Characterization and Water Balance of Internal Drainage Sinkholes

Abstract

Internal drainage microbasins were found constitute 10% of the 98 square km (38 square mile) portion of the Barton Springs segment of the Balcones Fault Zone Edwards Aquifer contributing to Barton Springs. Internal drainage basins have subtle geomorphological expressions that have historically been overlooked as a major recharge source within the Barton Springs segment. Internal drainage basins can be recognized and mapped based on large solution sinkhole bowls, internal contours on surface topographic maps, aerial photos, inspection of sinkholes during and after rain events, field delineation of catchment areas with GPS, and accounts from individuals such as property owners and cave explorers. We measured water balance components within a large internal drainage basin, the 19 -hectare (46 -acre) Headquarter Flat Sink to more directly quantify the amount of rechar ge contributed from internal drainage microbasins to the Edwards Aquifer. The monitored water balance components of HQ Flat that were measured for a year period (348 days), included the discrete recharge to the cave drain, evapotranspiration flux using an eddy covariance system mounted on a 15 -meter tower, and soil moisture using insitu sensors and gravimetric analysis. Over the year test period, where rainfall totaled 106 cm, 58% left as evapotranspiration, 8% recharged directly through the cave drain, an d 34% recharged diffusely across the internal microbasin of the cave site. The total recharge was measured to be 42% of the rainfall within the test basin. Only the discrete recharge portion was directly measured in a second internal drainage microbasin, the 22 -hectare (54 -acre) Flint Ridge Cave, since June 2003. The discrete recharge from Flint Ridge Cave was measured to be 9% over the same test year. During the 348 -day test period, it can be estimated that known internal drainage basins contribution co mprises about 5% of the Barton Springs flow reported by the USGS of 62,297,153 m 3 (2,200,003,200 ft 3 ).