The study unit divides into two distinct landforms along the Balcones escarpment, a line of low hills that locally separates the vast Great Plains physiographic province from the Coastal Plain. North and west of the escarpment is a rough, rugged terrain of upland plateaus and rolling hills dissected by many small creek beds that descend into the deeply incised valleys of several major streams. The land, covered by a thin veneer of rocky soils and ranging in altitude from about 1,000 to 2,300 feet above sea level, is used mostly for grazing cattle, sheep, and goats. Across the escarpment to the south and east, the terrain smooths noticeably into rolling prairies and broad stream valleys. Average annual rainfall in the study unit ranges from about 22 inches in the western part to about 34 inches in the eastern part.
Rainfall is highly variable.
Meteorologic and physiographic factors can combine to produce large, very intense rainstorms that result in rapidly rising flood flows. The Edwards aquifer is a sequence of extensively faulted, fractured, and dissolutioned limestone and dolostone that yields large quantities of water to wells and springs. The aquifer is exposed at land surface and under water-table conditions in its recharge area. As its rocks dip coastward, the aquifer becomes more deeply buried and under confined conditions in its artesian zone. Further downdip, the aquifer is much less permeable and contains moderately to very saline water.