Hydrology and Groundwater
Groundwater in the San Juan Creek basin, also referred to as the "San Juan Creek Groundwater Basin", has a total volume of roughly 900,000 acre feet (1.1×109 m3). (California State Water Resources Control Board, 1977.) Natural groundwater recharge in the San Juan basin is estimated to be roughly 160,000 acre feet (200,000,000 m3) per year historically, mostly from precipitation. This amount has been reduced due to extensive development of the lower segment of the watershed, which is continuing to reduce the amount of open ground that can potentially serve in groundwater recharge. Artificially sourced water from irrigation and other uses is responsible for recharging roughly 37,500 acre feet (46,300,000 m3) annually. The watershed is essentially split into half by the Christianitos and Mission Viejo fault zones, which result in the division of the "Upper" and "Lower" groundwater basins. The groundwater mostly lies in alluvium, which ranges from a depth of 200 feet (61 m) in the lower watershed to mostly none in the upper reaches of the watershed.
Historically, the total surface outflow from the San Juan basin into the Pacific was calculated at 5,200 acre feet (6,400,000 m3) annually. Increasing urban development, resulting in increased runoff from irrigation and other sources, brought the annual outflow to 7,800 acre feet (9,600,000 m3) as of 1993. The maximum annual yield is 9,000 acre feet (11,000,000 m3). Although water from San Juan Creek, tributaries and basin groundwater are increasingly used for a limited amount of agricultural and municipal purposes, the annual yield from the watershed is continually increasing. As the groundwater recharge rate of the watershed is relatively high, it is safe to assume that groundwater levels will recover quickly from human use and human-induced pollution.
Due to the lower amount of urbanization in the San Juan watershed as compared with other watersheds in the county, the 100-year flood inundation risk is also significantly lower than that of most of the nearby watersheds. It has been calculated that a 100-year flood in the watershed would only affect a roughly 0.5 mi (0.80 km) wide area for the lower reaches of San Juan Creek inside San Juan Capistrano, while for Trabuco Creek, only a 0.2 mi (0.32 km) wide area would be affected, mainly due to severe downcutting. A fact of note is that the failure of the earthen Trampas Canyon Dam (in Trampas Canyon, 2 miles (3.2 km) upstream of San Juan Capistrano) would actually create a floodwave slightly larger than that of the 100 year flood that would race down San Juan Creek to its mouth. However, many levees in the San Juan area are still inadequately suited to a 100 year flood, regardless of the extent.
Read more about this topic: San Juan Creek, Geology