17.1 Concept and Measurement
of Aridity
The concept of aridity is centred around inadequate rainfall and absence of adequate vegetation
cover. It is not of practical utility to define aridity
in terms of rainfall inadequacy alone. Aridity is
the cumulative climatological characteristic of
continued moisture deficiency when rainfall is
unable to meet the evaporative demands of a
region. The modern systems for defining aridity
are based on the concept of water balance. Water
balance is largely an equation between the precipitation and evapotranspiration. By definition,
there is an overall deficit in water balance in the
deserts. The amount of that deficit determines the
degree of aridity. The concept and delimitation of
hyper-arid zone is based on the largest gap
between water availability (rainfall) and the
atmospheric water need (potential evapotranspiration). When mean annual rainfall (water
availability) of a place is only around 5 percent
of the mean annual potential evapotranspiration
(atmospheric water need) it would be a hyperarid condition. For example, if the mean annual
potential evapotranspiration of a place is
2000 mm and the mean annual rainfall is only
around 100 mm, it would be a climatic condition
of hyper-aridity.
17.2 Evapotranspiration
and Climatic Demarcation
The concept of evapotranspiration is integrally
indispensable to climatic classification and
demarcation of environmental regions. One of
the most important factors related to water
management in the hyper-arid lands is evapotranspiration. Evapotranspiration is considered a
major factor in the water loss. It plays an
important role in the hydrological cycle (Jensen
et al. 1990). Evapotranspiration is a combined
term representing the transport of water to the
atmosphere in the form of evaporation from the
soil surfaces and from the plant tissue as a result
of transpiration (Allen et al. 1998).
Evapotranspiration is expressed in two forms:
potential
evapotranspiration
and
actual
evapotranspiration.
Potential evapotranspiration (PET) is defined
as “the maximum water demand of the atmosphere if it was available for evapotranspiration”.
The reference evapotranspiration (ET 0 ) is the rate
of evapotranspiration from a well-defined reference environment. In most cases, limited water is
available for evapotranspiration which is the
actual rate of water loss. Actual evapotranspiration is defined as the rate at which water is
actually removed by evapotranspiration to the
atmosphere. Transpiration helps the plants to
assimilate and transport mineral nutrients. It also
cools the leaves surface due to the removal of
latent heat for vaporisation. However, too much
transpiration can lead to plant stress in the hyperarid rangelands. The term “potential” is equivalent to the maximum possible level of water loss
under given climatic conditions. As soil water
depletes, evapotranspiration decreases and reduces from potential and actual evapotranspiration
in hyper-arid regions where the potential evapotranspiration is extremely high and the available
water is very limited.
There is a popular notion that the arid environment is monotonous in the western part of the
Indian Desert. Owing to inaccessibility and
scanty research in these extreme areas, the geographical variations within a broad range of arid
lands were often underestimated. In view of this
scientific tradition, arid condition was considered
the last limit of environmental degradation in the
Indian Desert. At macro- and meso-level enquiries, the aridity index in the Indian Desert has
been measured from 85 to 94 percent. However,
an environment is admitted to be hyper-arid if the
aridity index is measured as 95 percent or more.
There was an educated guess that the environment of the Indian Desert, on its last frontiers,
could be more than arid. The available literature
at Central Arid Zone Research Institute
(CAZRI), Arid Forest Research Institute (AFRI),
Central Research Institute for Dryland Agriculture (CRIDA) and International Crop Research
Institute for Semi-Arid Tropics (ICRISAT) does
226
S. Salahuddin
of Aridity
The concept of aridity is centred around inadequate rainfall and absence of adequate vegetation
cover. It is not of practical utility to define aridity
in terms of rainfall inadequacy alone. Aridity is
the cumulative climatological characteristic of
continued moisture deficiency when rainfall is
unable to meet the evaporative demands of a
region. The modern systems for defining aridity
are based on the concept of water balance. Water
balance is largely an equation between the precipitation and evapotranspiration. By definition,
there is an overall deficit in water balance in the
deserts. The amount of that deficit determines the
degree of aridity. The concept and delimitation of
hyper-arid zone is based on the largest gap
between water availability (rainfall) and the
atmospheric water need (potential evapotranspiration). When mean annual rainfall (water
availability) of a place is only around 5 percent
of the mean annual potential evapotranspiration
(atmospheric water need) it would be a hyperarid condition. For example, if the mean annual
potential evapotranspiration of a place is
2000 mm and the mean annual rainfall is only
around 100 mm, it would be a climatic condition
of hyper-aridity.
17.2 Evapotranspiration
and Climatic Demarcation
The concept of evapotranspiration is integrally
indispensable to climatic classification and
demarcation of environmental regions. One of
the most important factors related to water
management in the hyper-arid lands is evapotranspiration. Evapotranspiration is considered a
major factor in the water loss. It plays an
important role in the hydrological cycle (Jensen
et al. 1990). Evapotranspiration is a combined
term representing the transport of water to the
atmosphere in the form of evaporation from the
soil surfaces and from the plant tissue as a result
of transpiration (Allen et al. 1998).
Evapotranspiration is expressed in two forms:
potential
evapotranspiration
and
actual
evapotranspiration.
Potential evapotranspiration (PET) is defined
as “the maximum water demand of the atmosphere if it was available for evapotranspiration”.
The reference evapotranspiration (ET 0 ) is the rate
of evapotranspiration from a well-defined reference environment. In most cases, limited water is
available for evapotranspiration which is the
actual rate of water loss. Actual evapotranspiration is defined as the rate at which water is
actually removed by evapotranspiration to the
atmosphere. Transpiration helps the plants to
assimilate and transport mineral nutrients. It also
cools the leaves surface due to the removal of
latent heat for vaporisation. However, too much
transpiration can lead to plant stress in the hyperarid rangelands. The term “potential” is equivalent to the maximum possible level of water loss
under given climatic conditions. As soil water
depletes, evapotranspiration decreases and reduces from potential and actual evapotranspiration
in hyper-arid regions where the potential evapotranspiration is extremely high and the available
water is very limited.
There is a popular notion that the arid environment is monotonous in the western part of the
Indian Desert. Owing to inaccessibility and
scanty research in these extreme areas, the geographical variations within a broad range of arid
lands were often underestimated. In view of this
scientific tradition, arid condition was considered
the last limit of environmental degradation in the
Indian Desert. At macro- and meso-level enquiries, the aridity index in the Indian Desert has
been measured from 85 to 94 percent. However,
an environment is admitted to be hyper-arid if the
aridity index is measured as 95 percent or more.
There was an educated guess that the environment of the Indian Desert, on its last frontiers,
could be more than arid. The available literature
at Central Arid Zone Research Institute
(CAZRI), Arid Forest Research Institute (AFRI),
Central Research Institute for Dryland Agriculture (CRIDA) and International Crop Research
Institute for Semi-Arid Tropics (ICRISAT) does
226
S. Salahuddin