GIS Modeling of Significant Karst Areas for Purchase and Protection

In May 2000, the voters of San Antonio, Texas, approved a sales tax increase to raise $65 million over four years to purchase land over the recharge zone of the karstic Edwards Aquifer and local streams. A team of karst, hydrogeology, wildlife, GIS, and land management specialists was assembled to develop a strategy for identifying properties with highest hydrologic and aquifer protection value for possible acquisition. GIS methods were determined the most effective means of assessing the properties. A GIS model was constructed of three components: vulnerability, watershed, and biology. Each component comprised spatial data layers weighed according to their significance. Vulnerability layers were land slope, faults, caves, sinkholes, and the permeability of the geologic units. Watershed layers were property size, properties adjacent to existing preserved or protected areas, and areas that drain into known caves. Biology layers were the distribution of federally listed endangered bird and karst invertebrate species. The three components were respectively weighted at 50%, 30%, and 20% and combined. Sensitivity testing was conducted to assure the optimal quality of the model’s output, which was presented in three tiers of priority for acquisition based on the numerical values for the properties. The results were provided to land agents working for the city who checked the availability of the highest tier properties first. To date, approximately 11 square kilometers have been purchased and are creating what may prove an important buffer to mitigate impacts from the extensive urbanization occurring on the Edwards Aquifer recharge zone.