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A watershed model (Hydrological Simulation Program— FORTRAN) was developed, calibrated, and tested by the U.S. Geological Survey, in cooperation with the U.S. Army Corps of Engineers, San Antonio River Authority, San Antonio Water System, and Guadalupe-Blanco River Authority, to simulate streamflow and estimate ground-water recharge in the upper Cibolo Creek watershed in south-central Texas. Rainfall, evapotranspiration, and streamflow data were collected during 1992–2004 for model calibrations and simulations. Estimates of average ground-water recharge during 1992–2004 from simulation were 79,800 acre-feet (5.47 inches) per year or about 15 percent of rainfall. Most of the recharge (about 74 percent) occurred as infiltration of streamflow in Cibolo Creek. The remaining recharge occurred as diffuse infiltration of rainfall through the soil and rock layers and karst features. Most recharge (about 77 percent) occurred in the Trinity aquifer outcrop. The remaining 23 percent occurred in the downstream part of the watershed that includes the Edwards aquifer recharge zone (outcrop). Streamflow and recharge in the study area are greatly influenced by large storms. Storms during June 1997, October 1998, and July 2002 accounted for about 11 percent of study-area rainfall, 61 percent of streamflow, and 16 percent of the total ground-water recharge during 1992–2004. Annual streamflow and recharge also were highly variable. During 1999, a dry year with about 16 inches of rain and no measurable runoff at the watershed outlet, recharge in the watershed amounted to only 0.99 inch compared with 13.43 inches during 1992, a relatively wet year with about 54 inches of rainfall. Simulation of flood-control/rechargeenhancement structures showed that certain structures might reduce flood peaks and increase recharge. Simulation of individual structures on tributaries showed relatively little effect. Larger structures on the main stem of Cibolo Creek were more effective than structures on tributaries, both in terms of floodpeak reduction and recharge enhancement. One simulated scenario that incorporated two main-stem structures resulted in a 37-percent reduction of peak flow at the watershed outlet and increases in stream-channel recharge of 6.6 percent in the Trinity aquifer outcrop and 12.6 percent in the Edwards aquifer (recharge zone) outcrop. Introduction The U.S. Army Corps of Engineers (USACE), Fort Worth District, began a study in 2002 to address opportunities for ecosystem restoration (in part, through aquifer recharge), flood-damage reduction, and watershed management through Best Management Practices (BMP) in the upper Cibolo Creek watershed (study area) in south-central Texas (fig. 1). Phase 1 of the study (the first of three phases) consists of establishing and documenting the existing hydrologic, engineering, economic, and environmental conditions for the study area (U.S. Army Corps of Engineers, 2001). As part of phase 1, the U.S. Geological Survey (USGS), in cooperation with the USACE, San Antonio River Authority (SARA), San Antonio Water System (SAWS), and Guadalupe-Blanco River Authority (GBRA), developed, calibrated, and tested a watershed model of the study area using the Hydrological Simulation Program—FORTRAN (HSPF) to simulate existing hydrologic conditions. Phase 2 of the study involves evaluation of possible alternatives to address ecosystem restoration, flooddamage reduction, and watershed management opportunities. As part of phase 2, the USGS used the HSPF model developed in phase 1 to evaluate the effects of flood-control/rechargeenhancement structures on streamflow and aquifer recharge in the watershed. Phase 3 of the study is a detailed engineering analysis by the project cooperators of possible alternatives evaluated in phase 2. Part of the study area is in the Edwards aquifer recharge zone (outcrop) (fig. 2). The Edwards aquifer is one of the most productive carbonate aquifers in the Nation and is the major source of public water supply in south-central Texas. In addition, the Edwards aquifer supplies large quantities Simulation of Streamflow and Estimation of Ground-Water Recharge in the Upper Cibolo Creek Watershed, South-Central Texas, 1992–2004 By Darwin J. Ockerman

Simulation of streamflow and estimation of ground-water recharge in the Upper Cibolo Creek Watershed, south-central Texas, 1992-2004