Orrigation Return FE ws in Southern Idaho

The quantity and quality of irrigation return flows from a 203,000-acre irrigation tract in southern Idaho were measured and compared to the quantity and quality of the irrigation water. Return flows for a typical water year amounted to 929,350 acre feet representing 64 % of the total water input to the tract. The total salt con-centration in the subsurface drainage water was more than twice that found in the irrigation water. The mean electrical conductivity of the subsurface drainage water was 1040i.trnhosl cm which is as low as in some irrigation waters. The Na+ concentration increased more than four limes as water passed through the soil and be-came subsurface drainage water. Similar com-parisons were made for other cations and anions. Surface runoff water did not differ from irrigation water in chemical quality. Surface drainage water from a 3,000-acre subregion con-tained up to 3.06 tons of sediment per acre foot during the midpart of the irrigation season