Submitted to: American Geophysical Union
Publication Type: Proceedings
Publication Acceptance Date: 9/8/1999
Publication Date: N/A
Technical Abstract: Quantifying the components of the water balance for a watershed is crucial toward understanding the dominant hydrologic processes occurring in a basin. The water balance of snow-fed, semi-arid, rangeland watersheds presents some particularly interesting challenges. These watersheds, dominated by precipitation and evaporation, exhibit a high degree of variability in snow distribution and vegetation communities on scales much smaller than that addressed by most hydrologic modeling. The Upper Sheep Creek Watershed is a semi-arid, snow-fed rangeland watershed dominated by the processes of snowmelt, evapotranspiration, subsurface water flow, and ephemeral streamflow. Approximately 450 mm of annual precipitation is necessary to generate runoff from the watershed; above this critical threshold, runoff was linearly correlated to annual precipitation (r2 = 0.52). A ten-year water balance of this 26-ha watershed was computed by dividing the watershed into landscape units (low sagebrush, mountain big sagebrush and aspen), computing a partial water balance for each, and then aggregating these together to compute an overall water balance of the watershed. The average water balance error was 46 mm, or approximately 10% of the estimated 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, only 67% of the simulated percolation produced 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.