Mumbai receives its water supply from five lakes created by constructing dams across rivers, namely Tansa, Modak Sagar, Upper Vaitarna, Bhatsa and middle Vaitarna. It also draws a very meagre amount from Vehar and Tulsi lakes.
To receive unhindered water supply from these lakes during the dry periods, the withdrawals are fixed at 95% reliability level; this means such withdrawals may fail once in 20 years. The monsoon is active in this region from June 10 to September 30. So the dry period is taken from October 1 to June 30 next year, which works out to be 272/273 dry days.
The BMC records show that water levels in each of the reservoirs start increasing latest by the last week of June. At the 95% reliability rate, one level is fixed in each reservoir known as lowest drawable level (LDL). This LDL is expected to be reached by June 30.
Now there is the lowest outlet available in each reservoir which is still lower than the LDL, and is called Lowest Draw Down Level (LDDL). In each reservoir there are five to six outlets provided between full supply level (FSL) and LDDL. Water is always withdrawn from the topmost available outlet so as to ensure fresh water having maximum dissolved oxygen level.
The content between these two levels is called the ‘carry over quantity’, which is nearly 40% supply quantity or one can say the ‘reserve’ quantity. This ‘carry over quantity’ is to be touched only after June 30, in case of delayed monsoon and in no circumstances should exceed 50% ‘carry over’ content in a year. This is planned keeping in view that there can be two consecutive years of failed monsoon.
So if the monsoon is delayed beyond June 30, the authorities are required to impose a water cut. A failed monsoon for the BMC is if the rainfall is less than 1,500 mm (60 inches). Years 1987 and 1988 were the only incidents of two consecutive years of failed monsoon.
(The author was BMC’s chief hydraulic engineer; he is an expert on water resources)
