A ten-year water balance of a mountainous semi-arid watershed

Quantifying water balance components, which is particularly challenging in snow-fed, semi-arid regions, is crucial to understanding the basic hydrolog y of a watershed. In this study, a water balance was computed using 10 year s of data collected at the Upper Sheep Creek Watershed, a 26-ha semi-arid m ountainous sub-basin within the Reynolds Creek Experimental Watershed in so uthwest Idaho, USA. The approach computed a partial water balance for each of three landscape units and then computed an aggregated water balance for the watershed. Runoff and change in ground water storage were not distingui shable between landscape units. Precipitation, which occurs predominantly a s snow, was measured within each landscape unit directly and adjusted for d rifting. Spatial variability of effective precipitation was shown to be gre ater during years with higher precipitation. Evapotranspiration, which acco unted for nearly 90% of the effective precipitation, was estimated using th e Simultaneous Heat and Water (SHAW) Model and validated with measurements from Bowen ratio instruments. Runoff from the watershed was correlated to p recipitation above a critical threshold of approximately 450 mm of precipit ation necessary to generate runoff (r(2) = 0.52). The average water balance error was 46 mm, or approximately 10% of the estimated effective precipita tion for the ten-year period. The error was largely attributed to deep perc olation losses through fractures in the basalt underlying the watershed. Si mulated percolation of the water beyond the root zone correlated extremely well with measured runoff (r(2) = 0.90), which is derived almost entirely f rom subsurface flow. Above a threshold of 50 mm, approximately 67% of the w ater percolating beyond the root zone produces runoff. The remainder was as sumed to be lost to deep percolation through the basalt. This can have impo rtant ramifications in addressing subsurface flow and losses when applying a snowmelt runoff model to simulate runoff and hydrologic processes in the watershed. (C) 2000 Published by Elsevier Science B.V.