What is drought?
Drought is a temporary reduction in water or moisture availability significantly below the normal or expected amount for a specific period. This condition occours either due to inadequacy of rainfall, or lack or irrigation facilities, under-exploitation or deficient availability for meeting the normal crop requirements in the context of the agro-climatic conditions prevailing in any particular area. This has been scientifically computed as Moisture index (M I). There is a drought in jaisalmer (Average rainfall 200 mm) if rainfall is not sufficient to grow grass an paltry coarse-grains, whereas in Bolangir or Koraput (Orissa-rainfall above 1000 mm) there is a drought if there is not enough rainfall for bringing the paddy crop to maturity
Droughts can be of three kinds:-
(i) Meteorological drought: This happens when the actual rainfall in an area is significantly less than the climatological mean of that area. The country as a whole may have a normal monsoon, but different meteorological districts and sub-divisions can have below normal rainfall. The rainfall categories for smaller areas are defined by their deviation from a meteorological area's normal rainfall -
Excess: 20 per cent or more above normal
Normal: 19 per cent above normal - 19 per cent below normal
Deficient: 20 per cent below normal - 59 per cent below normal
Scanty: 60 per cent or more below normal
(ii) Hydrological drought: A marked depletion of surface water causing very low stream flow and drying of lakes, rivers and reservoirs
(iii) Agricultural drought: Inadequate soil moisture resulting in acute crop stress and fall in agricultural productivity
Droughts can be caused by a number of things. The most important drought cause is related to how much water vapor is in the atmosphere because water vapor in the atmosphere is what causes precipitation. When there are moist, low pressure systems, precipitation, such as rain, hail, sleet, and snow can occur. If the presence of dry, high pressure is above average, there will be less moisture available to create precipitation. This then results in a water deficit in areas they move over.
This can also occur when air masses are shifted by winds, resulting in dry, warm, continental air moving over an area instead of moist, cooler, oceanic air masses. El Nino, which affects the temperature of the ocean's water, also impacts levels of precipitation because during the years in which the temperature cycle is present, the air masses can be shifted above the ocean, often leading to places that are typically wet, dry and making places that are typically dry, wet.
Impact of Droughts:
There are always short-term and long-term consequences of a drought, regardless of its stage. The associated problems can have environmental, economic, and social impacts on the areas in which the droughts occurs, as well as the areas that have relations to the areas in which the drought occurred. The economic impacts tend to primarily impact the agricultural industry. The environmental impacts can include increased erosion, plant diseases, landscape and habitat degradation, insect infestations, and a decrease in air quality. There is also the risk of wildfires. The social impacts can include inequalities in water distribution amongst the poor and the wealthy, disputes between those who need to use the available water, a decline in health, and disparities in areas that need disaster relief.
Periods of drought can have significant environmental, agricultural, health, economic and social consequences. The effect varies according to vulnerability. For example, subsistence farmers are more likely to migrate during drought because they do not have alternative food sources. Areas with populations that depend on subsistence farming as a major food source are more vulnerable to drought-triggered famine.
Drought can also reduce water quality, because lower water flows reduce dilution of pollutants and increase contamination of remaining water sources. Common consequences of drought include:
- Diminished crop growth or yield productions and carrying capacity for livestock
- Dust bowls, themselves a sign of erosion, which further erode the landscape
- Dust storms, when drought hits an area suffering from desertification and erosion
- Famine due to lack of water for irrigation
- Habitat damage, affecting both terrestrial and aquatic wildlife
- Malnutrition, dehydration and related diseases
- Mass migration, resulting in internal displacement and international refugees
- Reduced electricity production due to insufficient available coolant for power stations,  and reduced water flow through hydroelectric dams
- Shortages of water for industrial users
- Snakes migration and increases in snakebites
- Social unrest
- War over natural resources, including water and food
- Wildfires, such as Australian bushfires, are more common during times of drought
- Cloud seeding - an artificial technique to induce rainfall.
- Desalination of sea water for irrigation or consumption.
- Drought monitoring - Continuous observation of rainfall levels and comparisons with current usage levels can help prevent man-made drought. For instance, analysis of water usage in Yemen has revealed that their water table (underground water level) is put at grave risk by over-use to fertilize their Khat crop. Careful monitoring of moisture levels can also help predict increased risk for wildfires, using such metrics as the Keetch-Byram Drought Index or Palmer Drought Index.
- Land use - Carefully planned crop rotation can help to minimize erosion and allow farmers to plant less water-dependent crops in drier years.
- Rainwater harvesting - Collection and storage of rainwater from roofs or other suitable catchments.
- Recycled water - Former wastewater (sewage) that has been treated and purified for reuse.
- Transvasement - Building canals or redirecting rivers as massive attempts at irrigation in drought-prone areas.
- Outdoor water-use restriction - Regulating the use of sprinklers, hoses or buckets on outdoor plants, filling pools, and other water-intensive home maintenance tasks.