Uncertainty in measuring runoff from small watersheds using instrumented outlet-pond

Authors: Nichols, Mary; Anson, Eric; Keefer, Timothy

Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/1/2014
Publication Date: 7/21/2014
Citation: Nichols, M.H., Anson, E.L., Keefer, T.O. 2014. Uncertainty in measuring runoff from small watersheds using instrumented outlet-pond. Transactions of the ASABE. 57(3):851-859. https://doi.org/10.13031/trans.57.10504.
DOI: https://doi.org/10.13031/trans.57.10504

Interpretive Summary: No measurements can be perfectly accurate. Runoff is measured on the Walnut Gulch Experimental Watershed in southeastern Arizona at ponds located at the outlet of subwatersheds. Uncertainty analyses were conducted to review current measurement procedures and to understand the effects of instrumentation, field methods, and data processing procedures on the accuracy of measured runoff. The largest measurement uncertainty was associated with flows that were less than 50 m3 in magnitude and with flows that exceeded the pond capacity and resulted in outflow through spillways. The largest source of measurement uncertainty was found to be related to establishing the position of the float sensor relative to the pond outlet elevation. This analysis is of practical importance for improving field measurement methods and for assessing the quality of runoff predictions that are based on simulation models developed using measured runoff data.

Technical 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.