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Meltwater and MAT (mutually assured thirst)

One of the things we have been taught by Hinduism, the majority religion of India, is that everything is connected to everything else. Two recent papers about groundwater extraction in north India offer a fine illustration of the truth of this proposition.

Matthew Rodell and colleagues, writing in Nature, analyse data from the GRACE satellites to show that in three north-western states of India more water is being pumped out of the ground than is being put back. The net extraction rate over six years is 18 gigatonnes/yr, give or take five, which translates to an equivalent depth of water of 40 mm/yr. The qualifier “net” is important because on top of the natural processes of recharge during the monsoon months there must be some recharge from the glacier meltwaters originating in the Himalaya to the north. Rodell and colleagues estimate this recharge as 3 Gt/yr. I reckon it is likely to be somewhat more but the point is that we have to take them together. The conversion of the two non-renewable resources, glacier ice plus groundwater, to ocean water makes an even bigger problem.

Most of the water irrigates cropland, from which it evaporates or finds its way into the rivers. One way or the other, most of it ends up in the ocean – which is what makes it non-renewable.

VM Tiwari and colleagues published on the same subject, apparently independently but relying also on GRACE, in Geophysical Research Letters. They focussed on the whole northern subcontinent, and found an extraction rate of 54 Gt/yr, give or take nine, equivalent to 20 mm/yr of depth of water removed. Add to this most of the 13 Gt/yr of whole-Himalaya loss from glaciers and you get an awfully big problem, affecting 600 million people according to Tiwari and colleagues. (This glacier number is so rough that it doesn’t even have an error bar, and it is not clear that the ice loss has been separated cleanly from the groundwater loss.)

One striking thing about these interconnections is that the two papers are the first firm evidence for significant contributions to sea-level rise from “terrestrial storage”, that is, aquifers, dams and the like. Up to now it has been conjectured that the terrestrial source is small, but it looks as though we will have to rethink that.

600 million is an awful lot of people. Rodell and colleagues quote the New York Times to the effect that most middle-class residents of New Delhi do not have a dependable source of clean water, from which I infer that a) all upper-class residents probably do, and b) probably no lower-class residents do.

Whatever its source, the water we are adding to the ocean spells trouble for people living near sea level. This includes the members of the government of the Maldives, who now hold their cabinet meetings wearing scuba gear; the 60,000-odd inhabitants of the coral islands of the tiny Indian union territory of Lakshadweep, just north of the Maldives; and the many more inhabitants of the delta of the Ganga-Brahmaputra in Calcutta and neighbouring Bangladesh, where it is rainy and swampy enough that they don’t have much call for irrigation but they inherit the problem of groundwater extraction in Rajasthan, Haryana and the Punjab by a roundabout route.

So another striking thing about the interconnectedness of Himalayan glacier mass balance, Indo–Gangetic extraction of groundwater, evaporation from irrigated fields, lack of clean water in New Delhi, and the impending submergence of the Maldives, Lakshadweep and the delta of the Ganga-Brahmaputra is that it also demonstrates that the Hindus, Moslems, Christians, Sikhs, Buddhists, Parsees and other adherents of the Indian subcontinent are all connected to each other – and to me.

Posted by Graham Cogley on December 14, 2009 3:00 PM |

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Comments (35)

  • 1 Vinny Burgoo December 15, 2009 1:33 PM

    You are forgetting that the Ganges/Brahmaputra delta isn't water-logged all year round. In fact a lot of water (25 cm/year by one esimate) is extracted for irrigation in the delta during the dry season, mostly for growing rice. Not all of this is replaced during the monsoon. A recent paper estimated that groundwater levels in parts of the delta are declining by 10 to 50 cm/year. They are rising slowly in most coastal regions but this is probably due mostly to subsurface sea-water intrusion - not good.

    So the inhabitants of the delta are not quite the innocent victims of someone else's water craziness, as you suggest.

    (Nor are the Maldivians entirely innocent, climatewise. Their economy is driven by jet travel.)

    And another thing ... Despite man's best efforts to raise sea levels, increase subsidence, reduce sediment deposition and destroy natural storm-surge barriers, the delta is still growing.

  • 2 Graham Cogley December 15, 2009 5:39 PM

    Dear Vinny - I am not forgetting it because I never knew it in the first place! Thank you for clarifying this aspect of a subject about which I don't know very much. But don't your points about lack of innocence in the Bengalese and Maldivians help to make my point that we are all in this together? Are the people in West Bengal and Bangladesh supposed to go thirsty, or die of arsenic poisoning; and how are the Maldivians to make a living if tourists stop going there?
    One thing by which I have been shocked recently is learning that the Indus has become a seasonal river. At the Kotri Dam, near Hyderabad, there were no days with zero flow in the 1950s. Nowadays the flow is zero or nearly zero except in August and September. See ch.16 in "Large Rivers: Geomorphology and Management", A. Gupta, ed., 2007, Wiley, a book which also has chapters on the Ganges and Brahmaputra.