In recent decades, drought years have become more frequent and more severe in California. Droughts are periods of unusually dry weather that last long enough to cause a shortage of water. The state’s highly variable precipitation is a main driver of drought. However, human-induced warming has made extreme droughts more likely.

Droughts can have widespread impacts on communities and ecosystems, often leading to significant economic costs. Water supplies for drinking, household use, agriculture, and power generation become scarce. Trees and other vegetation dry up, becoming more vulnerable to pests. Wildfire risks increase. Rivers and streams become less suitable for fish and other aquatic organisms.

Why is this indicator important?
Drought can have major environmental, social, and economic repercussions arising from its many impacts, including:

• Drinking water shortages. Reduced water supplies primarily affect small drinking water systems that often depend on a single water source. By late 2015, more than 100 small water systems lacked water and more than 2,000 domestic wells went dry, particularly in the Central Valley and Sierra Nevada foothills.
   
• Significant impacts on agriculture. During the recent drought, orchards and vineyards were abandoned, more than 500,000 acres of farmland (6 percent of irrigated acreage) were left idle, and the livelihoods of over 10,000 seasonal farmworkers disappeared.
   
• Increased tree mortality and wildfire risks. The recent drought has been linked to the die-off of water-stressed forest trees left more vulnerable to insect infestations. Dry vegetation, along with drought conditions, increase the risk for destructive, rapidly spreading wildfires.
   
• Threats to wildlife. Record-low flows and poor water quality threaten salmon and other native fish. Water shortages in wildlife refuges in the Central Valley and Klamath Basin have forced birds to gather in smaller areas, making them more vulnerable to disease outbreaks and predation.
   
• Human health impacts. Drought may affect human health by altering patterns of certain diseases like West Nile Virus and by increasing air pollution from wildfires and dust storms.
   
• Land subsidence. Reliance on groundwater increases during droughts. Over-pumping can cause the aquifer to become compacted, reducing its water-holding capacity and causing the land surface to sink. This lowering of the surface (known as land subsidence) can damage buildings, water conveyance systems, roads, railways, bridges and other infrastructure.