History, Drought Management
Drought is characterized by low precipitation compared to the normal amount for the region, low humidity, high temperatures, and/or high wind velocities. When these conditions occur over an extended period of time, drought causes low water supplies that are inadequate to support the demands of plants, animals, and people.
Drought is a temporary condition that occurs in moist climates. This is in contrast to the conditions of normally arid regions, such as deserts, that normally experience low average rainfall or available water. Under both drought and arid conditions, individual plants and animals may die, but the populations to which they belong survive. Both drought and aridity differ from desiccation, which is a prolonged period of intensifying drought in which entire populations become extinct. In Africa and Australia, periods of desiccation have lasted two to three decades. The loss of crops and cattle in these areas caused widespread suffering. Extensive desiccation may lead to desertification in which most plant and animal life in a region is lost permanently, and an arid desert is created.
Unlike a storm or a flood, there is no specific time or event that constitutes the beginning or end of a drought. Hydrologists evaluate the frequency and severity of droughts based on measurements of river basins and other water bodies. Climatologists and meteorologists follow the effects of ocean winds and volcanoes on weather patterns that cause droughts. Agriculturalists measure a drought's effects on plant growth. They may notice the onset of a drought long before hydrologists are able to record drops in underground water table levels. By observing weather cycles, meteorologists may be able to predict the occurrence of future droughts.
In addition to its duration, the intensity of a drought is measured largely by the ability of the living things in the affected vicinity to tolerate the dry conditions. Although a drought may end abruptly with the return of adequate rainfall, the effects of a drought on the landscape and its inhabitants may last for years.
Many factors affect the severity of a drought's impact on living organisms. Plants and animals are vulnerable to drought when stored water cannot replace the amount of moisture lost to evaporation. Plants and animals have several mechanisms that enable them to tolerate drought conditions. Many desert annuals escape drought altogether by having a short life span. Their life cycle lasts only a few weeks during the desert's brief, moist periods. The rest of the time they survive as dessication-resistant seeds. Some plants, such as cacti, evade drought by storing water in their tissues, while others, like mesquite trees, become dormant. Still others, such as the creosote bush, have evolved adaptations such as reduced leaf size and a waxy coating over the leaves that protect against water loss. Many animals that live in areas prone to drought like snakes and lizards forage and hunt at night, avoiding the desiccating effects of the sun's rays. Other animals have adaptations that allow them to survive without drinking, obtaining all of the water that they need from their food sources.