Estimating low flows for Coastal Plain watersheds

Abstract

Low-flow statistics and duration information are needed by Federal, state, regional and local governments for water resource and environmental planning, management and regulatory activities. The capability to estimate low-flow characteristics for streams on ungaged watersheds is an urgent need for addressing current water resource issues such as TMDL’s. Low-flow information can be developed for streams on gaged watersheds but is not readily developed for streams on ungaged watersheds where historical flow records are not available. Extensive streamflow data bases required for developing low-flow information have been notably lacking in the Coastal Plain of the southeastern US, a region characterized by low-gradient stream systems that feature broad, heavily-vegetated floodplains with large riparian storage. To meet this need, the USDA-Agricultural Research Service instrumented eight experimental areas within the headwaters of Little River watershed in central, south Georgia. These experimental watersheds range in area from 2.6 to 334 sq.km.and have fixed flow-measurement controls. This paper develops relationships for estimating low-flow characteristics for ungaged Coastal Plain watersheds based on low-flow statistics and duration information developed for the Little River watersheds. Low-flow information [including the 7-day, 10-year (Q 7,10) flow -- the most widely-used low flow index] is related to measured watershed characteristics including geomorphological and land use indices, using multiple linear regression analyses on log-transformed data. Regression models are presented for estimating low-flow characteristics based on critical geomorphologic and land use parameters. These regional regressions provide the simple approaches for estimating low-flow characteristics for streams in ungaged Coastal Plain watersheds that are needed to address a range of important water resources issues, including TMDL’s.