Final Evaporation Control in reservoirs

Kukkarahalli Tank : This tank had a limited catchment and the main source of 
water supply to the tank was from a feeder channel and had two draw-off points.
Cetyl alcohol was mainly used as evaporation retardant. Water level recording 
was done by means of Hook-Gauge. Contour survey was done as and when the 
water level got depleted. Seepage meters were also used to measure seepage 
losses. Cetyl alcohol in the solution form was dispensed either by the shore 
dispenser or by raft dispenser. Semi-field studies using pan evaporimeters were 
also done. The studies indicated saving in evaporation loss varying from 5.7 to 
24%.
b)
Varuna Tank: The methodology adopted for studies in this tank is similar to the 
methodology adopted for Kukkarahalli Tank. Semi-field studies using CS pan 
evaporimeter deploying all the 3 methods (emulsion, solution and powder form) 
at various film pressure were undertaken. It was observed that an appreciable 
savings could be achieved by an increase in surface film pressure using solution 
and emulsion techniques. The saving in water achieved was found to be varying 
from 8.9 to 36.91%.
c)
Kadavinahosahalli Tank: The studies done in this tank are similar to 
Kukkarhalli Tank. Cetyl alcohol was spread both in emulsion and solution form.
It was observed that the factors such as concentration of the retardant, 
temperature, rate of spreading and rate of evaporation have an important role to 
play in maintaining the optimum surface pressure of 40 dyne/cm. Emulsion 
technique was found to be more economical than the solution technique. 
d)
Lingambudhi Tank : Field and semi-field studies using all the three techniques 
are reported to have been conducted. Studies on correlation of evaporation 
observed in different evaporimeters namely Piche Evaporimeters, CS Pan, USWB 
Land Pan and two floating pans have also been reported. The percentage of 
saving in evaporation loss varied from 8.50 to 35.24%.
The inferences drawn from the study were:- 
i)
The quantity of chemical required to maintain a surface pressure of 40 
dyne/cm would be much more than to maintain 16 dyne/cm, but the cost of 
additional chemical is offset by the increase in the quantity of water saved.
ii)
Percentage saving is nearly the same at 40 dyne/cm by either of the techniques 
namely solution or emulsion. 
iii)
For corresponding concentrations of the chemical, emulsion techniques is 
found to be more effective than solution technique. The percentage of savings 
ranges from 9.5% to 28% in case of emulsion,19% to 34% in case of solution 
and 8.5% to 35% in case of powder. 
iv)
Optimum surface pressure of 40 dyne/cm lead to maximum evaporation 
reduction 

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