As shown, the greatest proportion, about 67 percent, returns to the atmosphere through evapotranspiration, about 29 percent is discharged from the conterminous United States as net surface-water outflow into the Pacific and Atlantic Oceans and across the borders into Canada and Mexico, about 2 percent is discharged as ground-water outflow, and about 2 percent is consumed by people, animals, plants, and industrial and commercial processes U.
For most of the United States, evaporation returns less moisture to the atmosphere than does transpiration. Evaporation is the loss from open bodies of water, such as lakes and reservoirs, wetlands, bare soil, and snow cover; transpiration is the loss from living-plant surfaces. Several factors other than the physical characteristics of the water, soil, snow, and plant surface also affect the evapotranspiration process. The more important factors include net solar radiation, surface area of open bodies of water, wind speed, density and type of vegetative cover, availability of soil moisture, root depth, reflective land-surface characteristics, and season of year.
Assuming that moisture is available, evapotranspiration is dependent primarily on the solar energy available to vaporize the water. Because of the importance of solar energy, evapotranspiration also varies with latitude, season of year, time of day, and cloud cover. The distribution of mean daily solar radiation for the United States fig. The areas that receive the maximum solar radiation and have the greatest lake evaporation in the conterminous United States are in the Southwest; the areas that receive the minimum solar radiation and have the least lake evaporation are in the Northeast and Northwest.
According to the Bureau of Census data U. Bureau of the Census, , p. Mean annual lake evaporation ranges from about 20 inches in parts of Maine, Oregon, and Washington to about 80 inches in parts of Arizona, California, and Nevada. Figure 2. Mean daily solar radiation in the United States and Puerto Rico. Source: Data from the U. Department of Commerce, Figure 3. Mean annual lake evaporation in the conterminous United States, Data not available for Alaska, Hawaii, and Puerto Rico.
Source: Data from U. Another important climatic factor that contributes to evapotranspiration is wind speed. Winds affect evapotranspiration by bringing heat energy into an area and removing the vaporized moisture. A 5-mile-per-hour wind will increase still-air evapotranspiration by 20 percent; a mile-per-hour wind will increase still-air evapotranspiration by 50 percent Chow, , p.
Maximum mean annual wind velocities, averaging more than 14 miles per hour, are recorded in the central United States. Minimum mean annual wind velocities, averaging less than 8 miles per hour, are recorded along the West Coast and in the mountainous part of the east-central United States Eagleman, , p.
The type of vegetative cover is not as important in the evapotranspiration process as is solar radiation if the vegetative cover is dense and sufficient soil moisture is available Kozlowski, , p. Most plants that have a shallow root system, however, will experience moisture stress, which results in decreased transpiration during prolonged droughts. The reflective characteristics of the land surface also have an effect on the magnitude of evapotranspiration. Coniferous forests and alfalfa fields reflect only about 25 percent of the solar energy, thus retaining substantial thermal energy to promote transpiration; in contrast, deserts reflect as much as 50 percent of the solar energy, depending on the density of vegetation Rosenberg, , p.
The seasonal trend of evapotranspiration within a given climatic region follows the seasonal trend of solar radiation and air temperature. Minimum evapotranspiration rates generally occur during the coldest months of the year; maximum rates, which generally coincide with the summer season, when water may be in short supply, also depend on the availability of soil moisture and plant maturity. However, the seasonal maximum evapotranspiration actually may precede or follow the seasonal maximum solar radiation and air temperature by several weeks.
In the conterminous United States, two major forested areas exist: the eastern forests, which include large areas of conifers and hard- woods, extend from the East Coast to the eastern edge of the central Great Plains; the western forests, which are predominantly conifers that grow in mountainous areas separated by semiarid basins, extend from the West Coast to the western edge of the central Great Plains. The forests of the eastern United States cover million acres; those of the western United States cover million acres and include about 24 million acres in Alaska U.
The severity of California's current drought is illustrated in these images of Folsom Lake, a reservoir in Northern California located 25 miles 40 kilometers northeast of Sacramento. The lake is formed by Folsom Dam, in the foreground, which is part of the U.
Bureau of Reclamation. White bleached rock show shoreline when Lake Powell is at capacity. This short video is one of a series of four total shorts highlighting USGS water science in California's Delta region.
