Drought and drought management strategies
4.1 Introduction
Low rainfall or failure of monsoon rain is a recurring feature in India. This has
been responsible for droughts and famines. The word drought generally denotes
scarcity of water in a region. Though, aridity and drought are due to insufficient
water, aridity is a permanent climatic feature and is the culmination of a number of
long term processes. However, drought is a temporary condition that occurs for a
short period due to deficient precipitation for vegetation, river flow, water supply
and human consumption. Drought is due to anomaly in atmospheric circulation.
Aridity Vs. Drought
Particulars Aridity Drought
Duration Permanent feature Temporary condition of
scarcity of varying duration
Factors Culmination of many long term
processes , considers all climatic
features
Caused by deficient rainfall
Aspect described Description of Climate Description of Water
availability
4.2 Definition of drought
There is no universally accepted definition for drought.
a) Early workers defined drought as prolonged period without rainfall.
b) According to Ramdas (1960) drought is a situation when the actual seasonal
rainfall is deficient by more than twice the mean deviation.
c) 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.
d) Prolonged deficiencies of soil moisture adversely affect crop growth indicating
incidence of agricultural drought. It is the result of imbalance between soil
moisture and evapo-transpiration needs of an area over a fairly long period so as
to cause damage to standing crops and to reduce the yields.
e) The irrigation commission of India defines drought as a situation occurring in any
area where the annual rainfall is less than 75% of normal rainfall.
4.3 Classification of drought
Drought can be classified based on duration, nature of users, time of
occurrence and using some specific terms.
4.3.1 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 evapo-transpiration losses, the result is
borderline water deficiency in soil resulting in less than optimum yield. This occurs
usually in humid regions.
4.3.2 Based on relevance to the users (National Commission on Agriculture,
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 absorption keeps pace with transpiration the 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 (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 as a consequence of scanty
precipitation or its uneven distribution both in space and time.
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.
4.3.3 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: Occurs due to long gaps between two successive rains and
stored moisture becoming insufficient during the long dry spell.
c) Late season drought: Occurs due to early cessation of rainfall and crop water
stress at maturity stage.
4.3.4. Other terms to describe drought
a) 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. For Eg. A condition may be a drought situation for growing rice, but the
same situation may not be a drought for growing groundnut.
b) 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.
4.4 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
4.5 Periodicity of drought
The Indian Meteorological Department examined the incidence of drought for
the period from 1871 to 1967, utilizing the monthly rainfall of 306 stations in the
country. It was seen that during 1877, 1899, 1918 and 1972 more than 40 per cent of
the total area experienced drought. General observation on the periodicity of drought
in respect of different meteorological sub divisions of India is given below.
Meteorological sub divisions Period of recurrence of
drought
Assam Very rare, once in 15 years
West Bengal, MP, Konkan, Coastal AP, Kerala, Bihar,
Orissa
Once in 5 years
South interior Karnataka, Eastern UP, Gujarat,
Vidharbha, Rajasthan, Western UP, TN, Kashmir,
Rayalaseema and Telangana
Once in 3 years
Western Rajasthan Once in 2.5 years
4.6 Effect of drought on crop production
a) Water relations: Alters the water status by its influence on absorption,
translocation and transpiration. The lag in absorption behind transpiration results in
loss of turgor as a result of increase in the atmospheric dryness.
b)Photosynthesis: Photosynthesis is reduced by moisture stress due to reduction in
Photosynthetic rate, chlorophyll content, leaf area and increase in assimilates
saturation in leaves (due to lack of translocation).
c) Respiration: Increase with mild drought but more serve drought lowers water
content and respiration.
d)Anatomical changes: Decrease in size of the cells and inter cellular spaces,
thicker cell wall , greater development of mechanical tissue. Stomata per unit leaf
tend to increase.
e) Metabolic reaction: All most all metabolic reactions are affected by water deficits.
f) Hormonal Relationships: The activity of growth promoting hormones like
cytokinin, gibberlic acid and indole acetic acid decreases and growth regulating
hormone like abscisic acid, ethylene, etc., increases.
g) Nutrition: The fixation, uptake and assimilation of nitrogen is affected. Since dry
matter production is considerably reduced the uptake of NPK is reduced.
h)Growth and Development: Decrease in growth of leaves, stems and fruits.
Maturity is delayed if drought occurs before flowering while it advances if drought
occurs after flowering.
i) Reproduction and grain growth: Drought at flowering and grain development
determines the number of fruits and individual grain weight, respectively. Panicle
initiation in cereals is critical while drought at anthesis may lead to drying of pollen.
