support systems are not currently available for most crops, climatic conditions, topography, and soil textures.

CPIS are designed to apply a relatively uniform amount of water to fields that are variable due to factors such as different soil types, topography, or multiple crops. This technology is feasible, but sometimes its cost may not be affordable for most irrigators. Moreover, in the short term, potential opportunities for water and nutrient conservation are promising if the concept of precision irrigation develops to include spatially precise irrigation. An increase in awareness of the need for water/energy conservation within scenarios of drought and short water supplies is foreseeable, as are tough regulatory actions to that end. Considering this framework, the use of precision irrigation could help in better monitoring of irrigation fields and better estimation of water needs, and, thus, improving water application uniformity and saving water, energy, and agrochemicals.

Water Application in CPIS

The application of water in CPIS is based on irrigation scheduling which fulfills crop water requirements. Nowadays, many farmers use sensors probes (soil water content, rain, radiation, temperature …), microprocessors, and computer-aided decision tools. These will provide support in their estimates of when to irrigate and how much water to apply, matching crop needs during all phenology states.

Crop water requirements fulfill the real crop evapotranspiration, which is estimated through climate variables: temperature, relative humidity, wind speed, and solar radiation (monitored in local weather stations), and the local crop coefficient for each plant phenology stage. The irrigation time is calculated by setting the proper limits to the available soil water content interval for plant growth. This can be determined in situ and on a daily basis through the measurement of soil water content by probes deployed at different field locations within the root profile.

Chemical Applications in CPIS

Chemigation has to do with the application of fertilizers, pesticides, herbicides, fungicides, and other chemicals dissolved in water. Soluble fertilizers, soil amendments, and pesticides are injected into the irrigation pipe, reducing labor costs and, in some cases, improving their application efficiency. Irrigation scheduling is intended to track with the optimal timing and rate of chemical applications for nutrient and pest management to best meet plant/soil needs.

Chemigation is applied to almost all major irrigated crops such as hay, corn, potato, cucumber, lettuces, peppers, tomato, cotton, and sugar beet, although the application of fertilizers is higher in high-value irrigated crops than in less-valued crops. Likewise, pesticides, herbicides, and fungicides, controlling weed and pest conditions, have also increased in irrigated areas. Consequently, careful nutrient and pest management would increase the effectiveness of water and dissolved chemicals, reducing their negative impacts.

Chemigation equipment must be corrosion-proof from soluble chemicals and a backflow protection device is required to protect against water source contamination. In general, liquid and dry fertilizers have been successfully applied with sprinklers. First, dry fertilizers are dissolved in a supply tank, and then they are injected to the irrigation pipe through electrical pumps, Venturi devices, or hydraulic valves. For center-pivot or other moving systems, the solution is supplied continuously across the field.

Chemigation efficiency increases in highly efficient irrigation systems. Water application uniformity in irrigation systems with a proper CPIS design can reach up to 90% or higher with an effective soil water monitoring program and appropriate management practices. In this regard, uniformities of 95% have been observed in the modified CPIS low-energy precision application (LEPA), although these systems are used in less than 1% of agricultural lands worldwide.

Nowadays, CPIS manufactured for variable-rate water application are also equipped with independent chemical application system. This is assembled in mini‐sprinklers and common commercial irrigation system components which are capable of spreading variable chemical applications both in tandem with and independently of selected water depths. Proper values for the Christiansen uniformity coefficient for water/chemical variable-rate applications range from 78.5% to 93%; however, these values are lower when these systems are clogged by suspended material in the water container and trapped air in lateral lines.

Clogging in CPIS

Clogging problems in sprinkler irrigation systems have been reported, particularly with primary and oxidation pond effluents. Biological growth (slimes) in the sprinkler head, emitter orifice, or supply pipes can cause blockages. High concentrations of suspended solids (e.g., algae) in the irrigation water may cause clogging and/or loss of pressure in pipes, valves, and pumps. As a general rule, water containing sediments must be filtered. Thus, not only may the use of adequate filters and their proper maintenance reduce clogging in the elements of the irrigation system, but they may help in keeping their longevity too. Likewise, chlorination of eutrophic waters is advisable to avoid blockages.

In the case of treated chlorinated wastewater, residual chlorine concentrations of less than 1 mg L−1 do not affect plant foliage. However, severe plant damage can be caused by chlorine concentrations over 5 mg L−1, when reclaimed wastewater is sprayed directly onto foliage.

Irrigation equipment (pipes, valves, pumps, hydrants, reservoirs) generally is made of materials such as concrete, iron, steel, bricks, asphalt, plastic, and synthetic fibers. The causes for concrete degradation can be classified as: intrinsic (material composition, manufacture, or aging); extrinsic chemical elements (CO2, mineral acids, pH, organic acids, bases, salts, NHC4, Mg, chlorides, sulfates); physical factors (elevated temperature), and mechanical factors (water velocity, abrasion, erosion). The presence of pollutants in the water can cause physical and chemical degradation processes in ferrous materials. These can suffer alterations and degradation by abrasion or erosion (suspended solids) or by corrosion (organic and inorganic chemical pollutants, pH). For all the above, it is advisable to use filtering systems in water carrying suspended material.

