from May through October. The climate is unfavorable to development in the sense that water demands for irrigated agriculture, air conditioning, outdoor domestic uses, and recreational purposes tend to peak in the warm dry season. However, precipitation throughout California is generally unreliable, and California is subject to persistent and sometimes severe droughts, even in the seasons when precipitation is expected.

The combination of rapid population growth and general aridity led to a 20th-century water resources development program punctuated by the construction of major water storage and conveyance projects. The Los Angeles and San Francisco metropolitan areas, the foci of urban settlement, outstripped local water supplies early on and began to import supplemental supplies from remote locations. Most famously, the City of Los Angeles acquired land and water resources in the Owens Valley on the eastern side of the Sierra Nevada and constructed conveyance facilities to bring the water to the Los Angeles basin (Kahrl 1983). At about the same time, San Francisco developed a storage and conveyance project to the east in the Tuolumne River basin, which drains a portion of the west side of the Sierra Nevada. There followed, in 1929, further development of the Mokelumne River basin, also a western Sierra drainage, to supply the growing demands of the East San Francisco Bay region and, in 1939, the Colorado River Aqueduct to bring water from the Colorado River to support growth throughout the South Coast basin of southern California (Hundley 2001).

During the 20th century, California also became the largest agricultural state in the nation. Although there had been extensive rain-fed (“dry-land”) farming in the late 1800s, it thrived only during an exceptionally wet period, and most subsequent agriculture was irrigated. Early irrigation communities relied on water from neighboring streams and groundwater. Dating back at least as early as 1855, California recognized the “prior appropriation doctrine” for the allocation of surface-water rights. This system, which follows the maxim “first in time, first in right,” allows the first water users (known as “senior” appropriators) on a stream system to divert their entire allotment before the chronologically next water user is entitled to divert a single drop. Because water rights are of theoretically infinite duration, many senior irrigators in California could argue that they hold more secure water rights than later-initiated uses, such as the application of water for the protection of the natural environment. Recent court decisions, combined with the state constitution, the developing public trust doctrine, and legislation have combined to create in practice a more rational method of allocation. The construction of large storage and conveyance projects, which began with the federal Central Valley Project (CVP) in the 1930s and 1940s, allowed the expansion of agriculture in both the Sacramento and San Joaquin valleys and offset, to some degree, the significant groundwater overdraft that was present in the San Joaquin

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