HYDRAULIC CONNECTIVITY, LATERAL FLOW, AND STREAMFLOW GENERATION IN A SEMI-ARID, SNOWMELT-DRIVEN WATERSHED

The Dry Creek Experimental Watershed was established in 1999 in the foothills adjacent to Boise, Idaho to serve as a research and education field site for the hydrology program at Boise State University. Hydrologic research is being conducted from pedon to watershed scales. Here, we present some recent research findings relating hillslope processes to streamflow generation. Recent studies have shown that hydraulic connectivity on hillslopes and the existence of preferred soil moisture states in a watershed have important controls on runoff generation. In dry conditions, soil moisture patterns are spatially unorganized and lateral subsurface flow is neglible, whereas in wet conditions soil moisture patterns are organized by downslope lateral flow. In this study we investigate the relationships between annual soil moisture patterns, lateral hillslope flow, and streamflow generation in a semi-arid, snowmelt driven catchment. We identify five soil moisture conditions that characterize the hydrologic seasonality of a semi-arid site and present a conceptual model, based on field studies and computer simulations, of the dominant mechanisms of streamflow generation for different soil moisture conditions. The five moisture conditions include 1) a summer dry period, 2) a transitional fall wetting period, 3) a winter wet, low flux period, 4) a spring wet, high flux period, 5) and a transitional late spring drying period. Transitions between the periods are driven by changes in the water balance between rain, snow, snowmelt and evapotranspiration. We show that the transition from a dry moisture state to a wet moisture state is necessary, but not a sufficient condition for hydrologic connection within the hillslope. We propose that lateral flow along the bedrock interface contributes a major component of streamflow once the hillslope is connected hydraulically and that this state depends on the degree of wetting of deep soil pockets throughout the hillslope. The time lag between the dry-to-wet switch and the vertical-to-lateral switch occurs when the snowpack delivers little water to the soil and delays the deep soil wetting.