Re: to reduce evaporation from lakes
- 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.