Hydrology in a California oak woodland watershed: a 17-year study

The western foothills of the Sierra-Nevada are some of the most rapidly developing lands in California. Use of these lands includes vineyards, retirement and family home construction, livestock grazing, and fuelwood harvesting. These many uses require varying levels of woodland conversion and oak tree removal that alters the nutrient cycling, wildlife habitat and hydrology of these watersheds. There is little long-term hydrologic data to help determine the impact of these land use changes on water yield or quality. To fill this gap, precipitation and stream flow data were collected for 17 years in a 103 ha California oak woodland watershed, from which oaks were removed from 14% of the land area. These data were combined with measured potential evapotranspiration (PET) to develop a simple water balance and to investigate changes in water yield from oak removal. Hydrologic data included continuous stage height records from a three-foot Parshall flume and a one-foot 90 degrees V-notch weir. Rainfall measurements were made using a tipping bucket rain gage. Average annual rainfall, runoff, and estimated evapotranspiration (ET) for the 17 years were 708, 344, and 364 mm, respectively. In this Mediterranean climate, ET is less dependent upon rainfall than is runoff because the majority of precipitation coincides with the period of lowest PET. Mean annual baseflow depth was 24 mm ranging between 15 and 40 mm. Depth of baseflow was more strongly associated with the annual rainfall than with rainfall from previous years, indicating that changes in soil moisture storage approaches zero on an annual time-scale. Effective depth for watershed soils was calculated to be 217 mm. Potential soil water storage between bedrock and the top of the clay-rich subsoil (Bt Horizon) was 52 mm. This quantity accounts for summer ET and stream baseflow. A weakly significant difference between the pre- and post-harvest mean monthly effective rainfall was observed, indicating that oak removal, from 1984 to 1986, had little influence on watershed hydrology. Peak monthly effective rainfall corresponded to peak monthly runoff. The threshold of response to significantly increase water yield from oak harvesting is greater than 14% of a watershed area for the Sierra-Nevada foothills oak woodlands.