USGS - science for a changing world

What is 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?

During times of drought, vegetation is visibly dry, stream and river flows decline, water levels in lakes and reservoirs fall, and the depth to water in wells increases. As drought persists, longer-term impacts can emerge, such as land subsidence, seawater intrusion, and damage to ecosystems. Unlike the immediate impacts of drought, however, long-term impacts can be harder to see, but more costly to manage in the future.

Short-Term Drought Impacts

During drought, declines in surface water flows can be detrimental to water supplies for agriculture and cities, hydropower production, navigation, recreation, and habitat for aquatic and riparian species. Several California Water Science Center streamgages have recently recorded streamflows that are below all-time record lows for specific days of the year. Annual runoff, which is calculated from this streamflow data, supplies many of our needs for water, Recent runoff estimates for California show measurements on par with 1930's and late 1970's droughts.

Unlike the effects of a drought on streamflows, groundwater levels in wells may not reflect a shortage of rainfall for a year or more after a drought begins. Despite reduced availability, reliance upon groundwater often increases during drought throughincreased groundwater pumping to meet water demands. If water is pumped at a faster rate than an aquifer is recharged by precipitation or other sources, water levels can drop, resulting in decreased water availability and deterioration of groundwater quality.

Ultimately, the surface water and groundwater form one interconnected hydrologic system. Nearly all surface water features - streams, lakes, reservoirs, wetlands, and estuaries - interact with groundwater. In addition to being a major source of water to lakes and wetlands, groundwater plays a crucial role in sustaining streamflow between precipitation events - especially during protracted dry periods. Although the contribution of groundwater to total streamflow varies widely among streams, hydrologists estimate the average contribution is somewhere between 40 and 50 percent.

Long-Term Drought Impacts

Excessive groundwater pumping and aquifer depletion can cause the aquifer system to compact, which can cause land to sink, permanent loss of groundwater storage in the aquifer system, and infrastructure damage. Overpumping in California's San Joaquin Valley aquifer system has caused renewed land-surface elevation drop that has resulted in serious operational and structural issues for surface water delivery networks, such as the Delta-Mendota Canal (DMC).

In coastal communities, the reversal of natural groundwater flows to the ocean as a result of groundwater pumping can cause seawater to enter the aquifer system. Seawater intrusion compromises groundwater quality and can be a costly problem to manage. The City of Santa Barbara is working with the California Water Science Center to update information on its groundwater supplies and to identify optimal water-resource management strategies to balance groundwater use with other sources of water.

Water allocations for river, wetland, wildlife, and fish restoration projects can be reduced or stopped altogether during severe drought. The California Water Science Center is part of the team tasked to restore river flows for salmon survival along the San Joaquin River. The Bureau of Reclamation reduced​ 2016​ flows earlier than planned because of critical water shortages.

Dry, hot and windy weather, combined with dry vegetation and a spark - either through human intent, accident or lightning - can start a wildfire. Drier-than-normal conditions can increase the intensity and severity of wildfires. In the aftermath of wildfires such as the 2013 Rim Fire, ash, woody debris and sediment can flow downstream from burn areas and contaminate water supplies. Flash flooding and mudslides in burn areas can also be damaging and deadly.

USGS science - conducted both in "real-time" and over the long-term - helps inform decision makers in communities across the country who have to deal complex issues and competing interests in times of drought. The California Water Science Center monitors the immediate impacts of drought on water availability and water quality through streamflow, surface water, and groundwater monitoring and data collection. Long-term data collection is needed to assess the effects of climate variability on water sources, to monitor the effects of regional aquifer development, and to obtain data sufficient for analysis of surface water and groundwater-level trends.