and much of southern California. Substantial amounts of water also are diverted upstream for use in the Bay Area and Central Valley cities and farms, and within the delta itself for local irrigation. Irrigation return flows are discharged upstream and into the delta itself. Water supplies are highly variable from one year to another.
Despite statewide water conservation efforts, which are particularly pronounced in the urban sector, increasing seasonal restrictions on diversions have been applied, although the total amount of water diverted for export by SWP and CVP has not decreased. The CVP withdraws water from the delta and conveys it southward into the San Joaquin Valley through a system of canals built and operated by the federal Bureau of Reclamation and various water user groups. Most of this water is used for agricultural purposes; a small amount is contracted for domestic use. The SWP withdraws water separately from the delta and conveys it southward to agricultural users on the west side and at the very southern end of the San Joaquin Valley and subsequently over the Tehachapi Mountains into the conurbation of the South Coast Basin. Total available supplies to both CVP and SWP have been constrained in recent years by court decisions restricting diversions because of environmental concerns. In addition, many of the levees have become weak and some of the natural riparian zones of the delta have been eroded. Resolution of these problems is complicated by water scarcity generally and because alternative solutions impose differing degrees of scarcity for the uses advocated by different groups of stakeholders. The risk of change in water supplies, which could be manifested either by increases in the already substantial intraseasonal and intra-annual variability or through an absolute reduction in available supplies, underscores the existence of water scarcity and illustrates ways in which such scarcity could be intensified.
In addition to serving economic purposes, delta water has been managed for other purposes. Since the beginning of CVP operations, water diversions to users outside the delta have been managed to reduce the effects of salinity intrusion on local water users in the western margins of the delta. Additionally, the constitution of California requires that the waters of the state be put to “beneficial use.” Although not defined, this criterion is subject to judicial review and determination. The enactment of both state and federal environmental laws has led to increased allocation of natural and stored water to environmental (instream) uses. The importance of environmental uses of water has been reflected further in many state regulatory decisions and, more recently, in judicial interpretations of the federal ESA and the California Endangered Species Act that have led to specific water allocations. Five taxa of fish residing in or migrating through the delta (one steelhead population, two populations of Chinook salmon, delta smelt, and green sturgeon) have been listed as threatened or endangered under the