Bangalore: While an energy crisis is looming large on one side, a new study cautions that India's water demand will outstrip supply a few decades from now - if not sooner.

The study by T N Narasimhan, a professor at the University of California at Berkeley, says the Indian government has "seriously overestimated" available and utilisable water resources.

Narasimhan's study is in the June issue of the Journal of Earth Systems Science published by the Indian Academy of Sciences here. It says that optimistic projections made by India's Planning Commission as late as in 2007 are incorrect.

He says these estimates were based on data provided by the water resources ministry in 1999 that "significantly underestimated" evapotranspiration (ET) - a term used to describe the amount of water lost due to evaporation of surface water and "transpiration" by plants and trees.

According to the study, the use of more realistic value for ET would reduce the amount of water available for human use by at least 37 per cent. This is the second time in 12 months that scientists have raised an alarm over water availability in India.

Last year N K Garg of the Indian Institute of Technology-Delhi (IIT-D) called for urgent action before water scarcity "becomes unmanageable". He said the government has overestimated utilisable water by as much as 68 per cent and that India is unlikely to meet the annual demand of 897 billion cubic metres (BCM) projected for 2050 "even after full development of utilisable water resources".

The source of water for all uses in India is the 3,838 BCM of rainfall it receives annually. Part of it enters rivers and streams, another part recharges the groundwater, and the third part is lost due to ET.

According to the Planning Commission's calculations, the surface flow and recharge components add up to 60 per cent - or 2,301 BCM - of the total rainfall and this is available for human use. This implies that the remaining 40 per cent is lost due to ET.

Narasimhan, however, argues that this figure of 40 per cent is "significantly" lower than published estimates of ET for a number of regions in the world. For instance, the ET is 90 per cent for Australia, 82.1 per cent for the Amazon basin, 82.8 per cent for France and between 60.5 and 66.4 per cent for the world's total land area.

Narasimhan says an independent study - using a higher ET of 69 per cent - has estimated the available water in India to be 1,460 BCM or almost 37 per cent less than the government projection.

Due to engineering and environmental constraints, only about half of the available water - or 712 BCM - is actually "utilisable," says Narasimhan. "If we compare this 712 BCM of utilisable water with the current use of 634 BCM, it is clear that India is already at the threshold of over-development of water resources," Narasimhan warns.

"Thus, India has to be seriously concerned about shortage of water right now rather than a few decades from now," a conclusion shared by the study carried out by IIT-D's Garg.

According to Garg's analysis published last year, almost all the basins in India would become water-deficit, thereby raising "a big question about the availability of water for inter-basin transfer".

Garg's analysis also showed that the groundwater has been overexploited as far back as 1997-98. Muthaia Perumal, a hydrologist at IIT-Roorkee, said the study by Narasimhan should serve as a warning and that a credible estimate of ET for India is urgently needed to revise the estimates of utilisable water.

"ET for India has been estimated so far by indirect means - subtracting run off and ground recharge from total rainfall - which is not accurate," Perumal told reporters. "A rigorous assessment of ET has not been made partly because water resource data is restricted by the government," he added.

Perumal says there is a strong necessity for precise data on water availability especially because power generation has been thrown open to the private sector. "There is a danger if private companies start building reservoirs for new power projects without knowing exactly how much water is available," he pointed out. "Impounding more and more water for power production means that ET will further increase, thereby accelerating water scarcity," Perumal maintained.

Source : IANS