Comparison of two methods for estimating ground-water recharge in 1978-80, Santa Maria Valley, California

Infiltration from the Santa Maria River and the lower part of the Sisquoc River constitutes most of the groundwater recharge to the aquifer system of Santa Maria Valley, California. The annual recharge volumes commonly are much greater or smaller than the long-term average annual recharge. A comparison of two methods of estimating annual recharge indicates that, in Santa Maria Valley, a seepage-loss method is comparable to a water-level-change method. Both methods indicate that recharge was about 600,000 acre-feet during 1978-80. By the seepage-loss method, annual recharges for 1978, 1979, and 1980 are estimated as 290,000, 130,000, and 190,000 acre-feet. Using the water-level-change method, annual recharges during 1978, 1979, and 1980 are estimated at 230,000, 190,000, and 160,000 acre-feet. A log-Pearson Type III probability distribution of annual flows gaged on the Sisquoc River indicates that average annual recurrence intervals of recharge events equal to those of 1978, 1979, and 1980 are 20, 4, and 10 years. (USGS)