Tuesday, May 20, 2008

Groundwater Under The Sahara and Thar Deserts

From NPR Science Friday (I've been getting a lot of material from NPR of late) a discussion of a study that tracked the desertification of the Sahara using palynological evidence from sediments of Lake Yoa. The study suggests that desertification was a gradual process taking thousands of years, apparently contradicting earlier work which relied on evidence from the Mediterranean sea and indicated that desertification was rapid, probably taking place in a few hundred years. What was of interest to me was the talk about massive amounts of groundwater in the Nubian sandstone buried under the Sahara sands. This sedimentary aquifer which is made up of sedimentary sequences ranging in age from lower Paleozoic to the Cretaceous was last recharged in the early Holocene period when the climate in the Sahara region was much wetter. The aquifer contains an estimated 150,000 cubic km of water and currently about 6.5 million cubic metres per day are being extracted over its entire extent covering parts of Sudan, Chad, Libya and Egypt.

I am thinking of the situation under the sands of the Thar desert and see some parallels. I wrote about the recognition of paleochannels associated with the Ghaggar river system in the Jaisalmer district of Rajasthan. These channels contain groundwater which also like the Sahara was last recharged in the early part of the Holocene when this part of Rajasthan was wetter. The ancient river system is thought to have dried out by around 2500 B.C. due to aridification of western Rajasthan. There are grand plans today to exploit these water resources by targeted drilling, i.e identifying ancient river channels and drilling into them. Besides paleochannels it is entirely likely that the bedrock below the Thar sands may also contain aquifers. Recognizing these aquifers is going to be more difficult than paleochannels which show up as distinct linear features in radar images. What is important here is that we realize that just like the Nubian aquifer system this water under the Thar desert, stored either in ancient river channels or bedrock , is currently a non-renewable resource and draw up plans of exploitation with that in mind. As part of the Indira Gandhi Nahar Project (IGNP), the government wants to extend the canals of the Punjab into Rajasthan and use excess water from the Sutlej, Yamuna and the Ghaggar to recharge these potential aquifers. The aquifers will act as giant underground storage tanks, potentially advantageous over new surface water storage areas since no land will be submerged and losses due to evaporation minimized. This is a geo-engineering plan on a massive scale and will likely face a long period of opposition from environmentalists and economists. It might take a couple of decades for the benefits of this project to be realized if geologists and hydrologists ascertain that it makes sense in the first place. Meanwhile it won't take much to start sinking tube wells in the paleochannels and pumping out water from aquifers which have at present no natural recharge potential. Undue haste in exploiting this resource might degrade the system beyond repair. I am not at all certain given the water scarcity in this part of Rajasthan that politicians will show the necessary patience until science determines a sustainable water management plan. Global warming is expected to reduce the supply of water to north Indian rivers from Himalayan glaciers in the future. A planning commission report on water resources which did not take into account the projected shortfall from glacial sources finds

“Currently, total water use (including ground water) is 634 BCM (billion cubic metres), of which 83% is for irrigation. The demand for water is projected to grow to 813 BCM by 2025 and 1447 BCM by 2050, against utilisable quantum of 1123 BCM – 690 BCM from surface water and 433 BCM from ground water. Clearly, the overall demand will outstrip availability in another 35 to 40 years, while ground water in particular will come under even greater pressure in the intervening years.”

Global warming will likely make the situation even worse than the official projections. The non-renewable water resources under the Thar desert will gain even more importance given the expected scarcity of water resources in the Gangetic plains. They will have to be managed carefully.

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