The Baseflow Recession Constant as a Tool for Water Availability Analysis

Average daily flow and other parameters such as annual recharge, 10-YR baseflow, and Q_7-10 are often used in water availability analysis to prescribe necessary withdrawal limits for protection of water resources.  In the case of ungauged basins, these parameters are typically evaluated through extrapolation from reference watersheds or regression methods developed using reference watersheds near the site of interest.  Such methods are needed where site specific data are limited or lacking.  The accuracy of this approach depends on how well the site specific conditions match those of the reference watershed.  Collection of site specific data greatly improves this process; however, even when site specific data are obtained it may be difficult to assess how well the site conditions match those of the reference watershed and what final estimates should be used.  The baseflow recession constant is an easily determined yet sometimes overlooked hydrologic parameter which greatly improves this process.  Because the baseflow recession index relates observed flows to such parameters as aquifer transmissivity, storativity, and groundwater contributing area, much of the information required for comparison of watersheds and prediction of flows is summarized in one parameter.  Because the recession index can be used to relate a measured flow to a volume of water held in storage, it provides a powerful tool for assessing impact from future withdrawals.  Examples from several field sites are used to demonstrate the usefulness of the baseflow recession constant in water availability investigations.  From selection of proper reference gauges to prediction of site flows through regression analysis, and assessment of impacts from future withdrawals, the baseflow recession constant is one of the most simple and versatile hydrogeologic parameters  available to the groundwater practitioner.