Elementary Errors in Analysing Water

Water analysis usually starts with estimates of water availability. According to India’s Central Water Commission:

"Precipitation (including snowfall) is the source of all water on the earth. The average annual precipitation over the country is estimated at 4000 BCM of which a part goes towards increasing ground water storage, a part is lost as evapo-transpiration and the remaining appears as surface water. The water resources potential of the country which occurs as natural run off in the rivers is estimated as about 1869 BCM, considering both surface and ground water as one system. Due to various constraints of topography, uneven distribution of resource over space and time, and geographic [sic] only about 1122 BCM of the total potential can be put to beneficial use, 690 BCM through surface water resources and 432 BCM by ground water."(pg. 13)

These numbers are widely used (see CSE and Iyer (2007)) and rather uncritically.

Note that just over 25 % of the precipitation is estimated of being put to beneficial use. So even a small increase in the utilisation percentage can lead to a big jump in available water.

The paragraph quoted above, however, is factually inaccurate, misleading and incomplete. To begin with the statement about precipitation being the source of all water is erroneous as oceans (97%), glaciers and polar icecaps (2.4%) hold the bulk of surface water. See here. Shallow and deep aquifers (in the aggregate) hold enormous quantities of freshwater.

The CWC statement can perhaps be re-read to indicate an estimate of sustainable water availability as water from rains is replenished every year. However, even so their estimate is incorrect, as we will see below.

Firstly, rainwater which seeps in to the ground is also (potentially) available for use so it should not be deducted from total precipitation. Secondly, India has commitments to supply (let water flow) to neighbouring countries and in turn it receives water from outside its boundaries. The net figure has to be deducted from overall precipitation. Finally, flow of water in rivers and out in to the seas serves many critical ecological and socio-economic functions, so even if all water could be captured and stored one wouldn’t do so.

So beginning with the annual precipitation over the country, a proper analysis must deduct the quantity of evapo-transpiration (strictly speaking this is the only quantity not available for use) and India’s net commitments to neighbouring countries. Water, which needs to flow to the seas to fulfil ecological and other functions, too needs to be subtracted.

Potentially all other water is available for use. However, it is not quite practicable to store all the water that falls as precipitation and much of it flows to the seas. The storable potential is not fixed and has and can be increased. Note, however, that this increase in storage capacity doesn’t necessarily have to come from the construction of large dams. Small storage structures and increasing ground water storage through increase in percolation of rainwater through the soil, to name just two measures, can be just as effective.

Even this analysis is incomplete. For water can be and is used again and again. This is true of the three major water-using sectors – domestic, agricultural and industrial. Return flows, as they are termed are extremely important and the bulk of water used is returned back to the hydrological cycle. Most of this happens naturally but can be enhanced by human efforts. A multiplier is at work here and recycled and reused water may increase manifold the effective utilisable water.

For an extremely illuminating discussion on the above see IWMI especially the section on Water Balance Analysis and Appendix A.

Finally, water supplies can be augmented by desalination of seawater. This is, of course, limited to coastal location and largely for industrial and domestic use but with a coastline of 7,500 kms this need not be a trivial source of supply. Such water is now available at very competitive rates without severe environmental damage (this is not the place to go into details, but I will discuss this in a later post). And rather than take out all the water from our rivers it may be far more sensible to let water flow into the seas and then desalt it.

The CWC analysis (which forms the basis for many others) seriously underestimates water availability in the country. I don’t have the model or the data to estimate the revised numbers but they must surely be much more than present CWC estimates.

{Note: BCM is billion cubic metres. I cubic metre = 1,000 litres. India’s average annual rainfall is 1200 mm (1.2 mts) and multiplied by the area of 328 million hectares gives an approximate figure of 4,000 BCM since a hectare = 10,000 sq. mts. Also 4,000 BCM=400 million hectare metres=4,000 cubic kms.}