Submitted to: American Society of Civil Engineers
Publication Type: Proceedings
Publication Acceptance Date: March 10, 1996
Publication Date: N/A
Interpretive Summary: Drainage and runoff water from irrigation is usually lower quality than the water supply. This review paper highlights the impact of water conservation on water quality in irrigation return flows. The basic effect is the reduction in the quantity of pollutant being transported to a surface stream as a result of water conservation. This is demonstrated in several examples of improved irrigation water management. When the applied irrigation is reduced to near the crop water requirement less nitrate is lost to ground water because of the reduction in deep percolation. Improvements in furrow irrigation management that reduce sediment transport may result in increased deep percolation. Matching the deep percolation losses to the leaching fraction through improved irrigation management will reduce the load of salt tranported to ground water or to subsurface drains.
Use of water for irrigation will result in the degradation of any water returning to the river system. The impact of conservation on water quality will depend on the site and use of agricultural water. Data from an experiment on the irrigation and fertilization management of corn demonstrated that nitrate transport by deep percolation was reduced when the applied irrigation matched the crop water requirement. Lysimeter experiments which evaluated salt transport as a function of leaching fraction demonstrated that the salt load was reduced as the leaching fraction was reduced even though the salt concentration in the leachate was highest for the lowest leaching fraction. Reducing sediment transport in furrow irrigation can be achieved by reducing tail-water runoff. However, methods to accomplish the reduction in tail-water can result in increased deep percolation. Each of these examples demonstrates that the impact of water conservation is to control the load of pollutant being transported. This is true for both surface and subsurface water.