One of the most effective methods of water conservation under flood irrigation systems is minimizing the tailwater or runoff water from irrigated fields. This can be accomplished by collecting the tailwater and reusing it for irrigation on the farm or minimizing the tailwater that leaves the field.
Cutback irrigation is another way to minimize runoff and improve water application efficiency. Cutback irrigation is simply reducing the inflow rate of water to graded furrows after the water reaches the lower end of the field. The basic objective of cutback irrigation is to reduce surface runoff from the field by matching the inflow rate with the average infiltration capacity of the soil. Cutback irrigation is one of the irrigation management practices that are commonly used to improve the efficiency of surface irrigation systems. Cutback irrigation increases irrigation efficiency by reducing the volume of runoff water or tailwater. Cutback irrigation is similar to tailwater recovery systems where irrigation efficiency is improved by reducing runoff losses.
Application efficiency is the ratio of the amount of water that is beneficially used by the crop to the total amount applied. Beneficial use includes crop water use and leaching. When cutoff irrigation is employed, the amount of runoff water generally decreases which improves application efficiency. Runoff can be eliminated when the rate of inflow equals the average infiltration rate over the entire length of the field.
It is not any easy task for an irrigator to adjust the inflow rate so it matches the average infiltration rate over the entire field. Cutback irrigation may require the installation of values at the head end of the field to control inflow rate. Infiltration rate varies with time and space (along the field). Therefore, cutback irrigation is commonly used to reduce runoff and not to eliminate tailwater. The width of irrigation set needs to be adjusted during the irrigation event in order to employ this method.
Since infiltration rate decreases over time, cutback irrigation is usually employed by cutting inflow rate several times during irrigation events. In general, multiple cutbacks are more effective in reducing runoff than a single cutback in flow rate. However, multiple cutbacks may require the installation of automated valves. As it is the case with other water conservation measures, implementation of cutback irrigation requires extra labor and improvements in the water delivery system.