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4.6 WATER RESOURCES Water Balance Analysis In order to evaluate the project's impacts to surface or groundwater sources, a water balance was constructed. A water balance is an equation that combines all the hydrologic aspects of an area, such as runoff,rainfall, evaporation,plant transpiration, irrigation needs and seepage from lakes. The water balance takes into account all of the inputs and outputs of the hydrology system to determine the required supply needs and any deficiencies (i.e., shortages). Water balances were completed for two different scenarios for the project, one simulating a normal rainfall year and the other representing the actual dry year conditions experienced during the period 1986 - 1990. The water balance calculations were made using monthly time steps. The results, along with a listing of the key assumptions, follow (Tables 4.6-2 and 4.6-3). In both cases, the input to the lakes was assumed to be only that from direct rainfall on the lakes and local runoff captured from the Jahant Slough watershed. No diversion from the Mokelumne River (i.e., under appropriative right) was assumed, since it has not been established (by the applicant or the State Division of Water Rights) that there is water available for appropriation in sufficient quantity and frequency to be of any significance to the lakes' water supply needs. The final (right-hand)column in Tables 4.6-2 and 4.6-3 shows the amount of make-up water required monthly to satisfy the lake level and golf course/pasture irrigation needs. This is the estimated amount of water that would need to be obtained from either on-site groundwater pumping,dry season diversion of water from the Mokelumne River (under riparian right), or a combination of the two. Over the course of an average rainfall year, Tracy Lakes receive a sufficient amount of runoff to fulfill the project's irrigation water needs and maintain the North Lake in a full condition,without the need for groundwater pumping or diversion from the river (Table 4.6-2). However,the water level in the South Lake would drop,leaving substantial exposed banks around the lake. The applicant has indicated a desire to maintain higher water levels at the western end of the South Lake for fish habitat and aesthetic reasons. This would require additional make-up water from groundwater pumping and/or diversions from the Mokelumne River. The water balance changes significantly during a below normal rainfall year and, to an even greater extent,during back-to-back below normal rainfall years. Actual rainfall data for the Stockton area during the last four years of drought,adjusted to the project site, were used to illustrate this scenario(Table 4.6- 3). In successive dry years, the project would require make-up water in the late summer and early fall months to meet lake management and water supply needs. 4.6-10