Surface water and groundwater
Groundwater is strongly related with surface water in the Edwards Aquifer. Water seeping through the limestone rock of the aquifer has created a karst landscape with many caves, fractures, and sinkholes that allow fast movement of water from the surface into the aquifer formation and within the subsurface (figure 1.4). The groundwater discharge from the San Marcos Springs forms the flow of the San Marcos River. As this river meanders downstream, it is joined by the Blanco River, whose flow is derived mostly from surface runoff, and these in turn are tributaries of the Guadalupe River, which carries their waters to the Gulf of Mexico. Normally a quiet, slow-moving river, the Guadalupe can turn into a raging torrent during severe storms and devastate the surrounding countryside. To accurately describe the water resources of the region, we must map both groundwater and surface water features together (figure 1.5) and define the relationships between them. This is part of the new design of Arc Hydro, which includes a framework data model with basic surface water and groundwater features and additional components describing unique aspects of surface water and groundwater.