South Atlantic Water Science Center Drought Monitoring

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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.


Information about drought
• Real-time drought maps:  Georgia  |  North Carolina  |  South Carolina

Lake Folsom, California. Comparing drought and  non-drought conditions.

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.

Visit our DroughtWatch pages by state:

DroughtWatch Maps

Drought Question and Answers


Drought Defined

Stillwater Creek in drought conditions

Stillwater Creek in drought conditions(Public domain.)

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.  Annual runoff, which is calculated from this streamflow data, supplies many of our needs for water. 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 through increased 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 often over 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. This has happened to a significant extent in California's San Joaquin Valley aquifer system. The land-surface elevation drop 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. Water allocations for river, wetland, wildlife, and fish restoration projects can be reduced or stopped altogether during severe drought. Dry, hot and windy weather, combined with dry vegetation and a spark - either through human intent, accident or lightning - can start a wildfire. 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 South Atlantic 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.

Drought and Groundwater

Groundwater, which is found in aquifers below the surface of the earth, is one of the nation's most important natural resources. Groundwater provides drinking water for a large portion of the nation's population, supplies business and industries, and is used extensively for irrigation. Groundwater can also contribute to surface water supplies. Some groundwater seeps into rivers and lakes, and can flow to the surface as springs.

The water level in an aquifer that supplies water to a well does not always remain the same. Droughts, seasonal variations in precipitation, and pumping affect the height of underground water levels. If a water is pumped at a faster rate than an aquifer is recharged by precipitation or other sources of recharge, water levels can drop. This can happen during drought, due to the extreme deficit of rain.

Long-term water-level data are fundamental to the resolution of many of the most complex problems dealing with groundwater availability and sustainability. Significant periods of time - years to decades - typically are required to collect water-level data needed to assess the effects of climate variability, to monitor the effects of regional aquifer development, or to obtain data sufficient for analysis of water-level trends.

Drought and Surface Water

Streams, rivers, lakes and reservoirs - collectively referred to as surface water - are important natural resources for irrigation, public supply, wetlands and wildlife. Surface water is also measured as annual runoff, which is the amount of rain and snowmelt drainage left after the demands of nature, evaporation from land, and transpiration from vegetation have been supplied. It supplies most of our basic water needs.

The South Atlantic Water Science Center measures the flow of surface water, and annual runoff, using a network of more than 800 streamgages that collect real-time data at locations across Georgia, North Carolina, and South Carolina. These gages are part of the larger USGS national streamgage network that contains about 8,000 continuous-record stream-gaging stations. The stream-gaging network provides a continuous source of well-archived, well-documented, and unbiased water data. Streamflow data assist water managers in making daily operational decisions for dealing with water requirements for municipal, industrial, and agricultural purposes, demands for hydroelectric power generation, and for controlling water storage in reservoirs to ensure adequate water supply.

Surface Water, Drought and Climate

The discharge and quality of water from rivers and reservoirs have broad effects on factors such as water availability, and ecosystem system health. The discharge and quality also fluctuates on timescales ranging from minutes to centuries in complicated ways that seem random at first glance. By taking a large-scale view of these fluctuations and linking them to global scale climatic processes, researchers are beginning to see more structure and predictability than has been previously recognized. For example, the Nation's water resources are tied together on regional scales by their shared responses to temperature and precipitation variations. Understanding these links provides a better scientific basis for predicting and planning for droughts, floods, and water supplies, months to years in advance.

How is water managed during a drought?

During severe drought, communities and water managers must often make difficult decisions about how scarce water resources will be used. How much water can be used to satisfy immediate water needs but minimize adverse impacts on future supplies? Can groundwater pumping be increased to augment depleted surface water supplies? If so, what are the long-term impacts of groundwater pumping? These are just a few of the questions that decision makers face during times of drought.

USGS science provides accurate, trusted, hydrologic data and scientific analysis to help decision makers who must address complex issues and competing interests in times of drought.