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4.6 WATER RESOURCES The water balance calculations show the make-up water requirement to be in the range of 200 to 400 acre-feet during the late summer and early fall of these dry years. In order to meet the water needs of the project, as presently planned,this water would be obtained either from groundwater pumping,from summer diversions from the Mokelumne River under riparian right (see following discussion), or from 1 a combination of these two sources. This amount of water, if obtained entirely from the Mokelumne River, could represent a diversion of as much as 10 to 15 percent of the summer streamflow in the river. Indications from the Department of Water Resources (DWR) are that no summer diversions are likely to be permissible for environmental reasons (Cassel, 1991). If obtained from groundwater, it would be equal to twice to three times the present amount of groundwater withdrawal, and would exceed the estimated local recharge of the aquifer,adding to the existing overdraft conditions in the area. Obtaining a portion of the required make-up from both sources would lessen the magnitude of the individual impacts on the river and on the aquifer. These calculations also show that one wet winter could provide enough water that could be stored to last through a dry following year. When the South Tracy Lakes is at capacity it has the ability to hold enough water, given below normal rainfall in the following year, into the summer of the second dry year. However, with successive dry years as the last four years, water deficit builds making it increasingly difficult to meet water demands in each subsequent year; and increasing the requirement for make-up water. Nater Rights Issues The proposed project raises a number of water rights issues. California water law makes a fundamental distinction between the right to use water from a stream on parcels of land adjacent to the stream (riparian right), and the right to store water for later use or take water from a stream for use on parcels of land that are not contiguous to the stream (appropriative right) (SWRCB, 1990). Riparian Right The owner of land adjacent to a stream has a riparian right to take water from the stream and apply it to reasonable use on his/her property,without obtaining a permit from the state. However,the riparian right does not extend to storing water from the stream, except for temporary storage associated with diversion(about 30 days). Also, unless riparian rights were reserved in an earlier parcel split, it applies only to the parcel of land actually adjacent to the stream, and not to neighboring parcels under the same ownership. This right applies to the natural flow which, in the case of the Mokelumne River, does not include water that may be stored in Camanche Reservoir and then later released. Riparian water use is not regulated by the state, in contrast with water use under an appropriative right (see discussion below). Consequently,the impacts of riparian water use on instream resources and other downstream water users are not subject to any environmental controls except through the courts (as a result of water right adjudication), or as a result of the CEQA process for individual development 4.6-12