Soil Water and Nitrogen Distributions in CPIS

In a center-pivot-irrigated area (cultivated with common beans) under non-tillage and seeded irrigated agriculture and for the same irrigation management, the incorporated crop residues zones resulted in lower soil water content than the zones where the residues were left on soil surface. Both soil managements had similar available water and crop production but for the same soil water content, water was hold tightly in the pore space in soils with non-incorporated residues thus, the root plant system will require higher energy to absorb it (Souza, Saad, Sánchez-Román, & Rodríguez-Sinobas, 2016).

Effective management of strategies for water and nitrogen N can minimize input costs and environmental damage maintaining proper crop performance or economic returns restrictions. An optimal combination of irrigation and nitrogen fertilizer improves crop yield and water use efficiency. On the contrary, an excessive application of irrigation and nitrogen fertilizer would produce water loss and nitrogen leaching, especially in light-textured soils. Likewise, an excess on nitrogen application does not significantly increase crop yields but it could increase the nitrogen loss, considerably.

In general, an excess of nitrogen fertilizer and nitrate nitrogen accumulation within the soil profile often results in environmental problems such as groundwater contamination. This is caused by leaching of soil nitrate nitrogen and ammonia's emission.

Wen, Li, and Li (2014) reported a spatial-temporal soil distribution of nitrate nitrogen (NO3-N) during all corn phenology states grown in very coarse sandy soil and under different irrigation and fertilization strategies. Within the 0–100-cm soil layers, its concentration was greater for the larger application of nitrogen. For a given nitrogen application rate in the root zone, the lower irrigation depths retained higher NO3-N concentration at the end of crop development. Irrigation depths fulfilling crop evapotranspiration in the interval from 40% to 100% significantly increased yield and nitrogen agronomic efficiencies; these efficiencies reduced when evapotranspiration was below 40%. Consequently, for semiarid regions, the authors recommended the application of irrigation depths up to 100% of crop water requirements with a dissolved nitrogen rate of 160 kg ha−1 (N2).

The variability of nitrate and nitrogen distribution will depend on the uniformity of water distribution in the CPIS. As is illustrated in Figure 11, water application varies along the main line: the higher the uniformity, the better the efficiency in distribution of fertilizers.

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Figure 11. Salts on soil surface caused by salinization of irrigation water.

Hubbard, Assmussen, and Allison (1984) observed significant differences in shallow NO3-N groundwater concentration between an agricultural CPIS site and nearby on sandy soils of non-agricultural areas. In their study, NO3-N concentration ranged from 1 to 133 mg/L in the first and from 0.1 5 to 1 mg/L in the second. The NO3-N concentration under the pivot area varied seasonally according to cropping and hydrological patterns. In the long term, these results raise concern about the environmental impacts both of conversion of forest areas to agricultural land and of the application of fertilizers under CPIS on extremely sandy soils and shallow groundwater. In physiographic areas of relatively impermeable subsoil horizons, shallow groundwater could flow laterally to a larger drainage network, but if they are surrounded by an alluvial natural forest, most NO3-N contamination may be removed. However, if these subsoils are lacking, the groundwater NO3-N contamination may become a problem.

Environmental Concerns in Irrigated Agriculture Facing Limited Water Resources

In general, it is assumed that irrigated agriculture has brought both positive and negative environmental impacts. On one hand, aquatic and riparian habitats may be modified by irrigation and its runoff may be a source for potential pollutants in surface waters. On the other hand, in arid regions, irrigated agriculture may increase wetland areas when irrigation depths exceed crop water requirements, which is perceived as a social benefit since it enhances wildlife and increases recreation activities. In addition, the over-irrigation doses increased water returns by feeding subsurface flows, which in turn feed stream flows. Likewise, these water returns will be available for other uses: recreation, domestic, industry, environmental … Conversely, any actions improving irrigation efficiency would reduce them and thus the artificially induced habitats.

In arid areas, salinity is one of the biggest threats for the sustainability of irrigated agriculture (see Figure 12). Therefore an additional water fraction is added to the crop water requirements. This fraction has the role of leaching salts beyond the root soil profile and maintaining a proper salt concentration for optimal crop production, although it may increase the salinity concentration in the groundwater. A high concentration of salts within in the plant root zone may cause crop yield reductions or even the crop’s death, depending on the crop sensibility to salts. Likewise, soil salinization induced by irrigation may also be prevented by adopting adequate drainage and appropriate water management, although drainage systems are expensive and their effluents may degrade water quality, which will turn have human and environmental health implications.

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Figure 12. Water distribution along CPIS.

The major environmental concerns of irrigated agriculture are with regard to the protection and management of water resources and water quality. These issues vary among regions, although in almost all cases, they have not yet been properly addressed by national environmental standards and policies. Likewise, societal awareness about the environmental side effects of irrigated agriculture has increased during the past few decades, although nowadays, the world is still searching for solutions to make irrigation sustainable, both environmentally and economically.

Even though best practices in agricultural lands have been imposed since the last decades of the 20th century, novel and improved strategies and procedures are still needed to reduce soil erosion and surface and groundwater contamination. Likewise, they would have to take world food production and food safety into account, while maintaining strategic, economic, and social benefits.

