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TECHNICAL REPORTS

Surface Water Quality

Nitrate and Sediment Fluxes from a California Rangeland Watershed

^a University of California Cooperative Extension, Santa Rosa, CA 95403
^b Department of Land Air and Water Resources, University of California, Davis, CA 95616
^c Department of Plant Sciences, University of California, Davis, CA 95616

^* Corresponding author (mjsinger{at}ucdavis.edu)

Received for publication January 27, 2006. Long-term water quality records for assessing natural variability, impact of management, and that guide regulatory processes to safeguard water resources are rare for California oak woodland rangelands. This study presents a 20-yr record (1981–2000) of nitrate-nitrogen (NO₃–N) and suspended sediment export from a typical, grazed oak woodland watershed (103 ha) in the northern Sierra Nevada foothills of California. Mean annual precipitation over the 20-yr period was 734 mm yr^–1 (range 366–1205 mm yr^–1). Mean annual stream flow was 353 mm y^–1 (range 87–848 mm yr^–1). Average annual stream flow was 48.1 ± 16% of precipitation. Mean annual NO₃–N export was 1.6 kg ha^–1 yr^–1 (range 0.18–3.6 kg ha^–1 yr^–1). Annual NO₃–N export significantly (P < 0.05) increased with increasing annual stream flow and precipitation. Mean daily NO₃–N export was 0.004 kg ha^–1 d^–1 (range 10^–5 to 0.55 kg ha^–1 d^–1). Mean annual suspended sediment export was 198 kg ha^–1 yr^–1 (range 23–479 kg ha^–1 yr^–1). There was a positive relationship (P < 0.05) between annual suspended sediment export, annual stream flow and precipitation. Mean daily suspended sediment export was 0.54 kg ha^–1 d^–1 (range 10^–4 to 155 kg ha^–1 d^–1). Virtually no sediment was exported during the dry season. The large variation in daily and annual fluxes highlights the necessity of using long-term records to establish quantitative water quality targets for rangelands and demonstrates the difficulty of designing a water quality monitoring program for these ecosystems.

Abbreviations: LOD, limit of detection • TMDL, total maximum daily load