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The Arid West—Where Water Is Scarce - Specter Of Inevitable Drought

climate weather precipitation united

Drought is a recurring and inevitable phenomenon. In arid and semiarid regions where water is particularly scarce, the effects of drought may be more immediately felt, but it happens everywhere in the world at some time, and all climates are susceptible. For example, an analysis of climate data for river basins in the United States from 1896 to 1995, based on National Oceanic and Atmospheric Administration data, shows that some part of the nation experienced an extreme or severe drought in every year in that period, and that in seventy-two of those years these conditions affected more than 10% of the United States. During that same period, the Pacific Northwest river basins experienced extreme or severe drought eighty-six times, while California river basins had these conditions fifty-three times. The Tennessee River Basin had the lowest number (thirty-one) of these drought events.

There is no such thing as "normal weather." The idea of normal or average weather is a mathematical construction used by the media to describe weather in terms of deviation from a mathematical norm. Close examination of climate records demonstrates that variation is normal in weather. Weather changes day to day, week to week, and one year to the next. Some weather patterns may last for years, with some decades being cool and wet while others are hot and dry. Drought is a naturally occurring part of the climate cycle.

The misconception that weather is normal and drought is an unusual circumstance is a very serious problem. Weather, in one form or another, is the source of all water for drinking, irrigation, power supply, industry, wildlife habitat, and other uses. When planners, managers, and citizens fail to recognize drought and its converse, flooding, as inevitable parts of the normal weather range, their plans fall short in anticipating water and societal needs.

A 2000 report from the U.S. Global Change Research Program stressed the importance of factoring in potential effects of climate variability when developing water conservation and supply strategies. The report, Climate Change Impacts on the United States, described various scenarios that would result from changes in climate in the United States. In addition, scientists and other researchers offered suggestions for water planners and managers to aid in efficient and effective water-supply strategies. Using historical climatic data, climate models, and sensitivity analyses (which ask how, and how much, the climate would have to change to cause major impacts on particular regions or sectors of the country) scientists suggested that in the coming years there will be more precipitation in the United States, with more of it coming in heavy downpours. In spite of this, however, scientists believe that some areas are likely to become drier as increased evaporation due to higher temperatures out-paces increased precipitation. They also suggested that droughts and flash floods are likely to become more frequent and more intense.

What Is Drought?

Drought can be defined simply as a deficiency of precipitation over an extended period, usually a season or more. This deficiency results in a water shortage for some group, activity, or part of the environment. Drought should be judged relative to some long-term average condition of balance between precipitation and evapotranspiration in a particular area. Drought is also related to the timing and the effectiveness of the precipitation. Timing refers to factors such as the period when drought is most likely to occur, delays in the start of the rainy season, and the occurrence of rain in relation to principal crop growth. Precipitation effectiveness refers to the duration, intensity, and frequency of rains or other precipitation events. In many regions of the United States and the world, high temperature, high winds, and low relative humidity are also associated with drought, increasing its severity. In normally arid and semiarid regions, the precipitation deficiency is over and above the more typical dry season.

The interaction between drought, a natural event, and the demand that people place on water supply affects society. Human activity often worsens the drought's impact. Changes in land use, land degradation, and the construction of dams all affect the hydrological characteristics of a water basin (the land area drained by a particular river and its tributaries). For example, changes in land use upstream may alter hydrologic characteristics such as water infiltration and runoff rates, causing more variable stream flow and a higher frequency of water shortage downstream.

Predicting Drought

Anyone can predict with absolute certainty that drought will occur because inevitably it will. It is the how, when, where, and for what duration that are difficult to predict. Drought is never the result of a single cause, but comes from the interaction, and sometimes compounding, of the effects of many causes. On the largest scale, global weather systems (teleconnections) play an important part in explaining global and regional weather patterns. These patterns occur with enough frequency and similar characteristics over a sufficient length of time to provide opportunities to strengthen our ability to predict long-range climate, particularly in the tropics. An example of these global systems is the El Niño/Southern Oscillation.

On a lesser scale, high-pressure systems inhibit cloud formation and result in lower relative humidity and less precipitation. Regions that are under the influence of high-pressure systems most of the year are generally deserts such as the Sahara and Kalahari Deserts in Africa. Most climatic regions experience high-pressure systems at some time, often depending on the season. Prolonged drought occurs when the large-scale deviations in atmospheric circulation patterns persist for months, seasons, or years. The extreme drought in 1988 that affected the United States and Canada was caused by the persistence of large-scale deviations in atmospheric circulation patterns, and is estimated to have cost the United States $40 billion.

There are too many variables for climatologists to accurately predict drought. Prediction depends on our ability to forecast two fundamental factors: precipitation and temperature. Scientists know that climate is inherently variable and that deviations in precipitation and temperature may last from several months to several decades. Other factors affecting their duration are air-sea interactions, soil moisture, topography, and the accumulated influence of dynamically unstable weather systems at the global scale. Until we can describe, interpret, and predict the interplay among all these factors, we cannot predict weather, including drought, with any real accuracy.

The USGCRP is actively working to promote understanding of climate change and the implications it has for the United States. According to the USGCRP's report Climate Change Impacts on the United States, advances in climate science are paving the way for scientists to project climate changes at the regional scale, allowing them to identify regional vulnerabilities and to assess potential regional effects. For example, the report suggested that the Earth's climate has changed in the past and that even greater climate change is very likely to occur during the twenty-first century. It also suggested that reduced summer runoff, increased winter runoff, and increased water demands are likely to compound current stresses on water supplies and flood management, particularly in the western United States. Understanding of the implications of these changes may help Americans adapt to an uncertain and continuously changing climate.

An emerging area of science that may help with drought prediction is the extraction of information from tree rings, the Arctic and Antarctic ice caps, prehistoric sites, and other naturally occurring "records." This information is being used to calculate the frequency of droughts in a region's history and to identify the normal precipitation in a given period. Using this information, climatologists may not be able to predict a drought, but they can calculate the probability of a drought occurring in a specific region under a particular group of climatic conditions.

The Arid West—Where Water Is Scarce - Drought Management: Too Little,too Late [next] [back] The Arid West—Where Water Is Scarce - Unreasonable Expectations

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