OVERVIEW OF SOIL-EROSION FROM IRRIGATION

Of the 15,000,000 ha (37,000,000 acres) of irrigated land in the U.S., 21% is affected by soil erosion to some extent. Irrigation-induced soil erosion has been studied, primarily in the Northwestern United States, since 1940. A number of studies have measured annual sediment yields from furrow-irrigated fields exceeding 20 t/ha (9 tons/acre) with some fields exceeding 100 t/ha (45 tons/acre). Under the center-pivot sprinkler method, sediment yields as high as 33 t/ha (15 tons/acre) have been measured. Annual sediment yields as high as 4.5 t/ha (2 tons/acre) were measured from irrigation tracts. Erosion is seldom excessive on slopes less than 1% and is often excessive on slopes greater than 2%. Erosion reduces the agricultural productivity of the fields and causes off-farm damages. In southern Idaho, crop yield potential has been reduced by 25% due to 80 years of irrigation-induced erosion. Some irrigation districts spend more than $50,000 annually to remove sediment from drains. Sediment in irrigation return flows causes major water-quality degredation problems in several rivers in the Western United States.