Rising mercury lowers lake levels

Poor Rains Left City With 30% Less Reserves Than Last Year, Early Summer Adds To Woes

 

   As if a poor monsoon last year was not bad enough, the steadily rising mercury levels in March and April have compounded the BMC’s woes.Even as the lakes that supply water to the city did not touch their brim last year and had 30% less water to start with, evaporation has started earlier this year with the sun shining harder in March than usual.
    In 2008-2009, between the end of monsoon 2008 and the start of monsoon 2009, the lakes lost nearly 50,000 million litres of water to evaporation. Now, civic officials are in a tizzy, expecting a much greater loss of water. To tackle the situation, as an immediate measure, the BMC has been forced to channelise the reserve quantity to Vaitarna, which is much deeper than other lakes but has a relatively smaller surface. This, say officials, reduces the rate of evaporation, which is directly proportionate to the surface area. For example, Tansa with a periphery of 44 km, has one the highest surface areas but is less deeper than others. “Evaporation here is highest since the lake has a flat surface as compared to a saucer-shape surface of Vaitarna,’’ said a civic official.
    The six lakes that supply water to the city collectively lose about 9% daily, or roughly 350 million litres of water per day (mld), to evaporation each year. Of these, Bhatsa and Tansa lakes, the largest suppliers of water, suffer maximum losses, of 136 mld and 84 mld respectively. While Vaitarna and Upper Vaitarna lose 64 mld and 47 mld respectively, Tulsi and Vihar—which have the smallest catchments—lose only 5 mld and 18 mld res p e c t ive ly. “This year, the lake levels were not up to the mark. To add to that, the heat has been very severe right from the beginning of March. Although we don’t know yet how much water will be lost to evaporation this year since the dry season has only just begun, we are anticipating a significant loss,’’ said a civic official.
    A BMC study in 1991 had found that the lakes lost 15 days of water in the dry period (after the monsoon) to evaporation. Civic officials pointed out that evaporation being a natural phenomenon, it is accounted for in the 20% water annually lost to theft, pilferage etc. But a poor monsoon and a harsh summer can upset calculations. After a delayed monsoon last year, the BMC was forced to impose a series of water cuts, between 10 and 30%. Currently, the city is reeling under a 15% cut in residential areas, and 30% cut in commercial establishments. The BMC is therefore only supplying 2,900 mld of water every day to the city as opposed to 3,300 mld that it supplied before the cut. However, officials said the current adjustment is enough to last the city up to July 15, and they are not toying with the idea of increasing the cut. Civic chief Swadheen Kshatriya said that an early monsoon is expected this year.