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Understanding and Monitoring Watersheds

Understanding and Monitoring California Hardwood Rangeland Watersheds

Oaks ‘n’ Folks – Volume 17, Issue 1 – February 2001

Watershed functions include the collection, storage, and transport of water. In California’s hardwood rangeland watersheds, these functions have distinct characteristics resulting from the prevailing Mediterranean climate and vegetation. The United States Clean Water Act and the State of California Porter-Cologne Act have been established, primarily to protect water quality in these and other watersheds. In many cases these regulations are developed through a political process, and in the absence of scientific understanding about the interactions between land use and watershed functions. Generating that understanding is imperative to making effective land use management and policy decisions.
One of the ways to improve our understanding of how land use influences watershed function is to conduct long-term watershed scale studies on water yield and quality. Beginning in 1980, professor Michael Singer established a long-term research watershed at the University of California Sierra Foothills Research and Extension Center. Known as the Schubert watershed, this 203-acre catchment has been managed for beef cattle grazing throughout the duration of this study. His work continues and has been incorporated into a network of eleven research watersheds under study by a multi-disciplinary team from the University of California.

The team’s goal is to develop an understanding of the complex interactions of soil, geology, vegetation, and land use management with watershed function within these watersheds. The team includes Barbara Allen-Diaz, range ecology professor at UC Berkeley; Ken Tate, rangeland watershed specialist at UC Davis; Randy Dahlgren and Mike Singer, soil science professors at UC Davis; and Rob Atwill, environmental health specialist with the School of Veterinary Medicine at UC Davis. The network of coastal and Sierran watersheds dominated by oak woodland was established with the support of the Integrated Hardwood Range Management Program, and respective staff at the Sierra Foothill and Hopland Research and Extension Centers.

Stream flow is monitored at flumes installed at the outlet of each watershed. Water samples are collected throughout the winter at 1-2 hour intervals during storms and 3-4 hour intervals between storms. In addition, vegetation composition, structure, and pattern are being documented at the watershed scale.

Currently, baseline data is being collected in order to ‘calibrate’ the watersheds before implementing treatments. During this calibration phase, the UC team is documenting relationships between the watersheds by comparing water quality (e.g. nitrate concentration) and yield measurements from each. Once the calibration phase is complete, the team plans to change grazing intensities and implement controlled burning within a subset of the watersheds, while maintaining current management in the others. The interactions of a treatment will be measured by detecting a significant change in the previously established relationship between the treated and control watershed.

Daily stream flow and precipitationWhile this approach requires a long-term commitment, the study is already providing an understanding of stream flow generation, as well as the seasonal variability of nutrient transport from these watersheds. Mirroring the delivery of precipitation during the winter months, these hardwood rangeland watersheds do not discharge water until they are sufficiently primed. Results from 17 years of data from the Schubert Watershed indicate that increases in stream flow occur after an average of 6.2 inches (SD = 1.9 inches) of cumulative precipitation. This closely matches the effective depth of 6.5 inches of water that can be stored in the watershed soils. When this storage capacity is reached and the watershed is primed, stream flow response to additional rainfall is rapid and elevated. For example, during the winter of 1996-1997, stream flow increases occurred in the beginning of December after 5.3 inches of cumulative precipitation (Figure 1).


Hydrological and biological processes influence the seasonal variability of nutrient transport in these watersheds. Nitrate concentrations from the Schubert watershed demonstrate this point. In the1996-1997 water year, concentrations reached a maximum of 12.1 mg/L in December 1996 and decreased to a minimum of 0.29 mg/L in September 1997 (Figure 2). This pattern results from the temporal breaks in annual nutrient cycles caused by distinct wet and dry seasons in these watersheds. As nitrate accumulates in the soil during the summer and fall dry season, it is not removed by plant uptake or leached into streams. As early winter storms arrive, watersheds become primed and stream flow is generated. This occurs at a time when oaks are dormant and germinated annual grasses are inactive. As a result, nitrate is available to leaching and is progressively flushed from the watershed until plant uptake can begin in the spring.

Understanding seasonal variability in nutrient transport is important when monitoring water quality. Inappropriate timing of sampling can over- or under-estimate nutrients transported from these hardwood rangeland watersheds. For example, if water quality samples are collected only in early winter, the concentration of nitrate would be overestimated in comparison to nitrate concentrations in spring and summer. The same concern is true for concentration variability during storms. Nitrate concentrations rise and fall with stream flow response to storms. If sampling does not successfully document this fluctuation, conclusions about the transport of nitrate would be inaccurate. Because results from monitoring programs have policy implications for drinking water standards and other beneficial uses, care should be taken to account for this variability by sampling once before, during, and after a storm, as well as throughout the season.

An expanded discussion of this seasonal and storm variability and the implications for water quality monitoring is presented in the November-December 1999 issue of California Agriculture magazine article entitled “Timing, Frequency of Sampling Affect Accuracy of Water-quality Monitoring”.

The team will continue to research the functions of California hardwood rangeland watersheds. Where insights are gained they will be shared with interested landowners, land managers, and resource management agencies so that more informed, science-based policy decisions can be made concerning the interactions of land use and watershed function.

David J. Lewis
UC Cooperative Extension Watershed Management Advisor

prepared and edited by Adina Merenlender and Emily Heaton