Suspended sediment, water discharges, and water temperatures were monitored in four small drains in the DID-18 subbasin tributary to Sulphur Creek and to the Yakima River. The purpose was to provide a basis for evaluating the effectiveness of Best Management Practices in reducing sediment discharge from irrigated areas.Sediment inflow for the 1979, 1980, and 1981 irrigation seasons was 298 tons, 119 tons, and 88 tons, respectively. The average sediment concentration of inflow water was 99 mg/L (milligrams per liter) in 1979, 32 mg/L in 1980, and 24 mg/L in 1981. Sediment outflow minus inflow for the 1979, 1980, and 1981 irrigation seasons was 362 tons, 370 tons, 142 tons, and -19 tons in Drains 59.4, 59.6, 60.7, and 61.0, respectively; discharge-weighted mean sediment concentrations averaged 2,460 mg/L, 440 mg/L, 290 mg/L, and 12 mg/L, respectively. The sediment yield for the same drains for the 1979 to 1981 irrigation seasons averaged 1.5 tons per acre, 1.0 ton per acre, 0.6 ton per acre, and -0.16 ton per acre, respectively. The sediment yield from the four drains combined during the three irrigation seasons was 0.89 ton per acre. Drain 61.0 yields less sediment than it receives in irrigation water and acts as a diluter of sediment concentrations.
Sediment discharges could not be correlated with changes in BestManagement Practices because Imhoff Cone readings taken in the interior of the basins showed no statistical differences between irrigation seasons. However, Drain 61.0, which acted as a sink for sediment, contained more sprinkler-irrigated land and had a smaller proportion of row crops than the other three drains, even though soils and slopes were similar. Sediment yield usually related best to acres of row crops.The only major storm (February 16-21, 1980) produced 11 to 51 percent of the sediment discharged during the study.A sediment pond in Drain 60.7 had an average trap efficiency of 70 percent for the three irrigation seasons.
The regressionrelations between Imhoff Cone readings and suspended-sediment concentration for the outflow sites on Drains 60.7, 59.6, and 59.4 had corresponding correlation coefficients of 0.90, 0.94, and 0.90, respectively, and standard errors of 1,070, 2,410, and 2,460 mg/L, respectively. Use of the Imhoff Cone/suspended-sediment relationship to estimate sediment discharge generally gives poor results.
