Climate Change Threatens Health: Drought

Drought: Threats to Water and Food Security

Water is life, and climate change is threatening this precious resource. Nearly every U.S. region is facing some increased risk of seasonal drought.

Climate change will significantly affect the sustainability of water supplies in the coming decades. As parts of the country get drier, the amount of water available and its quality will likely decrease - impacting people's health and food supplies.

Parts of the Western U.S. are already experiencing water crises because of severe dry-spells, but with climate change, the entire country will likely face some level of drought. NRDC's Climate Change, Water, and Risk report found that 1,100 counties - one-third of all counties in the lower 48 states - face higher risks of water shortages by mid-century as the result of climate change. More than 400 of these counties will face extremely high risks of water shortages.

As temperatures rise and precipitation decreases, water quality can be jeopardized. Shrinking amounts of water can concentrate contaminants such as heavy metals, industrial chemicals and pesticides, and sediments and salts. During drought, drinking water supplies are susceptible to harmful algal blooms and other microorganisms.

Of course, drought means more than not having access to clean drinking water. Changes in precipitation and water availability could have serious consequences for commercial agriculture – crops yield less and food security suffers. Drought conditions can also help fuel out-of-control wildfires.

Local communities across the country can prepare for drought by learning to conserve water and improving drinking water safeguards.

Nine states and several local governments have developed preparedness measures to address the drought impacts associated with climate change. The most common recommendation is improving general preparedness measures for droughts and ensuring an adequate water supply.

How are states addressing the threat of drought?

  • California's plan includes measures that focus on ensuring adequate water availability during times of drought. The city of Berkeley's strategy includes a city focused vulnerability assessment that will include assessing water resources, and Los Angeles has a measure to prepare for increased drought conditions. Find out more >>
  • Maryland's plan includes measures that focus on ensuring adequate water availability during times of drought. Find out more >>
  • New York's plan identifies measures to develop comprehensive drought monitoring programs and maintain emergency water stockpiles. Find out more >>
  • New Hampshire's plan identifies drought as a health-related threat due to climate change but does not include specific measures to address this threat. The city of Keene's strategy includes a measure to increase Keene's water storage capabilities in the face of drought. Find out more >>
  • Oregon's plan includes measures to improve capacity to provide technical assistance, and incentives to increase storage capacity during times of drought. Find out more >>
  • Pennsylvania's plan includes a general recommendation to implement measures to prevent and control adverse health effects caused by drought. Find out more >>
  • Washington's plan includes measures that focus on ensuring adequate drinking water resources and fire protections in areas likely to be affected by drought and improving drought-forecasting capability. King County's strategy includes a measure to develop emergency response protocols for events like droughts. Find out more >>
  • Wisconsin's plan includes a measure to minimize threats to public health and safety by anticipating and managing impacts resulting from extreme weather events like drought. Find out more >>
  • Virginia's plan identifies drought as a health-related threat due to climate change but does not include specific measures to address this threat. The city of Alexandria's strategy includes a measure to increase water storage capabilities in the face of drought conditions. Find out more >>

Map Methods

Background: Global warming is projected to alter precipitation patterns, increase the frequency and intensity of major storm events, and increase risks of floods throughout the U.S. and particularly the Midwest and Northeast.1 Over the period from 2000 to 2009, roughly 30 to 60% of the U.S. land area experienced drought conditions at any one time.2

Drought Vulnerability Indicators: Drought vulnerability is indicated by the extreme low flow days that are defined as less than the 5th percentile for each monitoring station. This category is classified by USGS as "severe hydrological drought".3 Current conditions (2000 -2009) were compared to historical conditions using a thirty-year reference period (1961-1990, in this case), consistent with other climate change-related studies.

Streamflow Data

Data Source: Data from all streamflow gauging stations for all years historically from the United States Geological Survey was collected. The data through 2009 was purchased from EarthInfo, a private vendor that collects USGS data and makes it available on DVDs. Watershed data also comes from the USGS ( The original watershed data was at the HUC12-level (high resolution). These watersheds were aggregated to HUC4 based on HUC ID.

Data Preparation: Data was assembled in an SQL data base. Each record represented a single site-month of data. The total was 7 million site-months or a total of more than 210 million site-days of data.

