Monday, November 13, 2006 - 2:00 PM

Surface Water Quality in Irrigated Watersheds.

Dave Bjorneberg, USDA-ARS, USDA-ARS NW Irrigation & Soils, 3793 North 3600 East, Kimberly, ID 83341

Irrigation has transformed arid land into productive agricultural land. However, much of this land is prone to erosion during irrigation, which often increases sediment and nutrient concentrations in water returning from the irrigation district. Management practices have been developed to control soil erosion on irrigated land and improve the quality of water returning to streams and rivers. Applying polyacrylamide (PAM) with irrigation water can reduce erosion from furrow irrigated fields more than 90%. Using PAM in combination with other practices, such as applying straw mulch in furrows and installing small sediment ponds on fields, can virtually eliminate sediment loss from fields. Once soil runs off a field, it can be removed by settling in sediment ponds, although soluble nutrients remain in the water. Applying 20 mg L-1 alum to irrigation return flow water can remove about 50% of the soluble phosphorus that will not be removed as suspended sediment settles in ponds. Monitoring on an 82,000 ha irrigation tract in southern Idaho in 2005 showed that implementing conservation practices has reduced suspended sediment loss from the tract. April through November 2005 monitoring showed a net loss of 102 kg ha-1 of sediment compared to 461 kg ha-1 measured in a similar study in 1971. Using conservation practices will allow irrigation to continue with minimal impact on water quality.