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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 March 23, 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?" 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 2017. As of March 23, 2017, statewide snow accumulation data indicate that snowpack in the Northern, Central, and Southern Sierra is 160 percent 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, among 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.

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 conducts its annual snowpack measurement. This benchmark is important because it provides a comprehensive examination of the snowpack’s water content. Historically, after April 1, snows begin to melt, producing runoff that recharges reservoirs and groundwater systems slowly throughout the ensuing months. If there is not excessive heat, as has been the case for the last 3 years, this slower runoff provides usable supply for the year and can enhance the aquifer recharge process. The April 1 measurement will help California better understand the effects winter snows have had upon the state’s water resources.

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.

Drought Facts

  • On January 17, 2014 California State Governor, Jerry Brown, declared a drought state of emergency.
  • As of March 23, 2017, the National Drought Mitigation Center estimates approximately 10.3 million people in California are currently affected by the drought. On that date, only 24% of the state was experiencing drought.
  • California's response to its ongoing drought has been guided by a series of executive orders issued by Governor Brown, the most recent ordering continued, long-term water savings as drought persists.
  • 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 February 16 - May 31, 2017, shows drought in southern California likely to resolve, and drought in the Central Coast region of California as persistent, but improving. The 1-month precipitation outlook for March 2017, issued February 16, 2017, suggests average precipitation for most of California, and above average precipitation for parts of southeastern 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.
  • As of March 23, 2017, regional snowpack from automated snow sensors is 160% of normal for this date (California Department of Water Resources).