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Pancheshwor Project Downstream Benefits By AB Thapa A high dam is proposed
to be built at Pancheshwor across the
Mahakali River that forms the border between
Nepal and India. The height of the dam is
expected to be over 300 meters. According to
the pre-feasibility study of this project the
volume of the storage reservoir would be
11.3 billion cu. m. The average annual flow
of the Mahakali River at the dam site
is 654 cu. m /sec. After the completion of the
Pancheshwor project the flow of the
Mahakali river would be fully regulated. It
would be possible to provide constantly throughout
the year an average flow estimated at 654
cu. m /sec if the water losses on seepage
and evaporation are not taken into account.
At present during the critical month in dry
season the 80% probability discharge of the
Mahakali River at the dam site comes down
to about 90 cu. m /sec. The annual electricity
generation of this project could be about
10,000 GWh. Big multipurpose projects
are expected to provide very large
downstream benefits. They could be irrigation,
flood control, navigation and also power benefits (
accruing from the additional generation of
electricity by hydropower plants that already
exist further downstream). In case of the
Pancheshwor project, the irrigation benefit is
expected to be the most significant out
of all other downstream benefits. For calculating
the net irrigation benefit, it is
necessary to know the current status of
the agriculture in the area to be served
by the Pancheshwor project. For its
part, the present status of
agriculture within the project area is very
much dependant on several factors such as
the actual water requirement for consumptive
use, meteorological conditions, availability of water
for irrigation etc. It might be possible
to have in an indirect way fairly a
good idea about the current status of the
agriculture within the project area by judging
from the perspective as to how far the
existing facilities would be able to support
the agricultural production. Irrigation Timing Water is the most
important input influencing the crop production.
Water plays several vital roles in the
life of plants. Biochemical processes such as
transpiration, building up of plant body and its
contents, hydration and photosynthesis are greatly
influenced by soil moisture. Water is an important
constituent of protoplasm and is present to an extent
of 85 to 90 percent in the fresh weight of
actively growing plant parts. We all know that
the water applied to crops is spent
in consumptive use by plant as well as
in deep percolation into the soil. In
order to derive to a reasonable extent
the benefits from irrigation, it is not enough
that the application of water is adequate
in quantity. It is equally important
that the crop roots would get the water
in required quantity at the right
time. In North India like anywhere else
the agricultural practices followed for
centuries by the farmers have helped
to establish the necessity of planting crops
during selected periods for getting optimum
yields. These rough indications applied by the
farmers in agricultural practices have gained
scientific validation since a long time.
Unfortunately, most of the simple
diversion type irrigation projects operating
for many years ( like the Sarada
Barrage Project at Banbasa ) have some
inherent weakness. These projects are not
able to supply even a small fraction
of the average flow of the river
during the summer dry season months
when the demand for irrigation water is
the highest, whereas they run into great
surpluses during the monsoon and post
monsoon periods. Consumptive Use of
Water Evaporation and transpiration
are two main processes of great significance.
Evaporation of water from the
storage reservoirs and ponds is a
commonly known phenomenon. A study of such
phenomenon is important for planning storage
reservoirs and ponds. Similarly the study of
evaporation from the bare soils is
important for determining the water requirement
in agriculture. Transpiration is the escape of
water from plants in the form of vapour.
During the crop growth period, it is
difficult to estimate the amount of water
lost from the soil and the plant
separately. As a result, both these processes
are looked at in combination and
estimated as evapo-transpiration. Blaney and Criddle
have derived an equation which is widely
used for estimating the water requirement of
irrigated crops in the USA. It is also
used in the Uttar Pradesh of India. In
the above equation often the monthly
evapo-transpiration in inches, referred to as
consumptive use is determined as the
product of crop constant and the monthly
evapo-transpiration factor. The agricultural research
institute at Pantnagar, Uttar Pradesh has carried
out extensive studies to make the Blaney and
Criddleís formulae applicable to Indian
condition. Thornthwaite has also deduced a
formulae slightly different from that of Blaney
and Criddle. Government of India has
published charts of consumptive uses of water
and also isohyets showing the rainfall
distribution during the monsoon, summer and
winter months for the whole India. Such
charts can be found in college text books
also. These charts could help to find out
to what extent the Mahakali river water
is being now used for irrigation in
India. According to the data published
by the WATER TECHNOLOGY CENTER of the
Indian Agricultural Research Institute, Govt. of
India, the consumptive use of water in
agriculture across the whole of the Uttar
Pradesh during the monsoon season from June
to September is only in between 20-40 cm.
The consumptive use for the winter season
from October to January is also in
between 20-40 cm. The consumptive use
for the summer season from February to May
is much higher and it is in the
range of 80-100 cm. Indian Agricultural Research
Center has also published data on water
requirements of field crops( other than rice) at 70
per cent field irrigation efficiency. According to
such data the KHARIF crop grown from July
to October requires from 50 to 60 cm
water. The RABI crop grown from
November to March requires from 40 to 50
cm water. Similarly the hot weather crop
grown from March to June requires
from 80 to 90 cm water. In case of monsoon
paddy cultivation some studies carried
out in the past have adopted
effective rainfall of 80%-85% of the mean.
