Quantifying Recharge via Fractures in an Ashe Juniper Dominated Karst Landscape

摘要

Few studies have quantified recharge into caves from overlying fractures. In areas such as San Antonio, Texas that rely primarily on karst (limestone) aquifers for potable water, the quantity and quality of recharge through these features could have significant implications for evaluating and managing groundwater. Vegetation cover also has the ability to influence recharge characteristics, when the vegetation is located directly above caves and inside the surface water and groundwater drainage basins. These studies in the Edwards Aquifer recharge zone north of San Antonio, evaluate the effects of woody cover on the water budget of two shallow caves. Use of large-scale rainfall simulation equipment above the caves allows re-creation of rainfall events where amount, rate, and duration of previously occurring events are simulated. One cave's footprint is instrumented with throughfall collectors and rain gauges to estimate canopy interception. Four trees are instrumented with stemflow collectors and transpiration measurement devices that record data, later scaled up to account for stemflow over the entire plot. Inside both caves, drip collectors constructed of PVC pipe and clear polyurethane plastic sheeting collect and route water to tipping buckets that digitally record recharge volumes onto a datalogger. In order to evaluate recharge rates and responses typical of the landscape, the collectors capture dripwater falling from the caves' ceiling, which travels into the caves through fractures, and not as focused recharge through sinkholes and cave entrances. Continuous automatic recording of recharge allows us to evaluate the effects of simulated and natural rainfall events on recharge rates, volumes, and durations. Preliminary analysis of natural and simulated data has shown that fractures can move large volumes of recharge quickly into the caves.