Soil moisture states, lateral flow, and streamflow generation in a semi-arid, snowmelt-driven catchment

Hydraulic connectivity on hillslopes and the existence of preferred soil moisture states in a catchment have important controls on runoff generation. In this study we investigate the relationships between soil moisture patterns, lateral hillslope flow, and streamflow generation in a semi-arid, snowmelt-driven catchment. We identify five soil moisture conditions that occur during a year and present a conceptual model based on field studies and computer simulations of how streamflow is generated with respect to the soil moisture conditions. The five soil moisture conditions are (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, and (5) a transitional late-spring drying period. Transitions between the periods are driven by changes in the water balance between rain, snow, snowmelt and evapotranspiration. Low rates of water input to the soil during the winter allow dry soil regions to persist at the soil-bedrock interface, which act as barriers to lateral flow. Once the dry-soil flow barriers are wetted, whole-slope hydraulic connectivity is established, lateral flow can occur, and upland soils are in direct connection with the near-stream soil moisture. This whole-slope connectivity can alter near-stream hydraulics and modify the delivery of water, pressure, and solutes to the stream.