Water consumption by forests - Chetan Pandit

I had asked a question about estimates of water consumption by forests, and this was published in IWP on 11February.

Today is 22 February, 10 days afterwards, and there is not a single reply to it.

On 14February I happened to be attending an official meeting where water availability and water use data was the topic of discussion, and the committee included a number of representatives from Civil Society/ NGOs. I raised the same query there too. As expected, some members took my query to mean as if this is a step towards questioning the very need for forests. I had to explain at length that just as estimating how much water is required for domestic use or irrigation does not mean questioning the very need for domestic use or irrigation, likewise, asking how much water is used by forests does not mean questioning the need for forest.

That done, the fact remained that no one in that committee also seemed to know how much water is consumed by forests. And after 10 days after the question was published on IWP, there is no reply on that forum either. This does not augur well for the water sector, because it means an important component of water budget is being ignored by the water resources analysts.

Some references were found on the internet, and these said - the annual water consumption by forests is about 1.25 to 1.5 times the annual evaporation from open surfaces. The evaporation in India ranges from 1500 to 3000 mm. Thus, the evapo-transpiration by forests would range from a minimum 1875 mm (1.25*1500) to 4500 mm (1.5*300). Taking the average value 3200 mm, that works out to 32,000 m3 from 1 Ha of forest, and 32 bcm from 1 mHa of forest. That, in any basin, is a significant quantum of water.

The annual water receipt in India is estimated as 4000 bcm; the annual freshwater resource 1869 bcm; and annual utilizable quantity is estimated as 1123 bcm. Suppose a forest is artificially planted on 1 mHa of area. The water it will consume, which for the time being can be taken as around 32 bcm, will it come from the slice 1123 to 1869 bcm or will it come from the 1123 bcm slice ? i.e. will it reduce 1869 to 1837, but keep 1123 unchanged? Or will it reduce 1123 also to some extent, and if so, how much. The answer to that is neither easy nor obvious. At this moment, all that can be said is – the question does not seem to be on the radar of water managers.

On the contrary, it is being advocated by some that forest cause increase in the rainfall. This is not supported by the science of meteorology. If forests caused more rains, then the increased rains would result in more forests, which would cause still more rains, which would result in still more forests, . . . . and all would be well. Unfortunately, nature is not that kind. A “rain forest” is a forest supported by the rain, and not a forest that supports rain.

We all are worried about the impact of climate change on water availability. But it seems that the impact of forest could be more than that of climate change, and more immediate. Water managers need to ponder on this question.

Chetan Pandit

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