drought and its types and effect on crop production
- 1. K. K. University, Bihar Sharif, Nalanda, Bihar, 803115 Rainfed Agriculture and Watershed management BSAG-2409 Lecture Prepared by- Mr. Naveen Kumar Assistant Professor, School of Agriculture Sciences
- 2. Drought and its types Early workers defined drought as prolonged period without rainfall. According to Ramdas (1960) drought is a situation when the actual seasonal rainfall is deficient by more than twice the mean deviation. American Meteorological Society defined drought as a period of abnormally dry weather sufficiently prolonged for lack of water to cause a severe hydrological imbalance in the area affected.
- 3. Classification of drought: • Drought can be classified based on duration, nature of users, time of occurrence and using some specific terms. 1.Based on duration • 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. • 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. • 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. • 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 optimum yield. This occurs usually in humid regions.
- 4. 2.Based on relevance to the users (National Commission on Agriculture, 1976) 1. 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). 2. 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 absorption keeps pace with transpiration the plants revive. (Mid-day wilt).
- 5. 3.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 for bringing crops to maturity. 4.Agricultural drought (soil 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 because of scanty precipitation or its uneven distribution both in space and time.
- 6. Agricultural drought is again classified into five major types ( in dryland areas) : a) Early season drought: •Generally occurs either due to delayed onset of monsoon or prolonged dry spell soon after the on-set of monsoon. b) Mid season drought: •This occurs in association with long gaps between two successive rain events, if moisture stored in the soil falls short of water requirement of crop during the dry period. •On other occasions the mid season drought may be associated with low and inadequate rainfall in the growing season to meet the crop water needs as per the phonological stage. c) Late season or terminal drought: If the crop encounters moisture stress during the reproductive stage due to early cessation (stop) of rainy season and rise in temperature the situation hastens (hurry up)the process of maturity.
- 7. d) Chronic drought: Content are common in extream dry areas where rainfall and subsequent soil moisture are inadequate to meet the water requirements of common field crops. Apart from these dry areas several other regions of low to medium rainfall , having undulating topography and shallow soils of very low moisture holding capacity, also shows the same chronic drought features. These areas are called drought prone areas. e) Apparent (clearly visible) drought: Conditions are seen in some areas with low to medium rainfall, in case of the crops which are selected for economic or other considerations, but do not match with the existing rainfall pattern i.e. intensity and distribution of place.
- 8. 3.Based on time of occurrence Early season drought: It occurs due to delay in onset of monsoon or due to long dry spells after early sowing Mid-season drought: Occurs due to long gaps between two successive rains and stored moisture becoming insufficient during the long dry spell. Late season drought: Occurs due to early cessation of rainfall and crop water stress at maturity stage.
- 9. 4.Other terms to describe drought Relative drought: The drought for one crop may not be a drought situation for another crop. This is due to mismatch between soil moisture condition and crop selection. Eg. A condition may be a drought situation for growing rice, but the same situation may not be a drought for growing groundnut. Physiological drought: 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.
- 10. Anna wary system: Crop condition assessed through visual estimates 1. Production above 75 % of normal: No drought 2. Production 50 to 75 % of normal: Moderate drought 3. Production 25 to 50 % of normal: Severe drought 4. Production less than 25 % of normal: Disastrous drought
- 11. Important causes for agricultural drought are Inadequate precipitation Erratic distribution Long dry spells in the monsoon Late onset of monsoon Early withdrawal of monsoon Lack of proper soil and crop management