Climate Change Indicators in the United States
Data source: USGS, 2014 1
Data source: USGS, 2014 2
Data source: USGS, 2014 3
Data source: USGS, 2014 4
- Over the past 73 years, seven-day low flows have generally increased in the Northeast and Midwest (in other words, on the days of lowest flows, streams in these areas are carrying more water than before). In parts of the Southeast and the Pacific Northwest, low flows have generally decreased (that is, streams are carrying less water than before). Overall, sites show more increases than decreases (see Figure 1).
- Three-day high-flow trends vary from region to region across the country. For example, streams in the Northeast have generally seen an increase or little change in high flows since 1940, while some West Coast streams have seen a decrease and others have seen an increase. Overall, sites show more increases than decreases (see Figure 2).
- The largest changes in annual average streamflow have taken place in the Northeast and Midwest. Other regions saw few substantial changes. Overall, sites show more increases than decreases (see Figure 3).
- Nearly half of the streams studied show winter-spring runoff happening more than five days earlier than in the mid-20th century. The largest changes occurred in the Pacific Northwest and Northeast (see Figure 4).
Streamflow is a measure of the rate at which water is carried by rivers and streams, and it represents a critical resource for people and the environment. Changes in streamflow can directly influence the supply of drinking water and the amount of water available for irrigating crops, generating electricity, and other needs. In addition, many plants and animals depend on streamflow for habitat and survival.
Streamflow naturally varies over the course of a year. For example, rivers and streams in many parts of the country have their highest flows when snow melts in the spring and their lowest flows in late summer. The amount of streamflow is important because very high flows can cause erosion, flooding, and ecosystem disruption, while very low flows can diminish water quality, harm fish, and reduce the amount of water available for people. The timing of high flow is important because it affects the ability of reservoir managers to store water to meet needs later in the year. In addition, some plants and animals (such as fish that migrate) depend on a particular pattern of streamflow as part of their life cycles.
Climate change can affect streamflow in several ways. Changes in the amount of snowpack and earlier spring melting (see the Snowpack indicator) can alter the size and timing of high streamflows. Because of the relationship between precipitation and runoff, more precipitation will potentially cause higher average streamflow in some places, while heavier storms (see the Heavy Precipitation indicator) could lead to larger peak flows. However, more frequent or severe droughts could reduce streamflow in certain areas.
About the Indicator
The U.S. Geological Survey measures streamflow in rivers and streams across the United States using continuous monitoring devices called stream gauges. This indicator is based on 193 stream gauges located in areas where trends will not be substantially influenced by dams, reservoir management, wastewater treatment facilities, or land-use change. The indicator also excludes stream gauges with substantially overlapping watershed areas.
This indicator examines four important measures of streamflow conditions that occur during the course of a year. Figure 1 shows trends in low flow conditions, which are commonly calculated by averaging the lowest seven consecutive days of streamflow in a year. In many locations, this method captures the year's driest conditions. Figure 2 shows trends in high flow conditions, which are commonly calculated by averaging the highest three consecutive days of streamflow in a year. Three days is an optimal length of time to characterize runoff associated with large storms and peak snowmelt. Figure 3 shows changes in the annual average streamflow, which is calculated by averaging daily flows over the entire year.
Figure 4 shows trends in the timing of winter and spring runoff. This measure is limited to 56 stream gauges in areas where at least 30 percent of annual precipitation falls as snow. Scientists look at the total volume of water that passes by a gauge between January 1 and June 30, then determine the date when exactly half of that water has gone by. This date is called the winter-spring center of volume date. A long-term trend toward an earlier date could be caused by earlier spring snowmelt, more precipitation falling as rain instead of snow, or other changes in precipitation patterns.
Streamflow measurements were used from gauges in areas where streamflow is not highly affected by human influences such as dams, land development, or changes in land cover. However, changes in land cover and land use over time could still influence streamflow trends at some streams. The gauges used for this indicator are not evenly distributed across the country.
Streamflow data were collected by the U.S. Geological Survey. These data came from a set of gauges in watersheds with minimal human impacts, which have been classified as reference gauges.5 Daily average streamflow data are stored in the National Water Information System and are publicly available at: http://waterdata.usgs.gov/nwis.
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