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USGS - science for a changing world

Drought Impacts

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. This year the Bureau of Reclamation will reduce flows a month 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.


Moreland, J.A., 1993 Drought: U.S. Geological Survey Water Fact Sheet, Open-File Report 93-642, 2

Drought - The Stealth Disaster Part 1, Overview

Drought - The Stealth Disaster Part 2, Science for Drought Planning

USGS Water Science School: Questions and answers about droughts


Drought Conditions Based on Current vs. Historical Data


This map shows current streamflow conditions as a percentile of historical averages (computed from the entire gage-station records compared to current day of the year records). Colored circles include stations containing at least 30 years of data. Gray circles are active stations, but are unranked because they contain less than 30 years of data, or parameters other than streamflow.

During the recent drought in California, below normal (less than the 25th percentile) streamflow conditions were recorded Statewide. For more information on streamflow, check out daily streamflow compared to historical streamflow for the day of the year in table or time series plot format.


California Below Normal Levels

Legend - Percentile classes for Below Normal Groundwater Levels in California

A well with below normal groundwater levels is identified on these pages when the most recent water-level measurement is in the 24th percentile or lower in the month of measurement over the period of record for the well. To be included on this map in red or orange, the well must be in an active measurement program (the well appears on the Active Groundwater Level Network) and the well must have 10 or more years of record in the month of the most recent measurement. These web pages present all active wells, regardless of measurement frequency, aquifer monitored, or the monitoring objective.