Nitrate and Sediment Fluxes from a California Rangeland Watershed

• Published in: Journal of environmental quality 2006-11, Vol.35 (6), p.2202-2211
• Main Authors: Lewis, D.J, Singer, M.J, Dahlgren, R.A, Tate, K.W
• Format: Article
• Language: English
• Subjects: Agronomy. Soil science and plant productions; Applied sciences; beef cattle; Biological and medical sciences; California; Chemical Precipitation; Dry season; Earth sciences; Earth, ocean, space; Ecosystem; Engineering and environment geology. Geothermics; Environmental Monitoring; Exact sciences and technology; Fish; Fundamental and applied biological sciences. Psychology; Geologic Sediments - analysis; grasslands; grazing; Load; monitoring; nitrate nitrogen; Nitrates; Nitrates - analysis; Nitrogen - analysis; Pollution; pollution load; Pollution, environment geology; precipitation; Quercus douglasii; Quercus wislizeni; Rain; Rangelands; Rivers; Seasons; sediment transport; sediment yield; Sediments; Stream discharge; Stream flow; streams; Suspended sediments; Time Factors; total maximum daily load; Water monitoring; Water Movements; Water pollution; Water quality; Water quality management; Water resources; Water Supply; Watershed management; Watersheds; Woodlands
• ISSN: 0047-2425; 1537-2537
• DOI: 10.2134/jeq2006.0042

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.