Most of California's agriculture is the Central Valley. Surely that was never desert? It's got rain all winter and runoff from the mountains all summer.
From Wikipedia[1]: "The northern Central Valley has a hot Mediterranean climate (Koppen climate classification Csa); the more southerly parts in rainshadow zones are dry enough to be Mediterranean steppe (BShs, as around Fresno) or even low-latitude desert (BWh, as in areas southeast of Bakersfield)."
Central Valley farms and the coastal cities are mostly fed by water from the Sierra's, via either rivers or one of California's many aquifers (e.g. Hetch Hetchy, Los Angeles, Colorado, California).
