Evaluation of Ground Water Overdraft in the San Joaquin Valley

Ground water overdraft for the San Joaquin Valley was evaluated for each planning subarea (PSA) using two independent methodologies: the specific yield method and the water balance method. The specific yield method examines changes in ground water storage over a long period; the water balance method is based on the balancing of water supplies and demands for each PSA.

In computing overdraft using the specific yield method, ground water level measurements from 1970 through spring 1983 were used. This period was chosen for the following reasons:

• The total water supplies and demands for this period were nearly the same as the 1990 normalized supplies and demands.
• On average, the local water supplies and deliveries during 1970-82 were quite similar to the long-term average supplies and deliveries. This minimizes the need to correct for any unusual ground water recharge and pumping. Also, local stream runoff during 1970-82 was very close to the long-term average runoff (about 102 percent of the long-term average). Ground water overdraft was computed based on 100-percent average local runoff and deliveries.
• The years preceding the ground water level measurements in 1970 and spring 1983 were both wet years and quite similar. This similarity reduces the potential for significant differences in ground water recharge during unlike years. Such an occurrence would complicate overdraft computations using the specific yield method.

The impact of subsidence on water level measurements and the loss of ground water storage were evaluated using pre-1970 subsidence rates. More recent, but limited, data from a few locations along the California Aqueduct were also used.

For the water balance method, the long-term average local and imported water supplies were tabulated, along with the long-term average annual natural percolation to ground water tables. These amounts were then compared to the normalized water demand for each PSA. Ground water overdraft was computed as the difference between water supplies and demands.

The two methodologies produced similar ground water overdraft computations for most of the PSAs in the San Joaquin Valley. One notable exception is the Kings-Kaweah-Tule Rivers PSA, where the specific yield method produced significantly smaller overdraft than did the water balance method. An extensive investigation was done to understand the reason for such a difference; however, no specific reason for the large difference could be found. Actual ground water overdraft in the Kings-Kaweah-Tule Rivers PSA is probably somewhere between the values produced by the two methodologies. For this PSA, the California Water Plan Update used the average of the ground water overdraft values computed using the two different methods.

Ground water quality degradation is another factor that must be considered when computing overdraft. Ground water overdraft in a basin may induce the subsurface movement of poor-quality water into higher-quality water. The resultant quality degradation may reduce the usable storage of a ground water basin. This adverse effect of ground water overdraft was evaluated and included in the ground water overdraft computations for the California Water Plan Update.

Back to Sidebars Index in Bulletin 160-93

Back to Volume I Index in Bulletin 160-93