Predicting Hydrologic Flow Regime for Biological Assessment at Ungauged Basins in the Western United States.

Hydrologic regime is vital to the composition, structure, and functioning of stream ecosystems. Quantifying streamflow variables that define the hydrologic regime is important to stream ecosystem management and restoration. Presently, stream flow variables can be quantified only at places where there is streamflow data. Regional hydrologic models developed by USGS (Streamstats, National Flood Frequency program) estimate some hydrologic variables that define low and high flows at ungauged basins. These models are based on regression equations that relate streamflow statistics to measured physical and climatic characteristics of the watershed above a gauge point. These models do not estimate all of the flow attributes important to the stream biota, and they are also based on a limited set of watershed attributes. The major goal of this work is to develop models for the prediction of a broader class of flow regime variables chosen for their relevance in biological assessment. The first step in this effort is to create a database of flow regime variables and watershed attributes at gauged locations to use in model development. We selected ten flow variables that, in our judgment, are sufficient to characterize the hydrologic regime of a stream relevant to biota. We are quantifying these variables from raw stream flow data obtained from USGS Hydro-Climatic Data Network (HCDN) stream gauges in the Western United States. Digital watershed boundaries and other topographic variables for the watersheds of these gauges are being developed from digital elevation models. Climatic, soils and geologic data attributes are being derived for these watersheds from nationally available geospatial information. In this paper, we describe the hydrologic variables selected for prediction and biological assessment. We also describe the emerging data base of streamflow variables and watershed attributes and discuss the current status of the predictive modeling. A comprehensive database of topographic/geomorphic, geologic and climatic variables for the watersheds of HCDN streamflow gauges should assist in assessing how these variables affect aspects of the hydrologic flow regime important to stream ecosystems.