Charts and Graphs

A small collection of trend plots that are ready to include in your term paper!  Annual recharge and discharge totals are usually not available until late in the following year.

If you prefer, you can view the actual numeric data.

For a chart showing J17 index well levels, see the J17 page.

Estimated Annual Recharge to the Edwards Aquifer

Average annual recharge for the 1934-2006 period is about 711,600 acre feet. However, averages mean little here...note how widely variable recharge can be from year to year. Also note how it appears that total recharge has been going up over time. It is possible that we have artificially increased the amount being recharged by pumping water out and making more room for recharge to flow in. Another possibility is that we have not been collecting good enough data for a long enough period of time to make any conclusions at all.

Annual Precipitation in San Antonio

Rainfall, the only source of recharge for the aquifer, is also highly variable from year to year.  Shortly after it stops raining, only small amounts of new water enter the aquifer, so there is less pressure on water already deep inside.  Lower pressure means that flow at Comal and San Marcos springs is reduced, sometimes to the point where endangered species are threatened.  Changes in pressure are reflected in rising or falling well levels and are monitored at the J17 index well.

Estimated Annual Discharge from the Edwards Aquifer

In many ways, this plot generates more questions than answers.  What is immediately obvious is that total discharge appears to be increasing over time because greater amounts are being pumped from wells.  The conventional wisdom is that discharge from wells decreases springflows; however, there are some years when springflows did not appear to suffer because more was being discharged from wells.  Also, springflows were lowest in the 1950's when discharge from wells was only half of what it is now.  This seems to suggest there is hardly anything we can do to keep springs from going dry periodically.  It also points out how we are at the mercy of nature and are usually in a state of being either water-rich or water-poor, as discussed in the section on regional climate.

Uses of Edwards Aquifer Discharge

This chart shows annual discharge volumes from the Aquifer since 1955 by type.  A few trends are evident: municipal and military uses grew steadily, peaked in the late 80's, and have declined somewhat since then.  Livestock/domestic uses have been replaced by industrial uses.  Demand for irrigation water has declined in the last decade and varies widely depending on how much it rained that year and if the rain fell at times beneficial to farmers.  Springflows also vary and in some years, such as 1991, declines in agricultural use seem to correspond to increases in springflow.