8.6: Drought

Drought Conditions

The word drought can mean different things to different people. To someone living in a city, it can mean there is a shortage of water for bathing, doing dishes, etc. from their provider and they are being asked to conserve. To a farmer, it means there has been less rainfall than is normal which is affecting the crops under cultivation. This can occur over a very short cycle (just a few weeks) if the moisture deficiency comes during certain periods of the growing cycle. To a meteorologist, a drought is a prolonged period when precipitation is less than normal. To a hydro geologist, a drought is an extended period of decreased rainfall and diminished stream flow.

Generically, drought is a period of reduced rainfall and/or snowfall that results in water-related problems. While precipitation (rainfall or snowfall) at a particular location varies from year to year, the average amount is constant over a period of years. This relates to the difference between weather which describes conditions at a point in time versus climate which is an average of those conditions. In the United States' southwestern deserts, the average precipitation is only a few inches per year. In contrast, the average yearly precipitation in some places in the southeast is over 50 inches. But, even if the total amount of rainfall for a year is average, rainfall shortages can occur during a period when moisture is critically needed for plant growth, such as in the early summer causing drought conditions.

The beginning of a drought can be difficult to determine, and weeks, months, or years may pass before people know that a drought is occurring. Droughts can persist for a decade or more, and the end can occur as gradually as it began. The first evidence of drought usually is seen in records of rainfall though this does not necessarily result in drought conditions. Some rain returns to the air as water vapor when water evaporates from water surfaces and from moist soil. Plant roots draw some of the moisture from the soil and return it to the air through a process called transpiration. When evaporation and transpiration rates are large, soils can lose moisture and dry conditions can develop. But if the weather is cool and cloudy, these rates may be small enough to offset the lack of precipitation and a drought may be less severe or may not develop at all.

Rainfall in any form will provide some drought relief. A light shower provides cooling, and revitalizes vegetation, but during the growing season, most of the rain that falls will be quickly evaporated or used by plants. The impact is short term. A thunderstorm will provide some of the same benefits as the light shower but can also be very damaging. Since thunderstorms produce large amounts of precipitation in a short period of time, most of the rain runs off into streams so it never soaks into the ground. If we are lucky, the rain falls upstream of a lake which captures the water and increases the available water supply.

Soaking rains, often produced by tropical storms, are best to alleviate drought. Water that enters the soil recharges ground water, which in turn sustains vegetation and feeds streams during periods when it is not raining. A single soaking rain will provide lasting relief from drought conditions, but multiple events over several months are usually required to break a drought and return conditions to within the normal range.

Even when a drought has been broken it may not be truly over. The benefits of substantial rainfall such as from a tropical storm may last for months, but a return to normal rainfall patterns and amounts is necessary for conditions in streams, reservoirs, and ground water to also return to normal.

Anthropogenic Effects on Droughts

Humans affect drought conditions in two major ways. First, especially in the current day, humans consume large amounts of water. Whether it is for household use, industrial use, or for agricultural use, water is often used at rates higher than what is naturally replaced. This leads to drought conditions as the amount required by a community is not available. This is particularly problematic for communities established in arid or semi-arid areas like southern California.

Second, according to a study from NASA's Goddard Institute for Space Studies (GISS) in New York City, human-generated greenhouse gases and atmospheric particles were affecting global drought risk as far back as the early 20th century. This is part of the global climate change crisis whereby industrialization affects temperatures as well as rainfall amounts. It should be noted that droughts do naturally occur. The amount of influence humans have had on drought conditions is a complex topic as there are many factors that contribute to changes in patterns.

How Drought Impacts Earth and Humans

Drought affects Earth and humans in many ways. When little or no rain falls, soils can dry out and plants die. When rainfall is less than normal for several weeks, months, or years, the flow of streams and rivers declines, water levels in lakes and reservoirs fall, and the depth to water in wells increases. If dry weather persists and water-supply problems develop, the dry period can become a drought.

The topic of well water depth is an important one as wells are used to access groundwater which is in turn used to provide a large portion of the Nation's population with drinking water, it provides business and industries water for their purposes and is used extensively for irrigation. The water level in the aquifer that supplies a well does not always stay the same. Droughts affect the height of the underground water levels as water continues to be pumped out and there is not enough water to recharge it. This obviously happens during drought due to the extreme deficit of rain.

Drought also affects humans through the destruction of crops and farms. The best modern-day example of this occurred during the 1930's. The US experienced what is referred to as the "Dust Bowl". During that time, most of the United States was much drier than normal. In California, the drought extended from 1928 to 1937. In Missouri, the drought lasted from 1930 to 1941. That extended dry period allowed wind to produced dust storms that destroyed crops and farms.