Uncertainty in Measuring Runoff from Small Watersheds Using Instrumented Outlet Ponds

Abstract. This study quantified the uncertainty associated with event runoff quantity monitored at watershed outlet ponds. Inflow and outflow depth data were collected from 2004 to 2011 at seven instrumented monitoring stations at the outlet of watersheds ranging in size from 35.2 to 159.5 ha on the USDA-ARS Walnut Gulch Experimental Watershed in southeastern Arizona. The effects of instrumentation, field methods, and data processing procedures were considered. Uncertainty was assessed separately for runoff that did not exceed pond capacity and for runoff that exceeded pond capacity and was discharged through overflow spillways. The largest relative measurement uncertainty was associated with runoff volumes that were less than 50 m³ in magnitude and with events exceeding pond capacity and discharging over the spillway. The largest source of measurement uncertainty was associated with error in establishing the position of the depth sensor relative to the pond spillway hydraulic control elevation. This analysis is of practical importance for improving field methods for measurement of pond inflow and outflow and was conducted to encourage similar analyses of flow data collected from all monitored watersheds.