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Long-term water balance investigations are needed to better understand hydrologic systems, especially semi-arid mountainous catchments. These systems exhibit considerable interannual variability in precipitation as well as spatial variation in snow accumulation, soils, and vegetation. This study extended a previous 10-year water balance based on measurements and model simulations to 24years for the Upper Sheep Creek (USC) catchment, a 26ha, snow-fed, semi-arid rangeland headwater drainage within the Reynolds Creek Experimental Watershed in southwestern Idaho, USA. Additional analyses afforded by the additional years of data demonstrated that the variability between streamflow and annual precipitation (r²=0.54) could be explained by the timing of precipitation and antecedent moisture conditions. Winter–spring precipitation and soil moisture deficit at the beginning of the water year accounted for 83% of the variability in streamflow, which was almost as accurate as applying the more complex physically-based Simultaneous Heat and Water (SHAW) numerical model (r²=0.85) over the three dominant land cover classes. A conceptual model was formulated based on field observations, numerical simulations and previous studies. Winter precipitation and spring snowmelt must first replenish the deficit within the soil water profile and ground water system before water is delivered to the stream. During this period, surface water and ground water are tightly coupled and their interaction is critical to streamflow generation. Shortly after snow ablation, however, water flux in the root zone becomes decoupled from the ground water system and subsequent precipitation does little to contribute to streamflow for the current year, but serves to offset ET and the soil moisture deficit at the beginning of the following year. This study demonstrates the merits of long-term catchment-scale research to improve our understanding of how climate and land cover interact to control hydrologic dynamics in complex mountainous terrain.
