Drought can be classified based on duration, nature of users, time of occurrence and using some specific terms. Demarcation between the classifications is not well defined and many times, overlapping of the cause and effect of one on the rest is seen.
1. Based on duration
(a) Permanent drought – This is characteristic of the desert climate where sparse vegetation growing is adapted to drought and agriculture is possible only by irrigation during entire crop season.
(b) Seasonal drought – This is found in climates with well-defined rainy and dry seasons. Most of the arid and semiarid zones fall in this category. Duration of the crop varieties and planting dates should be such that the growing season should fall within rainy season.
(c) Contingent drought – This involves an abnormal failure of rainfall. It may occur almost anywhere especially in most parts of humid or sub humid climates. It is usually brief, irregular and generally affects only a small area.
(d) Invisible drought – This can occur even when there is frequent rain in an area. When rainfall is inadequate to meet the evapotranspiration losses, the result is borderline water deficiency in soil resulting in less than the optimum yield. This occurs usually in humid regions.
2. Based on nature of the users (NCA, 1976)
(a) Meteorological drought – It is defined as a condition, where the annual precipitation is less than the normal over an area for prolonged period (month, season or year).
(b) Atmospheric drought – It is due to low air humidity, frequently accompanied by hot dry winds. It may occur even under conditions of adequate available soil moisture. It refers to a condition when plants show wilting symptoms during the hot part of the day, when transpiration exceeds absorption temporarily for a short period. When decreases, absorption keeps pace with transpiration and plants revive (mid day wilt).
(c) Hydrological drought – Meteorological drought, when prolonged results in hydrological drought with depletion of surface water and consequent drying of reservoirs, tanks etc. It results in deficiency of water for all sectors using water. This is based on water balance and how it affects irrigation as a whole for bringing crops to maturity.
(d) Agricultural drought – It is the result of soil moisture stress due to imbalance between available soil moisture and evapotranspiration of a crop. It is usually gradual and progressive. Plants can therefore, adjust at least partly, to the increased soil moisture stress. This situation arises as a consequence of scanty precipitation or its uneven distribution both in space and time. It is also usually referred as soil drought.
Relevant definition of agricultural drought appears to be a period of dryness during the crop season, sufficiently prolonged to adversely affect the yield. The extent of yield loss depends on the crop growth stage and the degree of stress. It does not begin when the rain ceases, but actually commences only when the plant roots are not able to obtain the soil moisture rapidly enough to replace evapotranspiration losses. Important causes for agricultural drought are:
• Inadequate precipitation,
• Erratic distribution,
• Long dry spells in the monsoon,
• Late onset of monsoon,
• Early withdrawal of monsoon, and
• Lack of proper soil and crop management
3. Based on time of occurrence
(a) Early season drought – It occurs due to delay in onset of monsoon or due to long dry spells after early sowing.
(b) Mid season drought – It occurs due to long gaps between two successive rains and stored moisture becoming insufficient during this long dry spell.
(c) Late season drought – It occurs due to early cessation of rainfall and crop water stress at maturity stage.
4. Other terms to describe drought
(a) Apparent drought – What is drought for one crop may not be drought for another crop; what is drought in red soil may not be drought in black soil.
(b) Physiological drought – It refers to a condition where crops are unable to absorb water from soil even when water is available, due to the high osmotic pressure of soil solution due to increased soil concentration, as in saline and alkaline soils. It is not due to deficit of water supply.
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