Calculations: A 95th and 5th percentile value for the reference period (1961-1990) was calculated for each site for each day of the year using a moving average of 7 days before and after the day. For example, the reference period percentiles for June 15th would be calculated by selecting all days June 8-June 22 for the years 1961-1990 (15 x 30 possible days = 450 days contribute to the percentile calculations). June 16th would be based on all days June 9-June 23 and so on. For the first and last six days of the year (Jan 1- Jan 6 and Dec 26-Dec 31), the reference period moving average included days from 1960 and 1991. Current flows at a site were then compared against the reference period percentiles day by day. For example, the ten possible June 15th values from 2000-2009 would be compared against the 95th and 5th percentiles for that day from the reference period. Watershed-level averages were then computed by averaging site-level data within the watersheds.

Analysis was limited to stations that make up the Hydro-Climatic Data Network, a subset of stations that are unaffected by artificial diversions. See appendix for a description of these sites below.

Sites were excluded from the analysis if A) they had less than 75% of possible reference days; B) two entire years from the 2000-2009 period were missing; or C) they had less than 75% of possible current period days.

Map: Color gradations reflect terciles of the data distribution.

Flood Stage Data

Data Source: Data was collected from the USGS WaterWatch website (

Data Preparation: We developed Python scripts to iterate through all states and download/format all available flood data 2000-2009. Tabular data was converted to geographic data by linking to the dataset described above on USGS station number.

Calculations: No additional calculations were performed. Graduated circles were mapped based on the "No. of days above flood stage" variable from USGS.

Map: Graduated circles reflect natural data breaks in the distribution. Note: flood stage information is not currently available for all Hydro-Climatic Data Network (HCDN) streamflow gauge sites.

Appendix: Definition of HCDN sites

HCDN Description: Pasted from USGS Hydro-Cliatic Data Network: Streamflow Data Set 1874-1988 by By J.R. Slack, Alan M. Lumb, and Jurate Maciunas Landwehr. USGS Water-Resources Investigations Report 93-4076

The potential consequences of climate change to continental water resources are of great concern in the management of those resources. Critically important to society is what effect fluctuations in the prevailing climate may have on hydrologic conditions, such as the occurrence and magnitude of floods or droughts and the seasonal distribution of water supplies within a region. Records of streamflow that are unaffected by artificial diversions, storage, or other works of man in or on the natural stream channels or in the watershed can provide an account of hydrologic responses to fluctuations in climate. By examining such records given known past meteorologic conditions, we can better understand hydrologic responses to those conditions and anticipate the effects of postulated changes in current climate regimes. Furthermore, patterns in streamflow records can indicate when a change in the prevailing climate regime may have occurred in the past, even in the absence of concurrent meteorologic records.

A streamflow data set, which is specifically suitable for the study of surface-water conditions throughout the United States under fluctuations in the prevailing climatic conditions, has been developed. This data set, called the Hydro-Climatic Data Network, or HCDN, consists of streamflow records for 1,659 sites throughout United States and its Territories. Records cumulatively span the period 1874 through 1988, inclusive, and represent a total of 73,231 water years of information.

Development of the HCDN Data Set: Records for the HCDN were obtained through a comprehensive search of the extensive surface- water data holdings of the U.S. Geological Survey (USGS), which are contained in the USGS National Water Storage and Retrieval System (WATSTORE). All streamflow discharge records in WATSTORE through September 30, 1988, were examined for inclusion in the HCDN in accordance with strictly defined criteria of measurement accuracy and natural conditions. No reconstructed records of "natural flow" were permitted, nor was any record extended or had missing values "filled in" using computational algorithms. If the streamflow at a station was judged to be free of controls for only a part of the entire period of record that is available for the station, then only that part was included in the HCDN, but only if it was of sufficient length (generally 20 years) to warrant inclusion. In addition to the daily mean discharge values, complete station identification information and basin characteristics were retrieved from WATSTORE for inclusion in the HCDN. Statistical characteristics, including the monthly mean discharge, as well as the annual mean, minimum and maximum discharge values, were derived for the records in the HCDN data set. For a full description of the development and content of the Hydro-Climatic Data Network, please take a look at the HCDN Report.

Related Links

  1. Karl TR, Melillo JM, Peterson TC, editors. Global climate change impacts in the United States. New York: Cambridge University Press; 2009.
  2. EPA "Climate Change Indicators in the United States (2010)", EPA 430-R-1—007.
  3. US Geological Survey (USGS). WaterWatch: Drought conditions website.
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