to reduce evaporation from lakes
To cut lake water lost to evaporation, civic body looks at Singapore modelStuti Shukla : Mumbai, Mon Jul 19 2010
Its plans of a desalination plant to convert sea water for potable use and a greywater recycling plant are yet to take off, the BMC has mooted a new proposal but this time to slow down evaporation from lakes.
The BMC is looking at a method adopted by Singapore, known for its water supply and conservation practices, which would involve spreading of a thin film of a potable, oil-based liquid over the surface of the six lakes that supply water to Mumbai.
This would reduce penetration of sunrays into the lakes to 5-10 feet, from 20 feet. An official from BMC's Bhandup water treatment complex said this could reduce evaporation by at least 20 per cent.
"The liquid is potable and can be consumed without any harm. It also does not cause any harm to aquatic life. The film of oil is extremely thin and does not affect quality of water at all," said the official.
The proposal was placed before the civic administration by Mayor Shradhha Jadhav and BMC political group leaders who recently visited Singapore to study the functioning of its Public Utilities Board that manages water supply. They also visited the desalination plant there.
The proposal is being studied for feasibility by the hydraulic engineering department. "While it is a useful technology for the city, considering much water is lost due to evaporation, it is also technically challenging to implement here. The catchment areas of our lakes are spread over a large area. We are therefore studying the technical and financial viability of the same," said hydraulic engineer Vinay Deshpande. Over 10 per cent of the water in lakes is lost to evaporation.
Reduce water surface area: Decrease of water surface area as compared to storage capacity of the lake.
Cover water surfaces: A plastic cover or floating substance can be used to act as air cushions around the lake to reduce evaporation.
Use of chemical substances to make a thin film over water surface: Chemical substances such as fatty alcohols can be sprayed periodically on water surface to reduce evaporation.
- Abstract—During the summers of 1957 and 1958 studies were conducted on two lakes in
central Illinois to determine a practical method of applying monomolecular layers and to
learn the effectiveness of the layers in reducing evaporation. Results showed a saving of 43
per cent in 1957 and 22 per cent in 1958 of the water normally lost to evaporation. A method
was developed for determining the strength of the monolayer by measuring the heat gradient
near the water surface.
- Pan evaporation is a measurement that combines or integrates the effects of several climate elements: temperature, humidity, rain fall, drought dispersion, solar radiation, and wind. Evaporation is greatest on hot, windy, dry, sunny days; and is greatly reduced when clouds block the sun and when air is cool, calm, and humid. Pan evaporation measurements enable farmers to understand how much water their crops will need.Pan evaporation is used to estimate the evaporation from lakes. There is a correlation between lake evaporation and pan evaporation.
- Some people don't realize that water evaporation is much greater in the winter than in the summer -- when water temperatures are high and air temperatures are cold (especially at night). This will just suck the water right out of the pool.
- The purpose of this brief paper is to describe some principles involved in estimating
evaporation with example data from large and small water bodies. Particular emphasis will be on
practical procedures and techniques that professionals can use to estimate evaporation from
shallow water bodies using pan evaporation and a combination equation using available weather
data and/or new data collected specifically for estimating daily evaporation.
- Lake water evaporation makes officials sweat
Sharad Vyas, TNN Oct 17, 2010
MUMBAI: The rain gods' generosity this year might have ended the need for water cuts, but there is still a problem that is making BMC officials perspire: how to tackle and minimize the massive loss of water from evaporation.
Between the end of monsoon in 2008 and the beginning of the rains in 2009, the city's lakes lost nearly 50,000 million litres of water due to evaporation. Now, with the mercury rising, the decline is expected to be greater. Yet, the civic officials remain at a loss for solutions.
"We know this is going to be a major problem for us during the summer months," said municipal commissioner Swadheen Kshatriya. "Though we have tried different technologies in the past, none has worked. Even our officials do not have the confidence to use them effectively in local conditions."
Every day, the catchment six lakes lose 350 million litres of water-roughly 9% of Mumbai's daily requirement-to evaporation. Of this, the city's main suppliers, Bhatsa and Tansa, suffer the biggest losses: about 136 million litres per day (mld) and 84 mld, respectively. Next are Vaitarna and Upper Vaitarna, which get depleted by 64 mld and 47 mld, respectively. Tulsi and Vihar-which have the smallest catchments-lose only 5 mld and 18 mld, respectively.
Yet, the civic body is bereft of answers. "The BMC has no technology in place, despite knowing that potential evaporation ranges between 150 cm to 250 cm over most parts of the state every year," said a civic official. "With the onset of monsoon, there is a marked fall in the rate of evaporation, but by then the BMC has lost a lot of water."
A BMC study in 1991 had found that the lakes lost about 15 days of water to evaporation during the dry period (between two successive monsoons). Civic officials argue that since evaporation is a natural phenomenon, it is accounted for in the 20% water annually lost to theft, pilferage, etc.
- Putting unusable old tyres on the surface of the lake to prevent evaporation.
- Evaporation Control in Reservoirs
report by Central Water Commission
The methods generally used or being tried are broadly:
i) wind breakers
ii) covering the water surface
iii) reduction of exposed water surface
iv) underground storage of water
v) integrated operation of reservoirs
vi) treatment with chemical Water Evapo RetardantsWind BreakersWind is one of the most important factors which affect rate of evaporation loss from watersurface. 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 checkingof evaporation loss. Plants (trees, shrubs or grass) should be grown around the rim of tanks in arow 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.