Comparison of Two Stream Gauging Systems for Measuring Runoff and Sediment Yield On Semi-Arid Watersheds.

Our ability to understand erosion processes in semi-arid ecosystems depends on establishing relationships between rainfall, runoff and sediment yield and determining the key factors that influence these relationships. This requires collection of extensive and accurate hydrologic data sets. A supercritical flume with traversing slot sediment sampler used on several sites at Walnut Gulch Experimental Watershed (WGEW) near Tombstone, AZ proved to be a reliable way to measure flow and sediment discharge from small watersheds. However, it requires installation of a costly permanent structure that interferes with erosion process and is only suitable for relatively small flows. CSIRO Land and Water (Australia) developed an alternative in-channel fully automated system for measuring water velocity, depth, turbidity and collecting runoff samples. A 3.7 ha arid watershed at WGEW was instrumented with both systems and hydrologic data was collected and compared during 2 year period (10 runoff events). Total sediment yield for the entire period measured by pump sampler (9.7 t ha^-1) was underestimated by 16% comparing to traversing slot sampler (11.5 t ha^-1). Pump sampler consistently underestimated the amount of coarse (>0.5 mm) sediment fractions. Median sediment diameter of samples collected by traversing slot and pump sampler were 0.35 and 0.28 mm respectively. Water turbidity was well correlated with concentration of fine (<0.5 mm) sediment (R2 = 0.71) and could be used to supplement sediment data for non-sampled and under sampled runoff events. The study outlines the limitations of the pump sampler based system and makes recommendations for improvement of its performance.