Water year 2007 was a dry year statewide, and especially in Central and Southern California. Much of Southern California had one of the driest precipitation years of record, surpassing the prior records set in 2001-02. In Northern and Central California, unimpaired runoff from Sierra Nevada watersheds was below average. The Colorado River Basin, an important source of water supply for Southern California, continued in drought conditions, having experienced below average runoff in seven of the last eight years.

With the spring rainfall season now essentially wrapping up, water year 2008 in California is also turning out to be dry, but not as dry as 2007. Although statewide precipitation for the water year to date is only about 10 percent below average, the depletion in soil moisture from a dry 2007 will result in reduced runoff. Runoff indices for the Sacramento and San Joaquin River systems -- the source of much of California's developed water supply -- are forecasted to be classified as "critical" for the Sacramento River at the 50 percent exceedance level and "critical" for the San Joaquin River at the 75 percent exceedance level. The combined impact of a dry 2007 and dry 2008 is also seen in reduced storage in the state's major reservoirs. See the link at left for a map with current reservoir storage conditions. Water supplies for customers of the State Water Project and federal Central Valley Project will be affected by court-imposed restrictions on diversions from the Sacramento-San Joaquin River Delta to protect the Delta smelt, as described here. The Department currently expects that 2008 State Water Project deliveries will be 35 percent of the amounts requested by Project contractors. Water year runoff in the Colorado River Basin is forecasted to be slightly above average, with contractors for Colorado River water expected to receive their normal deliveries. Total reservoir storage in the Colorado River system remains at roughly half of capacity, reflecting the preceding eight-year drought period.

Defining when a drought begins is a function of the impacts of dry conditions on water users. California's extensive system of water supply infrastructure -- its reservoirs, managed groundwater basins, and inter-regional conveyance facilities -- mitigates the effect of short-term dry periods. Hydrologic conditions constituting a drought for water users in one location may not constitute a drought for water users for water users in a different part of the state or with a different water supply. Individual water suppliers may use criteria such as rainfall/runoff, amount of water in storage, decline in groundwater levels, or expected supply from a water wholesaler to define their water supply conditions.

California’s last major statewide drought was 1987-92. At a regional level, parts of Southern California experienced a series of consecutive dry years in the late 1990s/early 2000s. Other significant droughts occurred during 1928-34 and 1976-77. 1977 has the distinction of being the single driest year of California’s measured hydrologic record.