The San Antonio and Hill Country areas are fortunate in having access to large aquifers that support large-scale agriculture and urban use. Overuse and declining water quality of the aquifers is an increasing concern, but so far, public water services such as the San Antonio Water System (SAWS) have maintained municipal supplies reliably and relatively inexpensively, so that alternate sources tend not to be cost effective.
In cases where public service is unavailable, water is pumped from privately owned wells drilled into an underlying aquifer. Historically, this has been a relatively easy and inexpensive matter, but the expansion of urban development in recent years has lowered water tables substantially in many rural areas. It is not unusual to have to drill (or redrill) to depths of 1000 feet to assure adequate flows in some locations.
Harvesting rainwater in areas supplied by public services may be desirable as a secondary source of high quality soft water for drinking or landscape use, but it is an increasingly attractive alternative to a deep well in rural areas.
The South Texas region gets on average 30+ inches of rain annually. Although this is a substantial amount, a collection system must be designed to take into account its uneven distribution over time (see below). To this end, we developed the Rainwater Harvest Estimator, a Microsoft Excel workbook consisting of two linked spreadsheets designed to estimate daily levels of rainwater storage that would have been possible in the San Antonio, Texas area from 1980 to the present under variable assumptions about household usage, roof catchment area, storage tank size, etc. Originally intended as the basis for planning a system that would provide all the water for a household, the program serves just as well in sizing more modest auxiliary systems that might supply landscape or stock watering, for example.