The Role of Soil Stratigraphy and Soil Moisture Dynamics On Stream Flow in a Mediterranean Catchment.

Soils with perched water tables are common in the metavolcanic terrain of California's Sierra Nevada Foothill Region. We hypothesize that the lateral redistribution of perched water is the dominant flowpath responsible for stream flow generation during storm events. An experimental watershed was developed to study the relationships between rainfall, temporal and spatial patterns in soil moisture, perched water flow, and stream flow. The following components of the hydrologic cycle were measured during the 2006-2007, 2007-2008 and 2008-2009 water years: 1) water content at 15 minute intervals in four genetic horizons (A, AB, Bt, and C/Cr) of 100 soil profiles distributed throughout the catchment; 2) lateral flow from the same horizons from a perched water monitoring trench; 3) stream flow; 4) rainfall amount and intensity; and, 5) water potential and perched water heights with tensiometers and peizometers positioned across major soil stratigraphic boundaries. Water samples were collected during one storm event in 2008 from each of these components including ground water. Perched water flow from the AB horizon represented 60% of the total lateral flow from the profile and coincided with the initiation of stream flow. This perched water was initially enriched (-45 ‰ for δD and -5 ‰ for δ¹⁸O) relative to the stream water and rapidly changed over the course of the storm event to match the value of rain and stream flow (-65 ‰ δD and -9 ‰ δ¹⁸O). Isotopic and hydrometric measurements therefore indicate that stream flow is dominantly new water as opposed to old water stored in the soil profile, because AB horizons are rapidly leached. The spatial extent of the source area for stream flow generation changed in response to rain intensity, evapotranspiration, degree of saturation, and horizon stratigraphy.