R scripts to Estimate Ungaged Basin Discharge Using Generalized Drainage-Area Ratio Method for the Medina and Diversion Lake System, Bandera, Bexar, and Medina Counties, Texas

To quantify seepage losses from the Medina and Diversion Lake system that provide recharge to the Edwards aquifer and upper zone of the Trinity aquifer, daily water budgets were analyzed for March 4, 2017–October 6, 2022, and over a range of stages in Medina Lake. In the water-budget analysis, the applicable components of the hydrologic cycle pertaining to Medina and Diversion Lakes were evaluated. The water-budget equation incorporates measurable terms of inflow and outflow to solve for (or otherwise scientifically estimate) unknown gains or losses, or both, from a lake or lake system. The measurable terms include precipitation, evaporation, surface water inflow and outflow, and change in lake storage. The solution to the water-budget equation is obtained by balancing the contribution of each term of the water budget for any given budget period. The net effect of the unknown gains and losses is represented by the residual of the measurable terms and is assumed to represent recharge to the Edwards aquifer and upper zone of the Trinity aquifer.
Inflow from the ungaged part of the drainage basin for the Medina/Diversion Lake system was estimated using a generalized drainage-area ratio method. Previously published R scripts created for the entire state of Texas were rescoped to only enclose an area encompassing 17 counties (Bandera, Bexar, Blanco, Burnet, Comal, Edwards, Gillespie, Hays, Kendall, Kerr, Kimble, Llano, Mason, Medina, Real, Travis, and Uvalde Counties) referred to hereinafter as the "Texas Hill Country," an informal name that is locally used for this part of Texas. The rescoping of the spatial extent was expected to improve the representativeness of the method for the study area. In the applied drainage-area ratio method, the ratio of streamflow at two streamgages is equal to the ratio of the respective drainage areas with exponent values that equate two ratios. The simplest drainage-area ratio method has an exponent of unity. A generalized drainage-area ratio method used for the study comes from an assumption that the drainage-area ratio exponent values vary for 34 streamflow percentile ranges. The delineation of the streamflow percentile ranges reflects the influence of streamflow probability. Three R script files were used to create a drainage-area ratio exponent value table using daily streamflow records from 130 streamgages from the earliest available records through October 15, 2022. Before December 4, 2017, the drainage area of the ungaged basin for the Medina and Diversion Lake System was 177 square miles. Beginning on December 5, 2017, the ungaged drainage area was reduced to 119 square miles because of the installation of the streamgage on Red Bluff Creek at Farm to Market Road 1283 near Pipe Creek, Tex (U.S. Geological Survey station number: 08179110). For computing ungaged basin inflow for the Medina and Diversion Lake system, three nearby streamgages were selected: Helotes Creek at Helotes, Tex. (U.S. Geological Survey station number: 08181400), Middle Verde Creek at State Highway 173 near Bandera, Tex. (U.S. Geological Survey station number: 08200977), San Geronimo Creek near Helotes, Tex. (U.S. Geological Survey station number: 08180586). The mean discharge estimate was calculated from the three streamgage values and used as the final daily mean inflow into the Medina and Diversion Lakes from the ungaged basin.

Tags: bandera-county, bandera-texas, bexar-county, blanco-county, burnet-county, comal-county, diversion-lake, edwards-aquifer, edwards-county, geoscientificinformation, gillespie-county, hays-county, helotes-creek, helotes-texas, hydrographic-datasets, hydrologic-processes, hydrology, inlandwaters, kendall-county, kerr-county, kimble-county, llano-county, mason-county, medina-county, medina-lake, medina-river, middle-verde-creek, pipe-creek, real-county, red-bluff-creek, regression-analysis, san-antonio-texas, san-geronimo-creek, statistical-analysis, stream-discharge, streamflow, surface-water-non-marine, texas, travis-county, trinity-aquifer, usgs-65c50daad34ef4b119caf4e3, uvalde-county, water-budget, water-cycle, water-resource
Organization: Department of the Interior