九乡新闻网逍遥模拟器多开教程:Drought - What is Drought?
来源：百度文库 编辑：九乡新闻网 时间：2024/04/24 00:34:58
Drought Occurs When Human Demand for Water Exceeds the Available Supply
By Larry West , About.com Guide
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Drought is not purely a physical phenomenon that can be defined by the weather. Rather, at its most essential level, drought is defined by the delicate balance between water supply and demand. Whenever human demands for water exceed the natural availability of water, the result is drought.
What Causes Drought?
Drought can be caused by too little precipitation (rain and snow) over an extended period, as most people assume, but drought can also be caused by increased demand for the available supply of usable water even during periods of average or above average precipitation.
Another factor that can affect water supply is a change in water quality.
If some of the available water sources become contaminated--either temporarily or permanently--that decreases the supply of usable water, makes the balance between water supply and demand even more precarious, and increases the likelihood of drought.
What are the Three Types of Drought?
There are three conditions that are generally referred to as drought:
- Meteorological drought—This type of drought all about the weather and occurs when there is a prolonged period of below average precipitation, which creates a natural shortage of available water.
- Agricultural drought—This type of drought occurs when there isn’t enough moisture to support average crop production on farms or average grass production on range land. Although agricultural drought often occurs during dry, hot periods of low precipitation, it can also occur during periods of average precipitation when soil conditions or agricultural techniques require extra water.
- Hydrological drought—This type of drought occurs when water reserves in aquifers, lakes and reservoirs fall below an established statistical average. Again, hydrological drought can happen even during times of average or above average precipitation, if human demand for water is high and increased usage has lowered the water reserves.
Which type of drought people mean when they talk about “drought” often depends on who they are, they kind of work they do, and the perspective that gives them.
Farmers and ranchers are most often concerned with agricultural drought, for example, and agricultural drought is also the type of drought that worries people in the grocery and meat business or people in farm communities that depend indirectly on agricultural income for their livelihoods.
Urban planners usually mean hydrological drought when they talk about drought, because water supplies and reserves are key components in managing urban growth.
The most common use of the term “drought” refers to meteorological drought, because that is the drought condition most familiar to the general public and the one most easily identified.