HYDROGEOLOGIC STUDY OF A COASTAL RANGE WATERSHED: SAN LUIS REY RIVER, CALIFORNIA

The San Luis Rey River watershed is representative of many small coastal range watersheds in the western US and Mexico. Its headwaters are in the Peninsular Ranges of Southern California, which locally reach elevations of 5,900 feet. Annual precipitation varies from 45 inches in the upper reaches of the watershed to as low as 12 inches in the trunk valley and coastal areas. The uncontrolled portion of the watershed is 55 miles long, 5 to 10 miles wide, and ~ 383 mi2 in area. Precipitation over this area delivers ~370,000 acre-ft of water into the basin every year. *** The watershed has a good (60-75%) vegetative cover of montane forest in the highlands; coastal sage and scrub species, grasslands, and oak woodlands in the valley floor; and thin riparian corridors along the major stream courses. Agriculture is intense throughout most of the low lying areas, where nurseries and avocado and citrus orchards are common. There is also considerable urban development in the western portion of the basin, including the inland city of Fallbrook (45,000 inhabitants) and the coastal city of Oceanside (65,000 inhabitants). Total water needs are ~20,000 acre-ft per year for urban uses, and ~60,000 acre-ft per year for agricultural purposes. In contrast, the stream system conveys about 110,000 acre-ft per year, which would leave only 30,000 acre-ft for environmental maintenance. The shortfall could be met by using groundwater resources. *** The bedrock of the Peninsular Ranges is formed by Mesozoic intrusions of gabbro, tonalite, and granite. Paleozoic and Mesozoic metasedimentary and metavolcanic rocks form screens between the plutonic masses. These crystalline rocks are cut by joint sets that provide secondary permeability, as does weathering, but in general the crystalline rocks behave like aquitards at a regional scale. In contrast, Quaternary alluvium present along the San Luis Rey River valley, and its tributary canyons, hosts viable aquifers. The alluvial prisms appear to be narrow (less than 1 mile) and comparatively thin (a few hundred feet), but they are also fairly continuous and transmissive. I estimate that average annual recharge into the basin is ~25,000 acre-ft, which could be regarded as the safe yield that would keep the basin in dynamic equilibrium.