This report presents the results of a study to quantify current (1983–93) mean freshwater inflows to the six bay systems (open water and wetlands) in the Corpus Christi Bay National Estuary Program study area, to test for historical temporal trends in inflows, and to quantify historical and projected changes in inflows. The report also addresses the adequacy of existing data to estimate freshwater inflows.

The six bay systems are the St. Charles, Copano, Redfish, Nueces and Corpus Christi, upper Laguna Madre, and Baffin. Each bay system has one or more adjacent contributing watersheds, for a total of 13 watersheds for purposes of this study, that together comprise about 6,000 square miles. All freshwater runoff to each bay system except the Nueces and Corpus Christi originates in adjacent watersheds. Freshwater that enters the Nueces and Corpus Christi Bay system is a combination of water that originates in the adjacent contributing watersheds and water that originates in the large regional watershed of the Nueces River (greater Nueces River Basin) upstream of the adjacent contributing watersheds.

The watershed simulation model Hydrologic Simulation Program—Fortran (HSPF) was used to generate simulated flow (runoff) from the 13 watersheds to the six bay systems because adequate gaged streamflow data from which to estimate freshwater inflows are not available; only about 23 percent of the adjacent contributing watershed area is gaged. The model was calibrated for the gaged parts of three watersheds—that is, selected input parameters (meteorologic and hydrologic properties and conditions) that control runoff were adjusted in a series of simulations until an adequate match between model-generated flows and a set (time series) of gaged flows was achieved. The primary model input is rainfall and evaporation data and the model output is a time series of runoff volumes. After calibration, simulations driven by daily rainfall for a 26-year period (1968–93) were done for the 13 watersheds to obtain runoff under current (1983–93), predevelopment (pre-1940 streamflow and pre-urbanization), and future (2010) land-use conditions for estimating freshwater inflows and for comparing runoff under the three land-use conditions; and to obtain time series of runoff from which to estimate time series of freshwater inflows for trend analysis.