Despite concerns of rapid urbanisation and real estate development in the vicinity of the Osmansagar and Himayatsagar reservoirs affecting the surface rainwater flow, experts say inflows also depend on the intensity and duration of the rain.
Nothing can stop water from flowing over its natural gradient to fill the reservoirs despite structures enroute, but what one should be more concerned about is the pollution of the reservoirs that could result with the violation of the G.O. 111, points out retired Engineer-in-Chief T. Hanumantha Rao.
Osmansagar and Himayatsagar were constructed for flood control after 1908 Musi floods submerged the then Hyderabad city and that is how their storage capacity was provided three times more than the expected yield of water in a year, he said. The expected yield to the reservoirs was only 1 tmcft each, he said.
The reservoirs were designed to be filled to the brim only once in three or four years. Never in the past, even in early decades, the reservoirs filled to their capacity every year.
According to the data given by the Hyderabad Metro Water Supply and Sewerage Board, Osmansagar and Himayatsagar reached their peak reservoir levels only seven times in the last 35 years: in 1981, 1983, 1988, 1989, 1990, 1998 and 2010.
Himayatsagar Reservoir alone reached its peak in 1982, 1991, 2001 while Osmansagar reservoir alone reached its full capacity in 1996 and 2000. In a conventional irrigation project, 0.7 tmc of stroage is provided for utilisation of 1 tmcft of water.
Because there will be outflows and it will be filled one-and-a-half times, Mr. Hanumantha Rao said.
The main reason for poor inflows into twin reservoirs of the city, he says is lack of intensity and distribution of rain as a consequence of global warming and climate change.
The run off occurs depending on the intensity of rainfall and absorption capacity of soil. There will be a good run off when 20 mm of rain occurs per hour.
In the urban areas, the run off water is ending up in storm water drains and the only way to prevent it is through rainwater harvesting in every house, open areas, play grounds by providing mini percolation tanks.
In the case of Himayatsagar and Osmansagar, the run off water first fills the chain of minor irrigation tanks and farm ponds in the catchment area and only after the last tank is filled, the run off water will form into a stream, he said.
But, due to urbanisation, construction of buildings on one hand and creation of farm ponds, mini percolation tanks in agriculture lands, watershed programmes and soil moisture factor, rainwater flowing into the streams will dwindle by about 20 per cent, he says.
Nothing can stop water from flowing over its natural gradient to fill the reservoirs despite structures enroute,T. Hanumantha Rao,Retired Engineer-in-Chief
