Final Evaporation Control in reservoirs

Guidelines for Testing Film Pressure 
The pressure that a monolayer builds on the treated surface of water can be indicated 
approximately by use of oils of known spreading pressures. The spreading pressure 
of various oils are given in Table 9.4. A small drop of any of these oils is applied 
over the monolayer, whose pressure is to be assessed, if the indicator oil drop spreads, 
it is evident that water surface carries a film at a pressure lower than spreading 
pressure of indicator oil. If the drop does not spread, it indicates that the film 
pressure is greater than the spreading pressure of the indictor oil. Oils of different 
spreading pressures in the working range may be kept ready at the site before field 
application of WER.

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12.0
 
ECONOMICS IN USE OF VARIOUS METHODS 
12.1
 
Chemical WERs 
The economic viability of evaporation control by chemical WER in drought/ scarcity 
conditions have been conclusively proved by the results of the works undertaken in 
Gujarat, Maharashtra and Rajasthan. However, the success of evaporation control 
work depends mainly on the site conditions. In unfavourable site conditions, the 
work may turn out costly, as reported in case of evaporation control measures in 
Dheku and Ambadi Reservoirs in Aurangabad District of Maharashtra State. The 
cost of evaporation control work done during 1984-85 in these reservoirs is reported 
to be around Rs. 10.00 to Rs. 13.43 per m
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of water saved. One of the reasons for 
this exorbitant cost is attributed to the high wind speed prevailing at the site.
12.2
 
Compartmentalisation of Reservoirs 
The compartmentalization work done in Bhogavo-1 reservoir during the year 1988 
involved a cost of Rs. 12.00 lakh for construction of bunds. Another amount of Rs. 
15.00 lakh was spent for pumping water from one compartment to another. This 
measure had resulted in saving of one month’s supply of water. This quantity of 
water , if provided through water tankers would have cost Rs. 32.00 lakh. The main 
advantage of compartmentalisation is that expenditure incurred on construction of 
bunds is a one time measure only. Another benefit from compartmentalization is that 
the emptied bed of the reservoirs could be used for raising various dry crops using the 
available soil moisture. In Bhogavo-1 reservoir two crops of green fodder of value 
Rs. 40.00 lakh were reportedly raised on the emptied bed of reservoir.
This is not to suggest that the chemical evapo-retardants are not to be used. However, 
other means of controlling evaporation could also be explored in areas with recurring 
droughts and scarcities and if found more economical and practical, the same could 
be beneficially resorted to. 

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