The absence of precipitation during summer months reinforces the seasonal variation in the demand for water in southern California. Furthermore, average annual precipitation is not only modest but also highly variable. Dry years often come in succession for a decade or more. Thus ground water basins have functioned as natural regulators of runoff and as storage reservoirs for daily, cyclical, and seasonal peaking requirements. These requirements must be met either from surface storage facilities or from ground water basins.
In southern California standby pump and well capacity is much more economical to develop and maintain than surface storage and distribution facilities. When the additional sizing costs necessary to meet peaking requirements in surface distribution facilities are considered, the critical economic importance of ground water basins for peaking purposes in southern California becomes apparent. If ground water storage is not continuously available for peaking purposes, alternative surface facilities would be required. Based on the present value and scarcity of land and construction costs, these facilities would represent a cost of hundreds of millions of dollars.
Underground storage is also preferable in several respects to storage in surface reservoirs. Water stored underground does not evaporate, as it does in surface storage and aqueducts. If the water needs to be stored for long periods, evaporation losses can be a serious concern, especially in arid regions where evaporation rates are high.
In addition, natural runoff that percolates into a ground water basin loses economic value if it flows into a basin degraded by sea water. This fresh water supply of approximately 270,000 acre-feet per year in Orange County would become unusable as a potable source.
The ground water basin acts as a distribution system because water may be extracted in a wide area overlying the basin. If the basin is lost, then a distribution system to deliver the alternative surface supply to the consumers must be constructed. In addition, the abandonment of the capital investment in wells and pumping facilities would represent a substantial economic loss.
The dependency on imported sources is becoming less desirable for southern California. The Metropolitan Water District provides the region with two sources of imported water. One is from northern California through the State Water Project and the other is from the Colorado River. Environmental concerns over the San Joaquin/Delta River system have had an impact on State Water Project