Linking Subsurface Lateral Flowpath Activity with Streamflow Characteristics in a Semiarid Headwater Catchment

Streamflow generation in semiarid Mediterranean environments is governed by the timing and amount of precipitation. Although topography is widely considered a dominant factor controlling the delivery of water to streams, we hypothesized that antecedent soil moisture and soil stratigraphy regulates the vertical dimension of hydrologic flowpaths and streamflow generation in a headwater catchment of the Sierra Foothill Region in California. We monitored soil moisture, subsurface lateral flow, streamflow, and meteorological conditions in a 33-ha catchment to study the linkage between hydrologic flowpaths in soil and streamflow characteristics. A total of 100 soil profiles were instrumented with soil moisture sensors at four depths in A, AB, Bt1, and 2Btss horizons. A hillslope perched-water collection system was used to measure subsurface lateral flow from these same horizons using tipping buckets. Streamflow was monitored using a V-notch weir for low flows and Parshall flume for high flows. Integrated hydrologic measurements show a close synchrony between streamflow and subsurface lateral flow in AB and Bt horizons overlying a hydraulically restrictive claypan (2Btss horizon). The thickness of the perched water table controlled the magnitude of subsurface lateral flow, which was greatest when AB horizons became saturated. Stream recession characteristics were controlled by lateral flow in less permeable horizons (Bt) directly overlying the claypan. Through the course of the water year, subsurface lateral flow from near surface horizons (A and AB) increased as antecedent soil moisture and thickness of the perched water table increased catchment-wide. The dynamic nature of hydrologic flowpaths in this system has implications for water quality as water is short-circuited to the stream through upper soil horizons. This study highlights the need for a detailed understanding of soil stratigraphic relationships to fully understand and model streamflow and water quality dynamics in semiarid soils with strongly contrasting physical properties.