Long-term perspectives point out that climatic change will affect the viability of water resources, and that the world population will reach about nine billion persons by 2050. Thus, food demand will double, and water demands for irrigation and human consumption will increase too. On a global scale, irrigation accounts for 70% of total freshwater withdrawals, but this is unevenly distributed among regions. Many areas in the world (the United States, the Middle East, North Africa, the semiarid tropics of India, China, Africa, and Central America) are suffering severe water scarcity. Moreover, the demand on groundwater resources for irrigation has increased in areas with surface water scarcity; however, its exploitation is sometimes costly in terms of energy (pumping costs) and the environment (groundwater is often nonrenewable in the short term), raising concerns about their environmental sustainability. In addition, it is sometimes unsuitable for irrigation due to its low temperature (thermic shock) or high salinity.

Agricultural water security is obviously a major part of sustaining irrigated agriculture. Factors such as competition for water, environmental concerns, continued urbanization, government policy, and the globalization of the economy may impact water security.

Within this framework, new water management strategies and more efficient delivery and on-farm water systems will be the key points for global food security. The adoption of improved irrigation technology can help to reduce problems regarding offsite water quantity and quality, and may increase water use efficiency at the farm level and achieve water savings.

Effects of CPIS in Soils, Plants, and Carbon Sequestration

One of the side effects of sprinkler irrigation is the impact of large droplets in soil compaction. When soil surface is compacted, the soil structure is modified, reducing soil infiltration and favoring runoff and erosion (see Figure 13). Thus, the total soil water available for the crop will reduce. Since this effect is more prompt in bare soil, its negative impact would be reduced in mulched soils. Likewise, the droplet energy absorbed by leaves could affect seedlings and other tender vegetation; hence, a proper selection of operating pressure and/or emitter orifice will benefit vegetation.

Environmental Benefits and Concerns of Center-Pivot IrrigationClick to view larger

Figure 13. Soil erosion in an irrigated area.

High temperatures produce heat stress on crops such as lettuce, potatoes, green beans, small fruits, tomatoes, cantaloupe, and cucumbers. Thus, irrigating during the day will reduce this stress and will maintain crop yield. Other factors affecting plant stress include low humidity, rapid air movement, and dry soils. When these conditions happen at a critical state of crop growth (emergence, flowering, or fruit enlargement), it is advisable to supply a low water depth at midday. Likewise in windy areas, irrigation at night will provide a more uniform coverage since winds are at a minimum speed.

In conditions of air ambient temperature higher than 50°C, emerging seedlings will often die as a result of high transpiration. Under these conditions, it is recommended to spread small water applications to ensure emergence and good stands. Likewise, it could improve herbicide effectiveness for weed control.

In organic or sandy soils cultivated with small seeded crops, the soil dries quickly and seeds may be blown away or covered with too much soil, making them incapable of germination. In addition, the small plants are prone to damage from wind-blown particles. In these conditions, crops can be protected by sprinkling at low rates, up to 2.5 mm/h.

CPIS are feasible for any soil type, although the sprinkler rate should be selected to suit a similar soil infiltration capacity. Water application rates exceeding the soil infiltration rate will result in ponding and runoff, producing non-uniformity in water distribution, water loss, and soil erosion. Water applications much lower than the soil infiltration rate have been reported as beneficial. Sprinkler nozzles operating with pressures providing a fine spray combined with water application rates that are half of the soil infiltration rate improved the maintenance of soil structure and minimized soil compaction (Huffman, Fangmeier, Elliot, & Workman, 2013).

Chemical elements such as sodium and chloride are absorbed by plant leaves. When they are absorbed over a certain value, plant growth and yield are reduced. Frequent irrigation increases foliar water absorption and create highly moist conditions, which may promote diseases, although sensitivities to high relative humidity differ among crops. In addition, the presence in the irrigation water of trace elements, such as arsenic, boron, cadmium, chromium, lead, molybdenum, and selenium, can also be toxic to some crops and human health. Nevertheless, some of these effects can be decreased by sprinkling at night, reducing evaporation losses, although it is recommended to determine the crop chemical sensitivity and double-check the composition of the water supply and the soil.

Gillabel, Denef, Brenner, Merckx, and Paustian (2007) reported that soil carbon stocks, through preferential sequestration of carbon inside micro-aggregates, increased in irrigation areas: there was about 25% more carbon storage in CPIS than in dryland. These results highlight the potential of the irrigation method for enhancing soil carbon sequestration in (semi)arid agricultural areas.

Soil erosion can be a serious problem for CPIS, particularly on steeply sloping fields and under the outer spans. These convey the highest water application rates and can contribute to offsite water quality problems. Thus, the application of small irrigation depths per irrigation unit, the selection of larger-pattern sprinkler heads, and booms to increase sprinkler head spacing are recommended for improving the uniformity of water application in CPIS: the higher the water application uniformity, the lower the soil loss. Similarly, other practices such as the maintenance of crop residue on the soil surface increase infiltration while protecting the soil. In some cases, deep tillage can reduce runoff through increased infiltration.

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