On average, 75 percent of California's annual statewide precipitation occurs from November through March. 50 percent occurs from December through February, coinciding with the timing of California’s largest winter storms. Our 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. A year will tend to be dry if a persistent Pacific high-pressure zone remains over California in midwinter and blocks storms that would otherwise reach us. Droughts occur when dry conditions persist long enough to create impacts.
Drought – and water conditions related to droughts – may be defined differently by different types of water users. Some key measurements that may be used to define drought conditions include precipitation, surface water storage (whether in reservoirs or in the form of snowpack), groundwater levels (as an indirect metric for groundwater storage), or supplies available from a water wholesaler.
The information below provides a quick overview of California’s basic conditions at a regional or statewide scale. More detailed information for precipitation and surface water information is available in the California Data Exchange Center(CDEC).
Groundwater conditions respond more slowly to drought than do surface water conditions and are heavily influenced by groundwater pumping, which increases during droughts. Drought’s impact on groundwater can be assessed by water level data submitted through the California Statewide Groundwater Elevation Monitoring (CASGEM) program for California’s more intensively used groundwater basins. Information about groundwater levels is available on CASGEM and in the Water Data Library. Many groundwater level measurements are only made or submitted twice a year (spring and fall). To analyze drought impacts on groundwater, we use this data to create maps such as:
- Groundwater Level Change: Fall 2011-Fall 2016 – This shows the cumulative impacts of five years of drought.
- Transition from drought to wet water year 2017 (from spring 2011 to spring 2017)
California’s largest wholesale water sources are the U.S. Bureau of Reclamation (USBR) Central Valley Project, USBR interstate Colorado River water contracts, and our State Water Project .
Information about their projected water deliveries can be found at:
- California Nevada River Forecast Center, Select City Precipitation
- Colorado Basin River Forecast Center
- National Weather Service California-Nevada River Forecast Center
- USGS Annual Runoff Estimate for California
- USGS Streamflow Conditions
- Western Regional Climate Center