RECHARGE TO THE WATER TABLE
Annual recharge from 1986 PWDWY to 1988 PWDWY was estimated (table 2) using equation 1. Recharge values for the remaining years of the study were not estimated because of inadequate data. During these years, which were years of reduced water deliveries, direct application of ground water pumped by individual growers is assumed to be a large component of total water applied to crops. Although variation in recharge may occur as a result of changes in management strategy or availability of imported water, reasonable estimates of recharge are not possible without good estimates of the quantities of ground water applied to the crops.
Other factors also affect the comparability of recharge estimates made in this study with those made in other studies in the area for the same period. For example, estimates of consumptive-use parameters (effective precipitation and crop coverages) used in this study differed from those of Fio (1994). Fio estimated average annual recharge for the years 1987 PWDWY and 1988 PWDWY to be 55.6 x 106 m3, in comparison with an average annual recharge of 65.1 x 106 m3 estimated in this study. The method used by Fio considered the total quantity of precipitation to be effective precipitation. Evaporation of a part of the total precipitation was incorporated into the calculation of consumptive use (Ayars and Schrale, 1989). This results in higher consumptive-use estimates than those reported in this study. The annual average consumptive use reported by Fio (1994) for 1987 PWDWY and 1988 PWDWY was 112.0 x 106 m3 in comparison with an annual average of 82.7 x 106 m3 in this study. In their consumptive use estimates, Ayars and Schrale (1989) also assumed that 95 percent of the field is covered by fully grown crops, whereas 86 percent (F in equation 2) was assumed in this study. If consumptive use used in this study were recalculated using a crop coverage of 95 percent, it would result in an annual average consumptive use of 90.8 x 106 m3 for 1987 PWDWY and 1988 PWDWY and an estimate of recharge of 56.8 x 106 m3, which is similar to the estimate of 55.6 x 106 m3 reported by Fio.
On the basis of a previous study, it is known that recharge to the water table is not uniformly distributed over the study area. Fio (1994) evaluated recharge for three subareas and showed that recharge was less in drained areas than in undrained areas. Estimates of spatial variability of recharge requires knowledge of the distribution of water applied and cropping patterns. Although the district maintains water-delivery data based on land ownership, these data were not available for this study. However, even if land owner delivery data were available, the actual spatial distribution within those land holdings is unknown and, therefore, distribution of recharge is uncertain.
An additional problem in making estimates of spatial distribution of recharge is the lack of actual crop maps, and therefore good spatial estimates of consumptive use. Although crop maps are developed by the water district, they are developed prior to the growing season for the purpose of projecting water demand (Marcos Hedrick, Panoche Water District, oral commun., 1995). However, comparisons of projected and actual crop acreages show significant differences. For this and the other reasons noted above the spatial distribution of recharge to the water table was not evaluated for this study.
