MODELING THE PRECIPITATION THRESHOLD REQUIRED FOR RECHARGE IN A KARST AQUIFER OF CENTRAL TEXAS

Karst aquifers play important roles by supplying water to aquatic ecosystems and for municipal use. Expanding communities place increasing pressure on aquifer systems and water requirements may not be matched by the available supply during periods of drought. A prime example of this type of strain placed on a karst aquifer can be found in the Edwards Aquifer of central Texas. Continued development along the San Antonio-Austin corridor requires more water, much of which is pumped out of the karstic Edwards and Trinity Aquifers. Improved management strategies are essential for sustaining both the communities and numerous spring-fed streams in the region. A more thorough understanding of the controls on groundwater recharge in these systems is required in order to improve their management.

To characterize controls on recharge in the karstic Trinity Aquifer of central Texas, continuous high frequency data have been collected since late 2009 in Cave Without a Name, located approximately 35 km northwest of San Antonio, Texas. Hydrologic and geochemical data, including drip rate at three sites, and discharge in a stream have been recorded continuously at ten minute intervals. During this same period, a full suite of environmental data were recorded above the cave. From these data, variables which are expected to be important in controlling recharge, or the change in stream stage and drip rate, have been examined. These include rainfall amount, rainfall intensity, PET, stream stage and drip rates prior to rain events, and the sum of rainfall over periods prior to each event. Preliminary data suggest that the region typically requires a minimum of ~13mm of rain during a single event in order for a response to be observed in the cave. In addition, rain events over ~13mm require certain antecedent moisture conditions in order for a response to be observed. Preliminary analyses also suggest that while the amount of rainfall is the most important predictor for a recharge response, the stage of the stream within the cave is the best indicator of antecedent moisture conditions. Statistical modeling of the preliminary data produced a best fit model using a relationship between the amount of precipitation that falls in a single rain event and the stage of the stream prior to the event to predict the in-cave hydrologic response.