Decreasing Sedimentation Loss from Surface Irrigation into Riparian Areas

Sediment carried in furrow irrigation return flows is a large contributor of sediment to riparian areas in the Pacific Northwest (PNW). About 3.3 million acres are surface irrigated in Oregon, Washington and Idaho, nearly two million in Idaho alone (Anonymous, 1991). Furrow irrigation is an inherently erosive process, but sediment losses in the PNW are worsened by the steeper slopes and high erodibility of typical furrow irrigated fields in the region. Irrigation-induced erosion is one of the most severe threats to sustainability of agricultural production in the region. Furrow irrigation-induced erosion commonly removes up to 22 tons/acre per year with a large part of this (as much as 3x the field averaged rate) occurring near the upper end of fields near furrow inlets (Berg and Carter, 1980; Kemper et al., 1985; Fornstrom and Borelli, 1984). Over 22 tons of soil loss per acre have been measured for a single 24 hr irrigation (Meth, 1959). Average field losses have removed over an eighth of an inch of topsoil per year from these irrigated fields. The magnitude of this problem is better appreciated when one recognizes that typical soil loss tolerance values for these soils are around 5 tons/acre per year. The net result has been that in the 80-100 years that PNW furrow irrigation has bee