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Dead vegetation on dry soil in Colorado during a severe drought.

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WHAT is a drought?

Sometimes, a natural hazard is a shortage of something, such as water in the case of drought. A drought is an extended period of depleted soil water. Usually, a drought occurs when more water is taken out of the soil than is added to it. This is often the result of a combination of many days without rain and excessive use of water for human activities. One of the greatest threats to humans from a drought occurs when agricultural regions receive unusually low amounts of rain, causing vegetation to become dry. When this happens, crop yields can decrease dramatically, often resulting in increased food prices, food shortages, and even famine. At the same time, decreased drinking water quality and availability puts additional strains on people's health. In developing countries, the ensuing malnutrition and famine can lead to outbreaks of disease and political conflict. As population and affluence increase, the demand for food and water increases, thereby increasing the probability of drought. Additionally, when vegetation becomes dry during a drought, wildland fire risk increases, threatening homes, crops, and the lives of humans and other species. Wind can reduce visibility during a drought by blowing loose, dry soil into the air, causing transportation delays and accidents.


WHERE do droughts occur?

In North America, several major droughts have occurred in the northern Rocky Mountain region including Montana, Idaho, Wyoming, southern Alberta, eastern Washington, eastern Oregon, and the northern portions of Utah, Colorado, and Nevada. Northern and central Mexico has also experienced many droughts. Perhaps the greatest area of concern is the Great Plains of the United States, which plays a vital role in agricultural production. Parts of Nebraska, Oklahoma, Kansas, Iowa, Missouri, and Illinois have all experienced several major droughts, each of which had noticeable impacts on food costs and food supplies throughout the United States.

Quick Fact:
Droughts caused at least $1 billion in crop damage per year in the U.S. for nine of the 25 years between 1980 and 2005.

Most agricultural crops are grown in the semi-arid and humid regions of the world. Droughts in either of these regions can pose an immediate threat to agricultural productivity and threaten human health. With global warming, droughts are expected to become a major problem for several agricultural regions. In semi-arid and arid regions, droughts frequently result in decreases in drinking water quality and drinking water availability.


WHEN do droughts occur?

In areas where most of the winter precipitation falls in the form of snow, the winter season is effectively a drought. Water at the surface, and just below the surface, is frozen for a long period of time and is therefore unavailable to plants. Many plant species drop their leaves and reduce growth rates in response to winter drought. From spring through early fall, plants require water for growth. During this time in arid, semi-arid, and temperate regions, irrigation is often required to supplement inadequate rainfall. Also during this time, increased temperatures cause human water consumption to rise markedly for such uses as drinking water and cooling. If precipitation remains below normal for an extended period of time and there is not enough water in stored in the ground for plant growth, a drought ensues. In general, droughts present the greatest threat from March through September for most of North America.

Anywhere in the world, droughts occurring during the growing season are the greatest threat to people. In sub-tropical and tropical climates, the growing season is often determined by precipitation, since temperature is usually high enough for plant growth all year. Monsoon failures, sometimes related to El Nino events, can cause extensive crop failures, resulting in drastic food shortages in regions such as South Asia.


HOW do we cope with droughts?

Coping with water shortages requires a basic understanding of how the Earth's water cycle operates. There are usually many inputs and outputs for any reservoir. When too much water is removed from a reservoir relative to how much is added, a drought may soon follow. Water conservation is the most effective method for mitigating droughts. In the United States, water conservation involves reducing the amount of water used for non-essential activities such as watering lawns and washing cars. Xeriscaping, which involves planting drought-tolerant species, is becoming increasingly popular in the dry western United States. In some regions of the world, water conservation is a way of life due to typically low rainfall and the absence of a need for certain non-essential activities (green lawns, etc.). In the long-term, successful drought mitigation requires assessing the amount of water available in a region and then adopting a plan of development that provides for ample reservoir water during dry periods.


WHO has more information on droughts?