Measured Sediment Yield Rates From Semiarid Rangeland Watersheds

Data describing long-term sediment yield rates on semiarid rangeland watersheds are relatively rare. To augment existing data and gain a better understanding of the controlling variables, sediment yields from 8 subwatersheds within the US Department of Agriculture-Agricultural Research Service Walnut Gulch Experimental Watershed in southeastern Arizona were computed from stock pond sediment accumulation measurements, water level records, and estimates of sediment transported in pond overflows. Sediment accumulation records ranging from 30 to 47 years were evaluated for subwatersheds ranging in size from 35.2 to 159.5 ha. Sediment yield ranged from 0.5 to 3.0 m3 ha-1 y-1, with a mean of 1.4 m3 ha-1 y-1 and a standard deviation of 1.0 m3 ha-1 y-1. As expected, runoff volume was a significant factor (P = 0.005) in explaining the variability in sediment yield, but regression analysis demonstrated other variables are important. For example, the ratio of watershed area to main channel length significantly described (P = 0.06) sediment yield, suggesting that more detailed measurements are needed to characterize channel networks to relate internal watershed sediment transport and deposition processes to sediment delivery at the outlets. To generalize sediment yield rates across rangeland regions, additional research is necessary to determine the relative influence of rainfall and runoff patterns, and watershed physiographic and geomorphic characteristics on sediment transport.