Loss of
reservoir water would mainly take place due to evaporation and a
number of methods have been suggest for controlling such loss. The
Bureau of Indian Standard code IS: 14654 - 1999 “Minimizing
evaporation losses from reservoirs- guidelines” describes the cause
of evaporation reduction methods in detail, some important aspects of
which are described in the subsequent paragraphs. As such,
percolation or seepage loss is small for most of the reservoirs and
progressively gets lowered with the passage of time since the
sediment getting deposited at the reservoir bottom helps to reduce
percolation losses. Of course, in some hills and valleys forming the
reservoir, there may be continuous seams of porous rock strata or
limestone caverns which cause huge amount of water to get drained out
of the reservoirs. The reservoir of the Kopili Hydroelectric Project
in Assam-Meghalaya border had faced similar problems due to the
presence of large caverns which had to be sealed later at quite large
cost at a later stage.
A number
of factors affect the evaporation from open water surface, of which,
the major factors are water spread area and frequent change of speed
and direction of wind over the water body. Other meteorological
factors like.
a) Vapour
pressure difference between water surface and the layer of air above;
b)
Temperature of water and air;
c)
Atmospheric pressure;
d)
Radiation;
e) Heat
storage in water body; and
f)
Quality of water, have direct influence on the rate of evaporation.
Since the
meteorological factors affecting evaporation cannot be controlled
under normal conditions, efforts are made for inhibition of
evaporation by control of flow of wind over water surface or by
protection of the water surface area by physical or chemical methods.
The methods generally used are as follows:
a) Wind
breakers,
b)
Covering the water surface,
c)
Reduction of exposed water surface,
(a)Wind
Breakers
Wind is
one of the most important factors which affect rate of evaporation
loss from water surface. The greater the movement of air over the
water surface, greater is the evaporation loss. Planting of trees
normal to windward direction is found to be an effective measure for
checking of evaporation loss. Plants (trees, shrubs or grass) should
be grown around the rim of tanks in a row or rows to act as wind
breaker. These wind breakers are found to influence the temperature,
atmospheric humidity, soil moisture, evaporation and transpiration of
the area protected. Plants to act as wind breakers are usually
arranged in rows, with tallest plants in the middle and the smallest
along the end rows, so that more or less conical formation is formed.
(b)
Covering the Water Surface
Covering
the surface of water bodies with fixed or floating covers
considerably retards evaporation loss. These covers reflect energy
inputs from atmosphere, as a result of which evaporation loss is
reduced. The covers literally trap the air and prevent transfer of
water vapour to outer atmosphere. Fixed covers are suitable only for
relatively small storages. For large storages, floating covers or mat
or spheres may be useful and effective. However, for large water
surfaces the cost of covering the surface with floats is prohibitive,
Further in case of reservoirs with flood outlets, there is also the
danger of floats being lost over spillway or through outlets. The
floating covers are thus of limited utility to larger water bodies.
(c)
Reduction of Exposed Water Surface
In this
method shallow portions of the reservoirs are isolated or curtailed
by construction of dykes or bunds at suitable locations. Water
accumulated during the monsoon season in such shallow portions IS
diverted or pumped to appropriate deeper pockets in summer months, so
that the shallow water surface area exposed to evaporation is
effectively reduced.
(Source:
CE, IIT Kharagpur)