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The Edwards and associated limestones, the principal aquifer in the San Antonio area, has been the subject of numerous geologic and hydrologic reports. Records of precipitation, water levels, and estimates of recharge to the aquifer during 1966 are summarized in this report, which is one of three basic-data reports published yearly by the Edwards Underground Water District. The gathering of these records (and data) is part of the continuing hydrologic investigation by the U.S. Geological Survey in cooperation with the Edwards Underground Water District, the Texas Water Development Board, and the city of San Antonio.

Table 1 shows the annual precipitation at selected stations throughout the San Antonio area for 1966 and the annual average for each of these stations. Most of the area stations for 1966 show rainfall to be below the long-term mean.

The recorded high and low water levels during 1966 in five selected key wells are shown in table 2. In 1966, the water levels fluctuated near or above the midpoint between the record high and low levels which are also shown in table 2. Ground water storage in the Edwards Limestone remained high, although rainfall in 1966 was below average.

Recharge to the Edwards and associated limestones is chiefly from streams which lose most of their base flow and a part of their flood flow as they cross the Balcones fault zone on the outcrop of the aquifer.  The recharge is estimated from records of continuous discharge at gaging stations located above and below the infiltration areas in each of most of the streams. The monthly mean discharges at the gaging stations during October-December 1966 are shown in table 3. Streamflow records for January-September 1966 have been published by the U. S. Geological Survey (U. S. Department of the Interior, Geological Survey, Water Resources Division, 1966).

Table 4 shows the recharge in each basin of the San Antonio area for 1966 and the average annual recharge for the period 1934-65.  The basic methods employed by Petitt and George (1956) and by Garza (1962) were used for estimating the 1966 recharge, which was well above the annual average. On August 13, 1966, greater than 10 inches of rain fell in the upper watersheds of the Nueces and Frio Rivers. This was followed by moderate rains during September. Recharge from the Nueces and Frio Rivers during August and September was 180 thousand acre-feet or 30 percent of all recharge for 1966. During these two months, the Nueces and Frio Rivers discharged 110 thousand acre-feet past the recharge area due to the high rainfall intensity.