This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Environmental Science. Please check back later for the full article.
Surface irrigation is the oldest and most-used irrigation method and is used on more than 83% of the world's irrigated area. It comprises traditional systems, developed over millennia, and modern systems with mechanized and often-automated water application, adopting precise land leveling. It adapts well to non-sloping conditions, low- to medium-soil infiltration characteristics, most crops and crop mechanization, as well as environmental conditions. Modern methods provide for water and energy saving, control of environmental impacts, labor saving, and cropping economic success—thus competing with pressurized irrigation methods. Surface irrigation refers to a variety of gravity applications of the irrigation water that infiltrates into the soil while flowing over the field surface. The ways and timings of how water flows over the field and is infiltrating the soil determines the irrigation phases—advance, maintenance, or ponding; depletion and recession—which vary with the irrigation method, namely, paddy basin, leveled basin, border, and furrow irrigation generally used for field crops; and wild flooding and water spreading from contour ditches, yet used for pasture lands. System performance is commonly assessed using the distribution uniformity indicator while management performance is assessed with the application efficiency or the beneficial water use fraction. The factors influencing system performance are multiple and interacting—inflow rate, field length and shape, soil hydraulics roughness, field slope, soil infiltration rate, and cutoff time—while management performance, in addition to these factors, highly depends upon the soil water deficit at time of irrigation, thus on the way that farmers are able to manage irrigation. The process of surface irrigation is complex to describe because it combines the surface flow process with the process of infiltration into the soil profile. Numerous mathematical computer models, therefore, have been developed for its simulation aimed at both design adopting a target performance and field evaluation of actual performance. The use of models in design allows the consideration of the factors referred to before and, when adopting any type of decision support system or multi-criteria analysis, to also take into consideration economic and environmental constraints and issues.
There are various aspects favoring and limiting the adoption of surface irrigation. Favorable aspects include the simplicity of its adoption at farm in flat lands with low infiltration rates, namely, when water conveyance and distribution are performed with canal and low-pressure pipe systems, low-capital investment, and low-energy consumption. Most significant limitations include high-soil infiltration and high variability of infiltration throughout the field, land leveling requirements, need for control of a constant inflow rate, difficulties in matching irrigation time duration with soil water deficit at time of irrigation, and difficult access to equipment for mechanized and automated water application and distribution. The modernization of surface irrigation systems and design models, as well as models and instruments usable to support surface irrigation management, have significantly impacted water use and productivity, and thus the competitiveness of surface irrigation.