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4.6 WATER RESOURCES Runoff Jahant Slough and its historical watershed are the only significant sources of runoff for Tracy Lakes. This watershed,as discussed above,encompasses approximately 16.5 square miles. Mean annual rainfall ' in the watershed varies slightly from 18 inches in the upper portions to 16 inches in the immediate vicinity of the project site. An estimated 2.5 to 3.5 inches (15 to 20 percent) of the mean annual precipitation is generated as runoff(Rantz, 1971). Runoff estimates for Jahant Slough can range from 1,088 acre-feet in dry years to 1,813 acre-feet in wet years (Table 4.6-1). Along with calculations based on Rantz's estimate, a slightly more conservative runoff estimate of 10 percent of mean annual rainfall is also presented to show the probable range of water yield from the watershed (Table 4.6-1). These calculations are only rough estimates; many factors, including the duration,timing and intensity of the storms, control the precise amounts of runoff. However, 10 to 15 percent of the yearly rainfall is a reasonable estimate of local runoff. TABLE 4.6-1 RUNOFF ESTIMATES FOR JAHANT SLOUGH Annual Rantz Runoff 10 percent Runoff Rainfall Estimates (ac-ft) (ac-ft) Normal (16") 2218 - 3062 1408 —: Dry (12") 1584 - 2112 1056 — Wet (20") 2640- 3484 1690 Water Quality The quality of surface water runoff entering Tracy Lakes is affected primarily by non-point source pollutants. Non-point source pollution includes materials and chemicals that are washed from the watershed areas into the drainage system from a variety of sources. Unlike water pollutants that come from discrete "point" sources such as factories or sewage treatment plants, non-point source pollutants are washed by rainwater and other means from streets, construction sites, golf courses and agricultural areas. Pollutants,mainly suspended sediment,fertilizers,herbicides,and other agricultural residues flow directly into surface waterways such as creeks, and eventually into the Tracy Lakes. No surface water quality sampling data are available for Jahant Slough or Tracy Lakes, thus the surface water quality of i the project area is unknown.However,it is expected to be similar to other agricultural runoff containing nutrients, trace amounts of pesticides and herbicides, and other constituents that accompany the agricultural use of the surrounding area. 4.6-4