|Cooley, Keith - ARS RETIRED|
Submitted to: Journal of Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 14, 2000
Publication Date: October 1, 2000
Citation: Flerchinger, G.N., K.R. Cooley, A ten-year water balance of a mountainous semi-arid watershed, 2000, v. 237. p. 86-99. Interpretive Summary: Quantifying the components of the water balance for a watershed is crucial toward understanding of the dominant hydrologic processes occurring in a basin. Although a number of water balance studies have been conducted for a variety of watersheds throughout the world, the water balance of snow- fed, semi-arid, rangeland watersheds presents some interesting challenges. The Upper Sheep Creek Watershed is a semi-arid, snow-fed rangeland watershed dominated by the processes of snowmelt, evapotranspiration and subsurface water flow with ephemeral streamflow. Spatially variable precipitation, snow accumulation and vegetation present interesting challenges in this small, mountainous watershed. A ten-year water balance of the 26-ha Upper Sheep Creek Watershed was computed. Approximately 450 mm of precipitation is necessary to generate runoff from the watershed; above this threshold, runoff increases linearly with precipitation. An estimated 46 mm, or approximately 10% of the annual precipitation was estimated to be lost to deep percolation losses through fractures in the basalt underlying the watershed. Water percolating beyond the root zone was directly related to measured runoff. Above a threshold of about 50 mm, 67% of the water percolating beyond the root zone produces runoff. This can have important ramifications in addressing subsurface flow and losses when applying a snowmelt runoff model to simulate runoff and hydrologic processes in the watershed.
Technical Abstract: Quantifying water balance components, which is particularly challenging in snow-fed, semi-arid regions, is crucial to understanding the basic hydrology of a watershed. In this study, a water balance was computed using ten years of data collected at the Upper Sheep Creek Watershed, a 26- ha semi-arid mountainous sub-basin within the Reynolds Creek Experimental Watershed in southwest Idaho. The approach computed a partial water balanc for each of three landscape. Precipitation, which occurs predominantly as snow, was measured within each landscape unit directly and adjusted for drifting. Spatial variability of effective precipitation was shown to increase with increasing precipitation. Evapotranspiration, which accounted for nearly 90% of the effective precipitation, was estimated using the Simultaneous Heat and Water (SHAW) Model and validated with measurements from Bowen ratio instruments. Runoff from the watershed was correlated to precipitation above a critical threshold of approximately 45 mm necessary to generate runoff (r2 = 0.52). The average water balance error was 46 mm, or approximately 10% of the effective precipitation for the ten-year period. The error was largely attributed to deep percolation losses through fractures in the basalt underlying the watershed. Simulated percolation of the water beyond the root zone correlated extremely well with measured runoff (r2 = 0.90). Above a threshold of approximately 50 mm, approximately 67% of the water percolating beyond the root zone produces runoff. The remainder was assumed to be lost to deep percolation through the basalt. This can have important ramifications in addressing subsurface flow and losses when applying a snowmelt runoff model to simulate runoff and hydrologic processes in the watershed.