Drought and types of drought

Introduction

 In Indian, South–West (S–W) monsoon is responsible for occurrence of rainfall during

June–September, while North–East (N–E) monsoon is responsible for occurrence of
rainfall during October–December
 Nearly 70% of the annual rainfall in India is received during the S–W monsoon.
 In India, nearly 60% of the net cultivated land is rainfed and the rest is irrigated.

Drought
I. Definition
There is no universally accepted definition for drought. Scientist has defined drought in various
way.

1) Drought is a situation when the actual seasonal rainfall is deficient by more than twice
the mean deviation.
2) 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. Prolonged deficiencies of soil moisture adversely affect crop growth
indicating incidence of agricultural drought. It is the result of imbalance between soil
moisture and evapotranspiration needs of an area over a fairly long period as to cause
damage to standing crops and to reduce the yields.
3) 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.

Causes of drought
 Inadequate precipitation
 Erratic distribution
 Long dry spells in the monsoon
 Early withdrawal of monsoon

Classification of drought
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. Agricultural drought appears to be a period of dryness
during the crop season, sufficiently prolonged to adversely affect the yield.

 Extent of yield loss depends on crop growth stage and degree of stress
 Does not begin when the rain ceases, but actually commences only when plant roots are
not able to obtain soil moisture rapidly enough to replace evapotranspiration losses.
e) Ecological drought occurs when primary productivity of natural or managed ecosystem
declines drastically due to reduced precipitation in a long run.
f) Socio–economic droughts are the aggregate of all the above droughts when precipitation is
not adequate to meet the needs of human activities
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

ARIDITY AND DROUGHT

 Drought is a temporary condition that occurs for a short period due to deficient
precipitation for vegetation, river flow, water supply and human consumption.
 Aridity is a permanent climatic feature and is the culmination of a number of long-term
processes.

Particulars Aridity Drought

Duration Permanent feature Temporary condition of
scarcity of varying duration

Factors Culmination of many long Caused by deficient rainfall

term processes
considers all climatic
features
Aspect described Description of climate Description of water
 availability

Indices of Aridity
1. Thornthwaite and Mather (1955): The classification of Thornthwaite (1948) was
modified for the Moisture Index (Im) and is given below:
Im = 100 [(P-PE)/PE]
Where, P = Precipitation, PE = Potential evapotranspiration

Im Quantity Climate classification

100 and above Per humid
20 to 100 Humid
0 to 20 Moist sub humid
-33.3 to 0 Dry sub humid
-66.7 to -33.3 Semi arid
-100 to -66.7 Arid

2. Troll (1965): Based on thermal and hygric variables and number of humid months, climate is
classified and said to be of agricultural use.

Humid month is one having mean rainfall exceeding the mean potential evapotranspiration.
Humid months Climate classification
12.0—9.5 Tropical forest
9.5-7 Humid Savannah
7-4.5 Dry Savannah
4.5 -2 Thornn Savannah
2-1 Semi desert
1-0 Desert
3. Papadakis (1961): They have calculated the moisture Index (H) based on precipitation, soil
moisture storage.
H = [P + W]/E
PET was developed.
P = Monthly precipitation
E = Monthly PET
W = Water stored from previous rainfall

Humid months Climate classification

Less than 0.25 Drought
0.25 to 0.50 Dry
0.50 to 0.75 Intermediate
0.75 to 1.00 Intermediate humid
1.00 to 2.00 Humid
More than 2 Wet

4. Hargreaves (1971): Moisture Availability Index (MAI) is used for the classification. It is
the ratio of dependable precipitation to potential evapotranspiration. It is a measure of adequacy
of precipitation in supplying crop water demand.
MAI: Precipitation (75% probable rainfall) /PET

MAI Climatic condition

0.0–0.33 during all months Very arid
More than 0.34 for 1–2 months Arid
More than 0.34 for 3–4 consecutive Semi arid
months

5. FAO classification is based on ‘growing period concept’

Growing period is the number of days during a year when precipitation exceeds half the
potential evapotranspiration, plus a period to use an assumed 100 mm of water from excess
precipitation (or less, if not available) stored in the soil profile.
Areas having a growing period between 1 and 74 days are classified as arid and those
with a growing period between 75 and 119 days are semi-arid.
6. Steiner et al. (1988) consider aridity index concept of the United Nations conference on
Desertification based on the balance between precipitation (P) and evapotranspiration

According to this classification, the areas with P/ETP ratio between 0.03 and 0.2 are arid
and areas with the ratio between 0.2 and 0.5 are semi-arid.

7. ICAR classification of agro-climatic zones: Used the simple formula of Thornthwaite (1955)
for estimating the moisture index. Thornthwaite and Mathur (1955) classified only six
categories, while the ICAR had eight moisture indices with eight moisture belt indicating eight
zones in India.
Moisture Index = 100 [(P-PE)/PE]

Moisture Index Climatic condition

1 < −80 Extremely dry
2 −60 to −80 Semi dry
3 −40 to −60 Dry
4 −20 to −40 Slightly dry
5 0 to −20 Slightly moist
6 0 to + 50 Moist
7 +50 to 100 wet
8 >100 Extremely wet