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Oak- and annual grass-dominated rangelands in California occupy 7.4 million acres, often occurring at the state's urban, wildland and agricultural interface. Rapidly changing land uses in these ecosystems have watershed-scale impacts that are the subject of intense debate among policy-makers, environmentalists and landowners. Watershed research conducted at the UC Hopland Research and Extension Center (HREC) between the 1950s and 1980s provided valuable information for predicting the effects of watershed management activities — such as converting oak and chaparral to grasslands — on water quantity and quality, slope stability and erosion. The research illustrated that conversion from woodland to grassland significantly impacts the hydrology and sediment dynamics of watersheds, suggesting that land-use changes such as vineyards and urban expansion must be evaluated carefully. Preliminary data from a new series of watershed studies initiated at HREC in 1998 indicate that livestock grazing does not significantly increase nutrient and sediment levels in stream water, but that current fecal coliform standards may be exceeded during storm events.