EVAPORATION CONTROL
Evaporation control has been first time used as a conservation measure in water supply storage dam at Yavatmal. Yavatmal water supply scheme is maintained by Maharashtra Jeevan Pradhikaran up to consumer including collection of revenue. Economics of dam water evaporation control by spreading chemicals in urban water supply is quite different than that for irrigation. In 1992-93 to 1995-96 lot of experimentation was done during scarcity. The funds were allocated by the Science and Technology wing from Govt. of Maharashtra. Complete mathematical approach was developed and a payment was related to the actual water saved. While getting the effect of conservation of water expenditure on the process for the whole summer period, was found to be much less than the revenue at the normal domestic water rates realizable from the saved water. Cognizance of this experience was taken by Maharashtra Jeevan Pradhikaran and Govt. of Maharashtra and Evaporation control was considered as one of the measures of water conservation. A concept has been included in the Maintenance Manual of Urban areas published by CPHEEO. A detailed case study for Yavatmal water supply scheme has been separately given.
Evaporation is a continuous process and it can not be totally eliminated, but it can be controlled. It is a function of surface area and difference of temperature in the atmosphere and the water body. Evaporation action is vigorous at the surface. There is a loss of quantity of water due to evaporation, as at interface transformation takes place from liquid phase to gaseous phase.
As evaporation is increased with the temperature difference the areas where the atmospheric temperature is beyond 40 degrees the rate of evaporation is cognizable. In the summer period only, the water depth depletes by 1.5 to 2.0 m. Quantity of water lost will be the average surface area multiplied by the depletion depth of water. This becomes substantial quantity and saving of possible maximum quantity should be an attempt, where the water is scarce.
Evaporation being a function of surface area attempt should be made to minimize the surface area. Recently chemical has been evolved having long chain organic compound. These warer evapro retardants are having chemical composition of n-alcohols and n-alcoxy ethanols.
Water evapro retardants area available in paste form and are normally white in color. It can be brought in the form of liquid by mixing in water in fixed proportions and can be spread over the surface of water body. This being a long chain organic compound it forms a monomolecular film over the surface of water and it spreads automatically. The film is normally retained on the surface for more than 24 hours and needs to be replenished by spreading the chemical again. The film is transparent and the sunrays can pass through the same and aquatic life is saved. The chemical has been tested by National Chemical Laboratory, Pune and it has been certified as the non-hazardous chemical as far as drinking water is concerned.
Local factors affecting the control on evaporation process are as following.
1. Local temperature
2. Relative humidity
3. Wind direction, speed and diurnal cycles
4. Soil structure
5. Surrounding topography and vegetation pattern
The film is likely to be disturbed by wind and covering efficiency is affected. The film can be drifted in the direction of wind and this has to be taken care of by spreading the chemical periodically. Normal coverage area is 200 gm/hector, for daily application.
Considering all factors it has been observed that the control on evaporation can be attained to the extent of 25 to 33%. In case of a medium dam quantity of water saved is substantial and economic analysis has to be worked out in each drinking water supply scheme. In case of irrigation utilization this cost is not economical but in case of drinking water this cost may be comparable with the different alternative available for creation of new source.
Measurement of saving in water by using chemicals for control of evaporation is possible with the use of pair of standard pan evaporators. (Please refer relevant I.S.).
METHOD OF EVLUATION
Following daily observations are to be recorded.
a) Evaporation to the accuracy of 0.1 mm in two standard pan evaporators. Pan A for untreated and pan B for treated with chemical.
b) Physical water level of dam to an accuracy of 1 mm by means of dumpy level.
c) Max, min., pan A and pan B temperatures.
Procedure of further calculations,
1. A data table shall be created for dam levels and corresponding capacity.
2. Computer programme can be developed for the best fit equation from the data table, for converting levels into capacities and capacities into levels. Similar best fit equation can be found for transformation of levels into areas and vice-versa.
3. Select the pan factor which is normally ranging 0.8 to 0.9, according to site conditions.
Next day, the readings are taken for the make-up water required for maintaining the level of water in pan A and pan B. This is done by a standard calibrated metal jar. Quantity of water used for making up the levels in the standard pan evaporators is indicated in ‘mm’ depth.
Reading in pan A will be than pan B, because in pan B evapro-retardent chemical is used. Use pan factors for field conditions readings. As both the pans are kept near the dam water and in the same physical conditions maximum control of evaporations will reflect in reading of pan B. However the actual water level depletion will be more in the dam as chemical film coverage efficiency will not be 100% in the field.
Standard depletion statement for dam water levels should be used to obtain the available quantity of water in the dam after a defined period. For daily readings maximum accuracy is required.
In some of the cases it has been observed that daily quantity supplied to the town is less as compared to loss of water due to evaporation. When water evaporation retardants are used, at least 25% daily lost quantity is made available for next alternative use. It will give some revenue. It is good conservation of water.
During scarcity tanker supply may cost Rs. 40 to 80 per 1000 lit of water. However, evaporation control measures may bring down this cost to 10% of the tanker cost.
STEPS TO CALCULATE ACTUAL SAVING OF WATER
Actual saving of water can be calculated by careful observations of dam water levels and pan readings, at the specified timings. Normally, early morning period is chosen, to avoid wind effect. Waves on water surface create disturbance in taking readings of water level.. A cast iron TEE placed in water body as shown having a side entry of water and arrangement to measure the level depletion by using a metallic tape floating between guides. Reading can be taken against the cross wires. These readings can be accurate to 1 mm. Top of TEE can be connected to a TBM or to GTS bench mark. Accordingly, water level in the dam can be calculated daily. Position of TEE can be suitably shifted on lower side as the depletion continues in the dam water level. This depletion can be by evaporation or by use of water externally. The external use quantity daily can be known from pumping records or letting out water records. From these records daily use of water in terms of ‘mm’ depth can be converted when the area of dam water surface at that level is known. The graph is generally available but accurate surface area from that graph is not identified. Hence, a best fit equation is required to be developed. This equation is possible to be developed on computer. Once this equation is known computer can calculate ‘mm’ depth to area and vice versa. Similarly, relationship between ‘mm’ depth and capacity can also be developed. These can be utilized in preparing depletion statement.