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The computer program SWGW automates the application of the recession-curvedisplacement method for estimating groundwater discharge to streams (baseflow) from streamflow data. SWGW was developed from an existing U.S. Geological Survey computer program, RORA, and provides the user with four enhancements useful for the analysis of streamflow data. These enhancements (1) compute an adjustment factor to account for the apparent over-estimation of the recession curve by RORA; (2) produce data files for graphical output of hydrograph separation results; (3) incorporate two recession indices one for the major rise period and one for the major recession period; and (4) consider streamflow data on a water-year basis. An adjustment factor is computed for each day following a streamflow peak that the computed recession discharge exceeds measured streamflow discharge. The adjustment factor equals the measured streamflow discharge divided by the computed recession discharge. For each peak, minimum adjustment factors are determined and are used to provide the user four choices for adjustment of the recession curve. The choices are (1) no adjustment (equivalent to RORA); (2) adjustment of the entire recession curve by the average of the minimum adjustment factors determined for each peak; (3) adjustment of the entire recession curve by the smallest of the minimum adjustment factors determined for each peak; or (4) adjustment of each segment of the recession curve by the minimum adjustment factor determined for the corresponding peak. Computed results from RORA and SWGW are evaluated by comparing them to published estimates of baseflow derived manually using the recession-curve-displacement method. Differences between manually derived estimates and computed estimates from RORA and SWGW using different adjustment factors are computed for selected streams. A statistical analysis of the differences indicates that the closest comparison of the computed estimates from RORA and SWGW with the manually derived estimates is achieved by SWGW using two recession indices and an adjustment factor between the minimum and the average adjustment factors computed for each stream. INTRODUCTION Problem Increasing and competing demands for water in the Apalachicola-Chattahoochee-Flint and the Alabama-Coosa-Tallapoosa (ACFACT) River basins (fig. 1) have created a need for evaluation of surfaceand ground-water resources. In 1992, the Governors of Alabama, Florida, and Georgia and the Assistant Secretary of the Army for Civil Works signed a Memorandum of Agreement establishing a partnership to address interstate and intrastate water-resource issues, and promote coordinated system-wide management of water resources. These study partners subsequently requested the U.S. Geological Survey (USGS) to provide the ground-water-supply element of the Comprehensive Study of the ACF-ACT River basins (U.S. Army Corps of Engineers, 1991). This element addresses ground-water resources in each basin, particularly the ground-water contribution to major rivers. Based on hydrologic and physiographic boundaries, the two basins were further divided into eight subareas. Individual reports describing the ground-water resources of each of the subareas are planned by USGS staff in the Alabama and Georgia District Offices. The surfaceand ground-water resources of the ACF-ACT River basins are major components of a dynamic hydrologic system comprised of a network of aquifers, streams, reservoirs, control structures, floodplains, and estuaries. To manage this system effectively, hydrologic relations between the surfaceand ground-water resources should be described and quantitatively evaluated. Streamflow hydrograph separation was identified as a useful methodology to estimate the groundwater contribution to streamflow. This contrbution is commonly termed baseflow. In this report, the term baseflow refers to groundwater discharge to a stream, which is the end result of the streamflow hydrograph separation procedures discussed. One of the primary objectives of the ACFstudy is to determine ground-water discharge to streams (baseflow) under drought conditions. To support that objective, estimates of baseflow under average annual conditions are needed to provide a point of reference for the drought condition analysis. The FORTRAN computer program RORA (Rutledge, 1993) separates streamflow hydrographs using the recession-curve-displacement procedure developed by Rorabaugh (1964). Only by using a computer-automated, hydrograph separation procedure like RORA could average baseflows be determined for a large number of streams. However, RORA, when applied to hydrographs typical of streams in the ACF-ACT study area, does not always provide completely satisfactory results. In particular, estimates of baseflow computed by RORA commonly exceed corresponding published estimates determined by manual hydrograph separation (Faye and Mayer, 1990; HooS, 1990). Differences range from about 4 to 2(K) percent. Also, inspection of numerous streamflow hydrographs from the ACF-ACT stud)| area for various water years, plotted with the corresponding ground-water recession curves computed by RORA, reveals that the computed recession curves commonly exceed measured streamflow. Subsequently, RORA is used to compute recession curves over a wide range of streamflow conditions and locations within the ACF-ACT study area. Results from this evaluation are similar to comparisons made between manually derived and RORA computed estimates of baseflow, in that computed recession curves exceed streamflow for many locations, particularly during summer and early fall.

SWGW; a computer program for estimating ground-water discharge to a stream using streamflow data