Following main types of drought may be recognized: 1. Meteorological Drought 2. Hydrological Drought 3. Agricultural Drought 4. Soil Moisture Drought 5. Socio-Economic Drought 6. Famine 7. Ecological Drought!
1. Meteorological Drought:
It describes a situation where there is a reduction in rainfall for a specific period (days, months, seasons or year) below a specific amount (long term average for a specific time).
The Indian Meteorological Department (IMD) has defined drought as a situation occurring in any area when the mean annual rainfall is less than 75% of the normal rainfall. IMD has further classified droughts into the broad categories viz., a severe drought when the deficiency of rainfall exceeds 50% of the normal rainfall and moderate drought when the deficiency of rainfall is between 25 and 50% of the normal rainfall.
It is worth noting here that the effectiveness of rainfall is more important than the amount of rainfall so far as meteorological drought is. On an average India receives 118 cm annual rainfall which is considered to be highest anywhere in the world for the country of comparable size. But the uncertain, unreliable and erratic nature of rainfall by south-west monsoons creates drought conditions in different parts of the country.
The major causes of meteorological drought may be summed up as under:
(i) Lean monsoon and below average rainfall due to absence of depressions over India.
(ii) Late onset or early withdrawal of monsoons.
(iii) Prolonged breaks in monsoon. s
(iv) Re-establishment of southern branch of jet stream.
2. Hydrological Drought:
Hydrological drought is associated with reduction of water. A meteorological drought often leads to hydrological drought. Generally it takes two successive meteorological droughts before the hydrological drought sets in. There are two types of hydrological droughts viz., (0 surface water drought and (ii) ground water drought.
(i) Surface-water Drought:
It is concerned with drying up of surface water resources such as rivers, streams, lakes, ponds, tanks, reservoirs etc. There are many processes, besides meteorological drought, which lead to surface water drought. Large scale deforestation is the main cause of surface water drought.
Some other unwanted human activities have led to the enhancement of flood/drought duo. Important among them are ecologically hazardous mining, indiscriminate road construction, averaging and spread of non-terraced agriculture. In the Doon valley limestone quarrying has drastically changed the surface water flow in the valley turning several perennial rivers into carriers of monsoon floods which go dry after the monsoon.
(ii) Ground-water Drought:
Ground-water drought is associated with the fall in the ground water level. This happens due to excessive pumping of ground water without compensatory replenishment and creates more or less irreversible ground water drought even in normal rainfall conditions.
Ground water replenishment depends upon the availability of surface water obtained from rainfall and nature of soil. The Northern Plain of India has soft and porous alluvial soils. These soils permit percolation of water and help in ground water replenishment. In contrast, the peninsular plateau area is made up of hard and impervious rocks which hinder the process of ground water replenishment.
3. Agricultural Drought:
Agricultural drought is concerned with the impact of meteorological/hydrological drought on crop yield. When soil moisture and rainfall conditions are not adequate enough to support a healthy crop growth to maturity thereby causing extreme moisture stress and wilting of major crop area, it leads to agricultural drought.
Agricultural drought may occur even when there is no meteorological drought and vice-versa. It is worth noting that agricultural drought is a relative category, depending upon the value of plant and soil. What could be a drought condition for the cultivation of rice could well be a suitable condition for wheat and a condition of excess soil moisture for dry crops like bajra or jowar.
Thus the choice of crops evolves according to variations of climatic and soil conditions. The indigenous dry-crops prove very high-yielding when there is optimum use of water. However, under extreme condition of soil water drought, no plant can survive and this condition is termed as desertification.
With the onset of green revolution in India in 1960s, High Yielding Varieties (HYV) of seeds has replaced the traditional drought resisting seeds. The use of fertilisers has also increased the water requirements of different crops.
The agriculture based on the Green Revolution technology is precariously dependent on irrigation and any delay in supply of water will cause serious agricultural drought. It has been found that the organic matter content of HYV seeds is quite less.
Organic matter input to soil increases its water holding capacity and soils rich in organic content don’t dry quickly. Hence the use of HYV seeds has increased the risk of agricultural drought to a great extent. Recent studies have revealed that with the addition of manure and organic fertiliser, the water retentivity in the soil increases by 2 to 5 times.
Change in cropping pattern, particularly with the introduction of Green Revolution technology has also led to soil water drought. The indigenous cropping pattern was in tune with the agroclimatic conditions of the concerned region.
With the introduction of an alien cropping pattern, more water demanding crops have gained importance which has led to soil water drought. For example, rice cultivation was practically unknown in Punjab, Haryana and Western Uttar Pradesh before Green Revolution and this region is not suitable for rice cultivation considering the amount of rainfall received by this region.
The area receives an average annual rainfall from 50 to 75 cm whereas minimum amount of rainfall for any successful cultivation of rice is 100 cm. This change in the cropping pattern has put heavy strain on meagre water resources leading to acute occasional droughts.
In the drought affected areas of Maharashtra, Karnataka and Andhra Pradesh there is a change in the cropping pattern in which cash crops are given more importance than the traditional crops. Thus while the staple crops are denied water, cash crops like sugarcane and grapes are given priority with respect to irrigation. This leads to a condition of soil water drought which is created not by absolute scarcity of water but by preferential diversion of limited sources of water.
4. Soil Moisture Drought:
This is a situation of inadequate soil moisture particularly in rainfed areas which may not support crop growth. This happens in the event of a meteorological drought when the water supply to soil is less and water loss by evaporation is more.
5. Socio-Economic Drought:
It reflects the reduction of availability of food and income loss on account of crop failures endangering food and social security of the people in the affected areas.
A famine occurs when large scale collapse of access to food occurs which, without intervention, can lead to mass starvation.
7. Ecological Drought:
Ecological drought takes place when the productivity of a natural eco-system fails significantly as a consequence of distress induced environmental damage.