Defining when drought occurs is a function of drought impacts to water users. Drought can best be thought of as a condition of water shortage for a particular user in a particular location. Hydrologic conditions constituting a drought for water users in one location may not constitute a drought for water users in a different part of California or for users with a different water supply. Individual water suppliers may use criteria such as rainfall/runoff, amount of water in storage, or expected supply from a water wholesaler to define their water supply conditions.
Drought is a gradual phenomenon. Although persistent drought may be characterized as an emergency, it differs from typical emergency events. Most natural disasters, such as floods or forest fires, occur relatively rapidly and afford little time for preparing for disaster response. Droughts occur slowly, over a period of time. There is no universal definition of when a drought begins or ends. Impacts of drought are typically felt first by those most reliant on annual rainfall -- ranchers engaged in dryland grazing, rural residents relying on wells in low-yield rock formations, or small water systems lacking a reliable water source. Criteria used to identify statewide drought conditions do not address these localized impacts. Drought impacts increase with the length of a drought, as carry-over supplies in reservoirs are depleted and water levels in groundwater basins decline.
Similarly, defining when drought ends is based on the moderation of drought impacts to water users. An urban water retailer may define the ending of drought as a full reservoir or as a full water supply from its wholesale water supplier. A rancher grazing livestock on non-irrigated rangeland may define the ending of drought as precipitation conditions that result in adequate pasture to support cattle over the summer grazing season. Because much of California’s developed water supplies are supported in part by snowpack in the Sierra and Cascade mountain ranges, recovery from drought conditions at the statewide scale is facilitated by having adequate snowpack to refill the major foothill reservoirs after the winter flood season has passed and storage of snowmelt runoff for later in the season can begin.
Storage, whether in surface water reservoirs or in groundwater basins, buffers drought impacts and influences the timing of when drought impacts occur. A single dry year does not constitute drought for most California water users because the state’s extensive system of water infrastructure and groundwater resources buffer the impacts of lack of a single winter’s precipitation. Following a multi-year dry period when precipitation (and snowpack) return to normal conditions, storage may not recover as quickly – especially storage in groundwater basins where longer time periods are required for recharge to occur. Water users whose supplies are heavily dependent on storage may not consider drought to have ended until their storage has returned to average conditions.
Historically, California’s significant multi-year droughts have been ended by an above-average water year where statewide precipitation was in the range of 150 percent of average. Because California’s annual water budget is determined by only a small number of winter storms, having a significantly above average year translates to having a winter season with a few very large storms. On average, about half of California’s average annual precipitation occurs from December through February, which coincides with the typical timing of the largest winter storms.
California's climate is highly variable both spatially (from temperate rain forest conditions on the North Coast to the extreme aridity of Death Valley) and temporally. Records for maximum annual precipitation range from more than 90 inches on the North Coast to a little over 2 inches in Death Valley. Droughts and floods can occur in close proximity. For example, the flooding of 1986 was followed by six years of drought (1987-92). At the beginning of the state's historical record the so-called "Noachian" floods of winter 1861-62 were followed by two severely dry years, a combination became the death knell for much of the cattle rancho economy.
Background data illustrating California's climate variability can be found on the Western Regional Climate Center website at: http://www.wrcc.dri.edu/CLIMATEDATA.html.
Measurements of California water conditions cover only a small slice of the past. Widespread collection of rainfall and streamflow information began around the turn of the 20th century. During our period of recorded hydrology, the most significant statewide droughts occurred during 1928-34, 1976-77, 1987-92, and 2007-09. The last significant regional drought occurred in parts of Southern California in 1999-2002. Historical data combined with estimates created from indirect indicators such as tree rings suggest that the 1928-34 event may have been the driest period in the Sacramento River watershed since about the mid-1550s.
Drought and Precipitation
Most of California’s precipitation (rain and snow) comes from storms moving across the Pacific Ocean. The path followed by the storms is determined by the position of an atmospheric high pressure belt that normally shifts southward during the winter months, allowing low pressure zones to move into the state. On average, 75 percent of California's annual precipitation occurs from November through March, with 50 percent occurring from December through February. California's average precipitation is dependent on a relatively small number of storms; a few storms more or less during the winter season can determine if the year will be wet or dry. If a persistent Pacific high pressure zone remains over California in mid-winter, there is a tendency for the year to be dry.
Drought and Groundwater
In an average year, about 30 percent of California's urban and agricultural water supplies come from groundwater. Reliance on groundwater increases during droughts due to reduced availability of surface water. During the six-year 1987-92 drought the total number of well driller reports filed with the Department were in the range of 25,000 wells per year for several years, up from fewer than 15,000 reports per year prior to the drought. Most of the new wells were for private residential use.