Drought usually refers to a period when a particular area or a region receives lower-than-average precipitation leading to prolonged periods of low water supply. Due to its diverse characteristics and effects across different areas of the world, it is difficult to give a precise and universally accepted definition of drought. Droughts often result in significant impacts, such as a substantial hydrologic imbalance, depletion of soil moisture, groundwater and streamflow, and crop damage. In recorded history, the world’s most prolonged drought has been continuing in Chile’s Atacama Desert for more than 400 years. At present times, due to climate change, drought is becoming a more powerful and less predictable recurring climatic feature in most parts of the world.
Types Of Droughts
Drought is believed to occur when the evaporation and transpiration rates exceed the precipitation rates for a substantial period. Depending upon where the moisture deficit occurs in the water cycle, there are three main categories of drought: Meteorological Drought, Hydrological Drought, and Agricultural Drought.
Meteorological Drought
This type of drought occurs whenever there is a precipitation deficiency from expected or normal levels over prolonged periods. Triggered by different meteorological factors such as a high level of reflected sunlight, winds carrying continental air masses, and above-average prevalence of high-pressure systems, this drought type precedes all other types of droughts. The severity of meteorological droughts depends on the immensity of the precipitation’s shortfall and the time of this shortfall event. Meteorological drought is further classified into two types: moderate drought – where the rainfall deficit is 26 to 50%, and severe drought – where the rainfall deficit is more than 50%.
Hydrological Drought
This type of drought occurs when water reserves in different sources, like lakes, aquifers, rivers, and reservoirs, become scarce, leading to a water deficit for common and specific purposes. Hydrological drought is usually preceded by several months of meteorological drought, where the lower amounts of rainfall and high temperatures cause the water bodies to dry up. This drought occurs quite slowly as it involves the stored water, which is used but not appropriately replenished, thereby diminishing the water reserves. However, hydrological droughts are not always caused by changes in weather conditions. For example, when a vital water source is diverted by a country toward its own territory, it leaves the neighboring country dry and leads to a hydrological drought in the latter region.
Agricultural Drought
Generally triggered by meteorological and hydrological droughts, agricultural or ecological drought takes place when inadequate rainfall and soil moisture during the crop growing season adversely affects the crop growth in an area. This type of drought is observed in non-irrigated agricultural regions, where the plant water demand depends on various conditions, including the prevailing weather conditions, the soil's physical and biological properties, and the specific plant's biological characteristics. Moreover, poorly planned agricultural practices can often lead to changes in the soil conditions, which in turn decreases the quantity of water required for the proper growth of crops.
Besides the above-mentioned three main categories of drought, some organizations mention another type of drought: the socioeconomic drought.
Socioeconomic Drought
This type of drought takes place when the demand for an economic good is much more than its supply due to a water supply deficit created by adverse weather-related changes. The supply of various economic goods like water, food grains, forage, fish, and hydroelectric power depends on suitable weather conditions for their sufficient production. However, sometimes due to the natural variability of climate, water supply is greater in some years but in lesser amounts in other years. For example, from 1988 to 1989, Uruguay suffered from a significant reduction in hydroelectric power production. This happened because a meteorological drought led to a water deficit in the streams which in turn provided water for hydroelectric power generation. Nevertheless, the government had to use more expensive fuel sources and enact strict energy conservation measures to meet the power needs of the country.
Causes Of Drought
Some of the significant factors that can lead to drought have been discussed below:
Extremely Low Levels Of Precipitation
One of the significant causes of drought is the extremely low levels of precipitation. Lower amounts of rainfall over a substantial period lead to the decline of streamflow, drying of the soil, and a drop in water levels in lakes and reservoirs. Abnormal weather conditions, such as extremely high temperatures and changes in wind patterns, can also result in less precipitation in an area.
Dry Season
The dry season is a period of low rainfall yearly in tropical areas. During this time, low humidity leads to the drying up of water holes and rivers. As a result, many grazing animals, such as elephants, wildebeest, zebras, rhinoceros, antelopes, etc., are forced to migrate in search of more fertile areas. Moreover, bushfires are also a common occurrence due to the water deficit in the plants.
El Nino-Southern Oscillation
This climate phenomenon plays a crucial role in droughts in different portions of the world. In the central Pacific Ocean, the El Nino-Southern Oscillation has two patterns: La Nina and El Nino. La Nina is linked with dry and hotter conditions and aggravating drought conditions in the Southwestern United States, California, and sometimes the American Southeast. In contrast, El Nino leads to drier and hot weather conditions in different parts of Central America, the Amazon River Basin, and Colombia. The direct effects of El Nino are also felt in parts of Africa, Northern Australia, and Southeast Asia.
Climate Change
Anthropogenic Climate Change induced by global warming is one of the major factors leading to droughts and significant agricultural impacts, especially in developing countries. Researchers predict that due to global warming, some places in the world will receive less rainfall in the future and, therefore, will be more prone to droughts. Moreover, higher temperatures and increased evaporation rates will lead to the drying up of soil and increased plant stress which will, in turn, seriously impact agriculture.
Human Activities
Several human activities, such as over-farming and over-irrigation, negatively impact the ability of the land to capture and store water and thus can lead to droughts. In addition, the loss of tree cover due to deforestation can also trigger droughts, as the absence of trees makes the soil more vulnerable to erosion by wind and water. Studies have estimated that in the Great Plains, the loss of soil due to wind erosion is about 6100 times more in drought years than in wet years.
Consequences Of Drought
The effects of drought can be divided into three broad categories: Environmental, Economic, and Social Effects.
Environmental Effects Of Drought
Some of the environmental effects of drought include the drying up of wetlands; increased surface water pollution and reduced water quality; land degradation and habitat damage; alteration of the functional diversity of plant communities; loss of biodiversity; frequent occurrence of severe wildfires; Dust bowls, and Dust Storms becoming more common; increased desertification; and forced migration of various animals.
Economic Effects Of Drought
Some of the economic effects of drought include lower agricultural output, reduced electrical power generation due to less flow of water through hydroelectric dams, shortage of water for industrial activities, higher costs of food production, and reduction in aqua tourism and transport revenues due to drying up of water bodies.
Social Effects Of Drought
Some social and health effects of drought include mass migration of people, stress caused by failed harvests, water scarcity leading to crop failure and famines, war over vital natural resources, violent activities becoming more common, increased accumulation of cyanotoxin in water supply and food chains, poor water quality causing dehydration, malnutrition, anemia, and other related diseases. Moreover, drought cripples the livelihoods of numerous people and keeps them below the poverty line forever.
Mitigation Of Drought
Adequate measures need to be taken in order to effectively mitigate the impacts of drought. Some of these drought mitigation strategies include rainwater harvesting, water recycling, desalination, outdoor water-use restriction, the building of aqueducts or redirecting rivers for irrigation in drought-prone areas, carefully planned crop rotation, use of cover crops, and no-till farming. It is estimated that approximately 55 million people worldwide are affected by droughts annually, and about 700 million people are at risk of being displaced due to droughts by 2030.
Considering the spread and severity of drought, its mitigation measures need careful attention from policymakers and higher authorities across the globe for their effective implementation on the ground. Moreover, anthropogenic climate change will further trigger drought conditions causing it to become more frequent, intense, and long-lasting worldwide. It is, therefore, of utmost necessity that government authorities must adopt new practices such as promoting water conservation, creating drought emergency plans, identifying alternate water supplies, and encouraging farmers to cultivate drought-resistant crops. The mitigation projects must also be complemented with correct monsoon forecasts and the development of other drought-proofing mechanisms.