Drought at grain development reduces yield while vegetative and grain filling stages
are less sensitive to moisture stress.
j) Yield: The effect on yield depends hugely on what proportion of the total dry
matter is considered as useful material to be harvested. If it is aerial and
underground parts, effect of drought is as sensitive as total growth. When the yield
consists of seeds as in cereals, moisture stress at flowering is detrimental. When the
yield is fibre or chemicals where economic product is a small fraction of total dry
matter moderate stress on growth does not have adverse effect on yields.
4.7 Crop Adaptations
The ability of crop to grow satisfactorily under water stress is called drought
adaptation. Adaptation is structural or functional modification in plants to survive
and reproduce in a particular environment.
Crops survive and grow under moisture stress conditions mainly by two
ways: (i) escaping drought and (ii) drought resistance (Fig. 4.1)
Fig. 4.1 Flow chart showing different mechanisms for overcoming moisutre stess
4.7.1 Escaping Drought
Evading the period of drought is the simplest means of adaptation of plants to
dry conditions. Many desert plants, the so called ephemerals, germinate at the
beginning of the rainy season and have an extremely short life period (5 to 6 weeks)
which is confined to the rainy period. These plants have no mechanism for
overcoming moisture stress and are, therefore, not drought resistant. Germination
inhibitors serve as safety mechanism.
In cultivated crops, the ability of a cultivar to mature before the soil dries is
the main adaptation to growth in dry regions. However, only very few crops have
such a short growing season to be called as ephemerals. Certain varieties of pearl
millet mature within 60 days after sowing. Short duration pulses like cowpea,
greengram, blackgram can be included in this category. In addition to earliness, they
need drought resistance because there may be dry spells within the crop period of 60
Adaptations to moisture stress
Escaping drought Drought Resistance
Drought avoidance Drought tolerance
Conserving water Mitigating Stress High tolerance
(Water savers)
Improving water uptake
(Water Spenders)
days. The disadvantage about breeding early varieties is that yield is reduced with
reduction in duration.
4.7.2 Drought Resistance
Plants can adopt to drought either by avoiding stress or by tolerating stress
due to different mechanisms. These mechanisms provide drought resistance.
4.7.3 Avoiding Stress
Stress avoidance is the ability to maintain a favourable water balance, and
turgidity even when exposed to drought conditions, thereby avoiding stress and its
consequences. A favourable water balance under drought conditions can be achieved
either by: (i) conserving water by restricting transpiration before or as soon as stress
is experienced; or (ii) accelerating water uptake sufficiently so as to replenish the
lost water.
4.8 Strategies for drought management
The different strategies for drought management are discussed under the
following heads.
4.8.1 Adjusting the plant population: The plant population should be lesser in
dryland conditions than under irrigated conditions. The rectangular type of planting
pattern should always be followed under dryland conditions. Under dryland
conditions whenever moisture stress occurs due to prolonged dry spells, under
limited moisture supply the adjustment of plant population can be done by
a) Increasing the inter row distance: By adjusting more number of plants within
the row and increasing the distance between the rows reduces the competition
during any part of the growing period of the crop. Hence it is more suitable for
limited moisture supply conditions.
b) Increasing the intra row distance: Here the distance between plants is
increased by which plants grow luxuriantly from the beginning. There will be
competition for moisture during the reproductive period of the crop. Hence it is less
advantageous as compared to above under limited moisture supply.
4.8.2 Mid season corrections: The contingent management practices done in the
standing crop to overcome the unfavourable soil moisture conditions due to
prolonged dry spells are known as mid season conditions.
a) Thinning: This ca be done by removing every alternate row or every third row
which will save the crop from failure by reducing the competition
b) Spraying: In crops like groundnut, castor, redgram, etc., during prolonged dry
spells the crop can saved by spraying water at weekly intervals or 2 per cent urea at
week to 10 days interval.
c) Ratooning: In crops like sorghum and bajra, ratooning can practiced as mid
season correction measure after break of dry spell.
4.8.3 Mulching: It is a practice of spreading any covering material on soil surface to
reduce evaporation losses. The mulches will prolong the moisture availability in the
soil and save the crop during drought conditions.
4.8.4 Weed control: Weeds compete with crop for different growth resources ore
seriously under dryland conditions. The water requirement of most of the weeds is
more than the crop plants. Hence they compete more for soil moisture. Therefore the
weed control especially during early stages of crop growth reduce the impact of dry
spell by soil moisture conservation.
4.8.5 Water harvesting and life saving irrigation: The collection of run off water
during peak periods of rainfall and storing in different structures is known as water
harvesting. The stored water can be used for giving the life saving irrigation during
prolonged dry spells.