Groundwater Availability Model for the Hill Country Portion of the Trinity Aquifer System, Texas

Ian C. Jones
Roberto Anaya
Shirley C. Wade


Mace and others (2000) constructed a groundwater availability model simulating groundwater flow through the Hill Country portion of the Trinity Aquifer System as a groundwater resource management tool. The purpose of this report is to document updates to this earlier model. The model is being updated by: (1) adding the Lower Trinity Aquifer as an additional layer to the model, (2) revising the spatial distribution of parameters, such as recharge and pumping, and (3) calibrating to steady-state water-level and river discharge conditions for 1980 and historical transient water-level and discharge conditions for 1981 through 1997. The calibrated model can be used to predict future water-level changes that may result from various projected pumping rates and/or changes in climatic conditions. Our conceptual model subdivides the Hill Country portion of the Trinity Aquifer System into three main components: the Upper, Middle, and Lower Trinity aquifers. The Upper Trinity Aquifer is composed of the upper member of the Glen Rose Limestone. The Middle Trinity Aquifer is composed of the lower member of the Glen Rose Limestone, Hensell Sand, and Cow Creek Limestone. The Lower Trinity Aquifer is composed of the Sycamore Sand, Sligo Formation, and Hosston Formation. The Middle and Lower Trinity aquifers are separated by the Hammett Shale which acts as a confining unit and is not explicitly included in the model. The model study area also includes easternmost parts of the Edwards-Trinity (Plateau) Aquifer.Recharge in the updated model is a combination of infiltration of precipitation that falls on the aquifer outcrop and infiltration from losing intermittent streams within the model area. Estimates of recharge due to infiltration of precipitation in this updated model vary spatially and are equivalent to 3.5 to 5 percent of average annual precipitation. The highest of these recharge rates coincide with the Balcones Fault Zone. In addition to recharge from precipitation, there is alsorecharge from streamflow losses in the downs