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USGS - science for a changing world

California Drought

Hydrologic science can help citizens and communities prepare for and cope with drought in two ways - through drought planning, and in helping communities make the best day-to-day management decisions while the drought is taking place.

The USGS closely monitors the effects of drought through data collection and research, and is studying the current drought in the context of long-term hydrologic, climatic, and environmental changes. These studies support successful planning and science-based decision-making by water managers who must address complex issues and competing interests in times of drought. They also and help decision-makers prepare for climate change and possible future drought.

Drought Defined

A drought is a period of drier-than-normal conditions that results in water-related problems. When rainfall is less than normal for several weeks, months, or years, the flow of streams and rivers declines, water levels in lakes and reservoirs fall, and the depth to water in wells increases. If dry weather persists and water-supply problems develop, the dry period can become a drought.

The term "drought" can have different meanings to different people, depending on how a water deficiency affects them. Droughts have been classified into different types such as:

  • meteorological drought - lack of precipitation
  • agricultural drought - lack of soil moisture, or
  • hydrologic drought -reduced streamflow or groundwater levels

It is not unusual for a given period of water deficiency to represent a more severe drought of one type than another type. For example, a prolonged dry period during the summer may substantially lower the yield of crops due to a shortage of soil moisture in the plant root zone but have little effect on groundwater storage replenished the previous spring.


What are the impacts of drought?

Water quality degradation, surface and groundwater level declines, land subsidence - all are impacts of drought. Understanding the impacts of drought can help mitigate drought-related issues and prepare for future dry periods.

Drought Impacts >>

How does drought affect groundwater?

Groundwater provides drinking water for a large portion of the nation's population, supplies business and industries, and is used extensively for irrigation. But what happens to this resource during drought?

Groundwater & Drought >>

How does drought affect surface water?

Careful observation and analysis of the movement and condition of surface water is essential for understanding this resource, especially during times of drought. The California Water Science Center uses a network of more than 500 streamgages to collect real-time data on surface water at locations across the state.

Surface Water & Drought >>

How is water managed during a drought?

Water shortages during drought are not only a concern for humans, but for ecosystems in the Bay Delta and Central Valley as well.

Managing Water Resources during Drought >>

Is the drought over?

Updated April 24, 2017

After more than five years of drought in California, water year 2017 has seen above-average precipitation and snowpack, inspiring many to ask, "is the drought over?" On April 7, 2017, Gov. Jerry Brown issued Executive Order B-40-17, officially ending the drought state of emergency in all California counties except Fresno, Kings, Tulare, and Tuolumne. Hydrologically, however, the answer to this question requires consideration of California's three primary sources of water: surface water, snowpack, and groundwater.

small graphic of a stream

Surface Water: Precipitation in water year 2017 has filled the majority of California's major reservoirs to above-historic average levels. Likewise, as the USGS streamgage network shows, flows in the majority of the streams have been at or above average for most of the last 4 months. This indicates that most of California's rivers, creeks, lakes and reservoirs are in good condition.

snow along the Pit River in California

Snowpack: On average, the Sierra Nevada snowpack supplies about 30 percent of California's water needs as it melts in the spring and summer. A series of back-to-back atmospheric river storms blanketed the Sierra Nevada in January and February 2017. As of April 24, 2017, statewide snow accumulation data indicate that snowpack in the Northern, Central, and Southern Sierra is 190% of normal for this date.

small graphic of a fissure in parched land

Groundwater: Groundwater aquifers recover much more slowly than surface water and are limited, amoung other things, by how much and how fast water can recharge. Unlike surface water, which can recover during a few days of heavy precipitation, groundwater aquifer recovery often takes years or decades. Groundwater systems are also relied upon more heavily during times of drought. In addition, in many areas of the state, groundwater systems have been depleted for long periods - even between droughts - that they have not recovered from. Excessive, long-term groundwater over-use resulting in groundwater depletion can cause subsidence and permanent loss of groundwater storage as well as water quality degradation and seawater intrusion. These long-term impacts on groundwater have not been remedied by the recent weather. If recovery is possible, it will likely take several to many years to accomplish. Executive Order B-40-17, which ends the drought state of emergency in most of California, stresses the importance of emergency drinking water projects that will help address diminished groundwater supplies.

The long-term outlook for California’s drought can be better assessed in the months ahead. On April 1, the California Department of Water Resources conducted its annual snowpack measurement. This benchmark is important because it provides a comprehensive examination of the snowpack’s water content. Typically, April 1st marks the start of significant snowmelt, producing runoff that recharges reservoirs and groundwater systems slowly throughout the ensuing months. Unless there is excessive heat – which has been the case for the last three years – this slower runoff provides significant usable supply for the year, and can enhance the aquifer-recharge process. On April 1, 2017, DWR measured the snowpack to be 164 percent of average for the date, and determined the snowpack water content to be 183 percent of average. This measurement likely signifies that runoff in Water Year 2017 will be above average, particularly in comparison to recent drought years. However, the rate at which that runoff occurs is particularly sensitive to temperature. A rapid rise in temperatures may cause snowpack to melt too quickly, contributing to excessive runoff in a short period of time; which could lead to flooding. A slow, steady rise in spring and summer temperatures is ideal from a water supply perspective because the snowpack will melt slowly, allowing for optimal replenishment of aquifers and reservoirs.

When compared with historical, long-term data, analysis of surface-water runoff data now being collected by the USGS streamgage network will also help scientists better understand the effects that winter storms of 2017 have had upon California’s drought conditions. Runoff is an important component in maintaining healthy waterways and ecosystems and also contributes to groundwater replenishment through groundwater surface-water interactions. The USGS California Water Science Center – along with cooperating local, state, and federal agencies - continues to collect long-term data that are needed to assess the effects of climate variability on water resources.

While the drought state of emergency is officially over in most of California, the hydrologic effects of the drought will take years to recover. Dedicated science from the U.S. Geological Survey and its cooperators will help California understand the long-term effects the recent drought has had upon California's hydrologic framework.

Drought Facts

  • On January 17, 2014 California State Governor, Jerry Brown, declared a drought state of emergency. On April 17, 2017, Brown issued Executive Order B-40-17, officially ending the drought state of emergency in all California counties except Fresno, Kings, Tulare, and Tuolumne.
  • As of April 18, 2017, the National Drought Mitigation Center estimates approximately 10.3 million people in California are currently affected by the drought.
  • California's response to its ongoing drought has been guided by a series of executive orders issued by Governor Brown, the most recent officially ending the drought in most of California, but stressing the importance of emergency drinking water projects that will help address diminished groundwater supplies.
  • The time period of January 2016 - December 2016 has been the 3rd warmest on record for California. California saw 2014 as the warmest year on record.
  • The latest National Weather Service Climate Prediction Center seasonal drought outlook, valid for April 20 - July 31, 2017, shows drought in areas of southern and Central Coastal California likely to persist throughout those dates. The 1-month precipitation outlook for May 2017, issued April 20, 2017, suggests average precipitation for most of California, and above average precipitation for parts of northern California.

Snowpack Status

  • Snowpack, through runoff, provides about one-third of the water used by California's cities and farms. The snowpack at the beginning of April is crucial because this is when the snowpack is normally at its peak and begins to melt into streams and reservoirs.
  • On April 1, 2017, the California Department of Water Resources measured regional snowpack at 164% of normal for this date, and snowpack water content to be 183% normal for this date.
  • As of April 24, 2017, statewide snow accumulation data indicate that snowpack in the Northern, Central, and Southern Sierra is 190% of normal for this date.