Similarly the paddy percolation rate, field
channel efficiency and canal conveyance
efficiency (lined canal) have been taken to
be 4.6-6.1 mm/d, from 0.7 to 0.9 and from 0.9 to 095 respectively. Rainfall Distribution The rain that falls
on the earthís surface is the major source
of water for agriculture and also
for various other types of uses. South-west
monsoon and cyclonic depressions contribute to
rainfall in the north-eastern region of the
Uttar Pradesh within the command area of
the Pancheshwor project. Due to high
temperature of summer, the moisture-laden south-west
monsoons originate from the vast expanse
of the Indian ocean and enter the Indian
sub-continent from the south-west. These
south-west monsoon winds increase from June to
July. By the first week of July the
monsoon is established all over the North India. The withdrawal
of the monsoon starts in September in north
India and it continues until the beginning
of the October. Precipitation during the cold
weather season is due to a low pressure
systems in north India called the
western disturbance which move from west
to east. Looking at the rainfall
isohyets maps of India it can be
said at a glance how much rainfall
the cultivated lands in the Uttar Pradesh
within the command area of the
Pancheshwor project could expect to receive.
It is also possible to find out
separately the total rainfall during each
of the three seasons of a year. In the
monsoon period from June to September the
total rainfall within the command area of
the Pancheshwor project is about 100 cm.
Thereafter during the winter season from
October to January the total rainfall sharply
decreases and it comes down to only
about 5 cm. Similarly during the summer
months also from February to May
there is very limited precipitation
and the total rainfall continues to be
only about 5 cm. Cropping Pattern An extensive study of
the present agricultural practices within
the irrigation command area of the
Karnali Multipurpose project had already been
carried out some years back under the
aegis of the World Bank. Incidentally
the irrigation command areas of the Karnali
project and the Pancheshwor project overlap.
Thus some of the Karnali project data
on cropping pattern could be directly
applied to the Pancheshwor project. The present cropping
intensity within the irrigation command
area of the Pancheshwor project is only
96%. The main crops grown in the project
area are the monsoon paddy, wheat, sugarcane
and potatoes. From the view point of
availability of water for irrigation there are
two critical periods. The first one is in
the monsoon period itself when there is
a demand for water in huge quantity
to irrigate lands under paddy and maize
cultivation . Similarly the next critical period
is in the months of February, March and
April. During this period the crops grown
such as the wheat, sugarcane and potatoes
also need water in large quantity for
irrigation. Mahakali River Water
Availability The flow of the
Mahakali river during the monsoon period is
very big. The average monthly flow of the
Mahakali river in the last 29 years
during the monsoon season had
exceeded the average dry season for the
same period by almost 15 times.
At present the huge monsoon flow of the
Mahakali River is hardly being used in
irrigation despite the fact that the diversion
structure like the Sarda Barrage at Banbasa
equipped with canal system had been built
a long time ago. During the monsoon
season according to the data published
by the Government of India the effective
rainfall exceeds the water demand for
consumptive use in irrigation in the command
area of the Mahakali River project. It is
thus obvious that the function of the
irrigation barrages like the one at Banbasa
is merely to supplement the rainfall with
supplies from the river whenever there is
a prolonged draught. At present very
limited volume of the Mahakali River water
might be available for irrigation in the
command area of the Pancheshwor project
during the dry season. The average monthly
discharge of the Mahakali river in the
months of February and March with a
recurrence probability factor of 80% could be
somewhere around 90 to 100 cumecs. Indiaís
share in such dry season flow of the
Mahakali river could be roughly around
50% if the water to be allocated
for different purposes according to the
Mahakali Treaty is set aside. The cultivation
period of the main crops grown during
this season extends from the middle of
winter to the middle of summer when the
water needed for consumptive use is
very high. It implies that the Mahakali
River might now be providing very
limited quantity of water for irrigation
during the dry seasons. Pancheshwor Storage Reservoir
The present debate about
the existing consumptive uses of the Mahakali
River water in India might be irrelevant
in the context of building a high
dam at Pancheshwor to provide big storage
reservoir. The share of Nepal and India
in Mahakali river water for dry season
irrigation might be virtually even. At
present India is using to a certain
extent the Mahakali River water to
supplement the rainfalls during the monsoon
season. Such supplementary irrigation is
extremely important to save the
paddy crops during the prolonged dry
spells despite the fact that on the whole
the irrigation water requirement might
be very close to effective rainfall. During the monsoon
seasons the Pancheshwor storage reservoir would
be in the process of being filled
up. It might even be necessary towards
the end of the monsoon season to
spill the water from the storage reservoir
when it is already full. Moreover even
during the monsoon period the Pancheshwor
hydropower stations would be non intermittently
discharging water in very big quantity into
the river after the power generation.
Thus the limited use of the Mahakali
River water to supplement the rainfalls might
hardly have any adverse impact on
irrigation benefits accruing from the
Pancheshwor project. |
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