California’s hydrologic conditions remain dry as February draws to a close.
California is no stranger to drought; it is a recurring feature of our climate. We recently experienced the 5-year event of 2012-2016, and other notable historical droughts included 2007-09, 1987-92, 1976-77, and off-and-on dry conditions spanning more than a decade in the 1920s and 1930s.
Paleoclimate records going back more than 1,000 years show many more significant dry periods. The dry conditions of the 1920s-30s, however, were on a par with the largest 10-year droughts in the much longer paleoclimate record.
Droughts cause public health and safety impacts, as well as economic and environmental impacts. Public health and safety impacts are primarily associated with catastrophic wildfire risks and drinking water shortage risks for small water systems in rural areas and private residential wells. Examples of other impacts include costs to homeowners due to loss of residential landscaping, degradation of urban environments due to loss of landscaping, agricultural land fallowing and associated job loss, degradation of fishery habitat, and tree mortality with damage to forest ecosystems.
Unfortunately, the scientific skill to predict when droughts will occur – which involves being able to forecast precipitation weeks to months ahead – is currently lacking. Improving long-range weather modeling capabilities is an area of much-needed research.
Defining drought is based on impacts to water users. California is a big state and impacts vary with location. Hydrologic conditions causing impacts for water users in one location may not represent drought for water users in a different part of California, or for users with a different water supply. Individual water agencies 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, occurring slowly over a period of time. 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 isn’t a drought for most Californians because of the state’s extensive system of water infrastructure and groundwater resources buffer impacts.
Drought impacts are felt first by people most dependent on annual rainfall – such as ranchers using dryland range or rural residents relying on wells in low-yield rock formations. Drought impacts increase with the length of a drought, as carry-over supplies in reservoirs are depleted and water levels in groundwater basins decline.
Provisions of California’s Emergency Services Act have been used to declare a statewide drought emergency for only two of our droughts, the 2012 to 2016 event and its immediate predecessor in 2007-09.
Declaring an End to Drought
Defining when drought ends is based on the moderation of drought impacts to water users. A city may define the end of a drought when its reservoir is full or it receives a full supply from the wholesale water agency. A rancher might define the end of a drought after enough precipitation falls to adequately support livestock grazing. Recovery from some drought impacts, such as declines in groundwater storage, can take multiple years.
- Experimental Seasonal Precipitation Forecasts
- Experimental Sub-Seasonal to Seasonal Precipitation Forecasts
- Using long-term tree-ring data to understand drought risks