are protected by levees. The islands themselves were historically converted from marshlands as agricultural lands2 and most of them still are farmed.
Unimpaired inflows of water to the delta originate in the watersheds of the Sacramento and San Joaquin rivers. In an average year those flows are estimated to be 40.3 million acre-feet (MAF) or 48.8 percent of California’s average annual total water resource of approximately 82.5 MAF. Of the total unimpaired average inflow, 11.4 MAF are diverted upstream of the delta for agricultural (83.8 percent), urban (15.0 percent), and environmental (1.2 percent) uses. Diversions from the delta average 6.35 MAF, a little more than one-third of all diversions in the Sacramento–San Joaquin system. Diversions from the delta are dominated by the exports to the irrigation service areas of the federal Central Valley Project (CVP) and the State Water Project (SWP), which include southern portions of the San Francisco Bay Area, the western side of the San Joaquin Valley, and much of southern California. Significant amounts of water are diverted to irrigate delta lands, and irrigation return flow is discharged into delta channels. The average yearly outflow from the delta remaining after diversions equals 22.55 MAF (Lund et al. 2010).
The quantities of water reported above are for an average water year, but hardly any water year in California is average. Water supplies are highly variable from one year to another. Thus, for example, in the Merced River, which drains the watershed including most of Yosemite National Park and is a tributary of the San Joaquin River, the average annual flow is 1.0 MAF. Yet the low flow of record for the Merced River is 150,000 acre-feet, only 15 percent of the average flow, whereas the high flow of record is 2.8 MAF, 280 percent of the average flow. The variability in flows, which is characteristic of all of the state’s rivers, is largely a function of the interannual variability in amount and patterns of California’s Mediterranean climate, which has a wet and a dry season with precipitation falling mainly in the late fall and winter months. In addition, there is considerable variability in the proportion of the precipitation that falls in the mountains as snow, which adds to the variability of the hydrologic regime.
Until recently, planning for water shortage was based on a 5-year dry cycle from the 1930s, or on 1977, the driest year of record. However, recent analyses by the California Department of Water Resources (CDWR 2008, 2011) and Hanak (2012) indicate that changes in precipitation resulting from different anticipated climate conditions (see Chapter 4) will affect water availability for all users. Despite statewide conservation efforts, particularly in the urban sector, increasing seasonal restrictions have been
2 Recent historical ecology studies at the San Francisco Bay Institute are revealing that the original delta landscape was more complex than formerly thought, and had been modified by humans long before the 19th century (http://sfei.org/node/1088).