The Sacramento-San Joaquin Delta is the hub of the state's water system. Water quality touches on all aspects of life. Teams of U. Geological Survey scientists along with their partners monitor water quality and identify sources of pollution and.
The surface level of Lake Mead in Nevada and Arizona has fallen to a historic low as 16 years of ongoing drought in the American Southwest continue to impact the Colorado River Basin. A scientist from the University of California, Berkeley climbs a giant sequoia to measure its drought stress.
The USGS is collecting data at hundreds of sites on rivers and streams in six western states to document the drought. USGS scientists will analyze the data to identify which rivers and streams may be most vulnerable to future droughts. Skip to main content. Search Search. A drought is a period of drier-than-normal conditions that results in water-related problems.
Drought Portal. Apply Filter. What are the long-term effects of climate change? Scientists have predicted that long-term effects of climate change will include a decrease in sea ice and an increase in permafrost thawing, an increase in heat waves and heavy precipitation, and decreased water resources in semi-arid regions.
Below are some of the regional impacts of global change forecast by the Intergovernmental Panel on How can climate change affect natural disasters? With increasing global surface temperatures the possibility of more droughts and increased intensity of storms will likely occur. As more water vapor is evaporated into the atmosphere it becomes fuel for more powerful storms to develop.
More heat in the atmosphere and warmer ocean surface temperatures can lead to increased wind speeds in tropical What is the difference between global warming and climate change? Although people tend to use these terms interchangeably, global warming is just one aspect of climate change. What are some of the signs of climate change? Why doesn't a drought end when it rains? Rainfall in any form will provide some drought relief. A good analogy might be how medicine and illness relate to each other.
A single dose of medicine can alleviate symptoms of illness, but it usually takes a sustained program of medication to cure an illness. This is generally not a good thing, especially if these molecules are free radicals. Once again, drought tolerant plants have some very cool strategies to fight this problem.
At the first signs of drought, the cells of these plants will accumulate a bunch of molecules involved in what is called osmotic adjustment OA [ 3 ]. OA is the change is solute concentration in a cell. This is like when you dissolve sugar in water, where sugar is the solute. These molecules solutes can be sugars, amino acids or small proteins. The purpose of these molecules is to limit the movement of water out of the cell.
What makes these OA molecules unique in drought tolerance is that they serve many functions. They can also bind water itself, preventing it from moving out of the plant cells. These OA molecules also bind to membranes, stabilizing the structure of the plant when water is restricted. Resurrection plants are able to survive complete loss of water.
They accumulate vast quantities of OAs, release free radical scavengers and produce special protective proteins to survive long and severe droughts. They do all of this while they also fold their leaves away and wait until rain falls Figure 3. The process can be compared to bears going into hibernation. Keep in mind that we have discussed these processes used to protect plants from drought in a very simplified manner. Looking closely at these processes is actually very complicated.
Substances necessary to survive drought will be produced by accessing this code at the right time. This accessing of the genetic code to help a plant survive a drought is called the genetic response of the plant. The genetic responses of a plant experiencing the stress of a drought are very complex—lots of genes are switched on or off.
Using advanced computer technologies, scientists are now able to identify most of the genes that play a role in protecting a plant from drought. This technology has found that literally hundreds of genes are switched on and off, depending on where and when they are needed! What we will say is that these genes fall mainly into three groups: 1 genes that control other genes important for switching genes on and off; 2 genes that produce substances that help with drought protection in the plant; and 3 genes involved in water uptake and transport.
Why do you think it might be important to know which genes play a role in helping plants avoid or tolerate drought? Most of our crops are actually not able to survive droughts. How are we going to protect our crops or make them more resistant to these droughts?
We need to use the knowledge of the genes that are turned on or off during drought conditions to produce plants that are more resistant to drought. Over the years, plant scientists have had some success in producing drought-resistant crops. These drought-resistant crops were produced mainly by selecting and breeding individual plants that survived well under drought conditions. Over the past few decades, scientists working on genetically modified GM plants also started to focus on producing drought-resistant crops [ 4 ].
To produce a GM plant, a new gene from any source! Imagine being able to choose from hundreds of helpful genes in a resurrection plant and introduce some of them into wheat! Much more work needs to be done, including convincing the general public that GM plants are not dangerous! Plants are really vulnerable when it comes to water scarcity. However, plants do have some built-in protection against drought. Press ESC to cancel.
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