Estimating Natural Flows for Hydrologic Reference Streams

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Scientists develop metrics of expected hydrological conditions at ungaged sites throughout Callifornia 

The general study area

The general study area. Triangles indicate a subset of current and historic gage locations to illustrate geographic distribution of sites.​​​​​​​

Although a basic understanding of the relationship between flow alteration and ecological response exists, it is difficult to make general statements about the importance of hydrology because relatively few streams are gaged and gages are often installed to monitor flows after human alteration begins rather than establishing a long period of record in anticipation of human alteration. Such baseline information is needed to calculate the degree of streamflow alteration in a specific stream, which in turn can be associated with changes in biological condition. These issues can be addressed by developing metrics of expected hydrological conditions (hereafter, "flow metrics") at ungaged sites with minimally altered flow regimes (hereafter, "reference sites"). These expected values can then be compared to observed values of the hydrologic metrics at altered sites to determine the degree of hydrologic alteration.

The basic goal of this project is to develop statistical models that predict natural hydrologic characteristics from 1950-2012, for all California streams based on a subset of gaged sites identified as having an unaltered hydrologic regime. Specific tasks include:

  • Determine final list of sites appropriate for use as hydrologic reference sites.
  • Develop and evaluate statistical models that predict expected baseline hydrological characteristics.

The approach is based on statistical relationships in observed data rather than mechanistic, process-based models. The spatial domain of the study includes three areas that exist partly or entirely within California, USA: "xeric" (dry areas), "interior mountains" (Sierra Nevada; Eastern Cascades Slopes and Foothills), and "north coastal mountains" (Klamath Mountains; Coast Range). Gages with records from 1950-2012 and 10 or more years of records were considered.

Task 1: We will determine a final list of sites appropriate for use as hydrologic reference sites by evaluating: 1) an index that combines several geospatially-derived indicators, including total upstream reservoir storage, freshwater withdrawal, pollution discharge, and impervious land cover; 2) published site-description records from annual data reports; and 3) imagery of each site and its contributing watershed.

Task 2: Develop statistical models for calculating expected baseline values for select hydrologic metrics. Metrics will include mean monthly flow for each month and approximately 6 other hydrologic characteristics mutually agreed upon by the study team and the cooperator, The Nature Conservancy.

As well as addressing an important issue facing California resource managers, the project is directly relevant to the Understanding Ecosystems and Predicting Ecosystem Change component of the USGS Science Strategy. In addition, this project represents a new collaboration with The Nature Conservancy and will contribute to ongoing collaborations of the CAWSC with the USGS-NAWQA Program, the Southern California Coastal Water Research Project, and the California Department of Fish and Wildlife