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USGS - science for a changing world
California Water Science Center

California Drought

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.

small graphic of a streamflow datamap

What are the effects 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.

small graphic of a streamflow datamap

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?

small graphic of a streamflow datamap

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 over 400 streamgages to collect real-time data on surface water at locations across the state.

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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.