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19th century. How water is used and distributed in California is at the heart of the ongoing conflicts in California's water law and policy. The major water projects discussed later in this chapter provide much of California's water supply. Dependable supplies from the State Water Project were estimated under Decision 1485 at 2.3 million acre-feet per year. However, future State Board or federal standards and the requirements for endangered species will likely reduce this fIrm yield. Almost half of this water comes from Lake Oroville in Butte County, and the rest is developed from surplus flows in the Sacramento-San Joaquin Delta. Some of these flows are re-regulated in the San Luis Reservoir. The State Water Project has contracted to deliver 4.2 million acre-feet. The facilities originally planned to meet these requirements have not been completed, and completion is now uncertain. The Central Valley Project has contracted to deliver 9.3 million acre-feet of water to over 250 contractors of the federal Central Valley Project. Average year deliveries have been about 7.3 million acre-feet of water per year. California is apportioned 4.4 million acre-feet of Colorado River water per year, although in wet years it is possible for California to receive more water. California has generally received more than its entitlement because of a number of consecutive wet years and Arizona's failure to fully develop its Colorado River use. However, as Arizona takes more water, California cannot depend on receiving more than 4.4 million acre-feet per year without changes in the laws and regulations governing the Colorado RiverS The largest user of California's water is agriculture. In an average year, agriculture uses about 31.1 million acre-feet of water or approximately 80 percent of the state's developed water supply. Agriculture's share of California's total water supply, as opposed to the amount "developed" for use, is approximately 40 percent. Agriculture boomed in California from the World War I era to the 1980s. The approximately 5 million acres under irrigation in the 1930s grew to 9.7 million acres by 1981, although acreage in agri- cultural use has declined somewhat since 1981 to about 9.2 million acres in 19906 With the increasing prevalence of water transfers from agricultural to urban use, this trend is likely to continue. It is important to note that a significant percentage of the water used by agriculture percolates back into the ground or flows into streams, where it can be used again for other uses. Thus, 5 California Water Plan Update, volume 1, pages 43. 63-64, 69. 6 California Water Plan Update, volume l,page 8. Agriculture is the largest user afCalifor- nia's water supply. Chapter 1 A Brief History 5