Tuesday, June 01, 2010

Some rudimentary water-related calculations

I posted this status on Facebook the other day: "If you take the number of centimeters of rain that fall on Bangalore or Pune every year, multiply by the land area, and divide by the population, you get so much water per person that we should all have enough and more of it every day.
(This is what I've been doing today afternoon, aided by the numbers from Wikipedia)"

A couple of folks wanted me to post the calculations, so here's a post explaining it.

According to this site, one cubic metre of water is equivalent to 1000 litres. So, if your city has an area of one square meter (small city, I know), and it has one meter of rainfall in a year, the city is getting a thousand litres of water. I.e., one centimetre of rainfall over one square metre is 10 litres of water.

Wikipedia lists Bangalore as having an area of 710 sq. km. approximately. 1 sq. km. is about 10^6 square metres. Therefore, if one centimetre of rains falls on Bangalore, we have 10 litres * 710 * 10^6 of water. That's 710,00,00,000 or 710 crore litres of water.

The official government site on Karnataka lists Bangalore as having about 900 mm of annual rainfall, or about 90 cm. That means that about 63,900 crore litres of water falls to the ground within Bangalore city limits every year.

Again, Wikipedia lists Bangalore's population at 65 lakhs. 63,900 crore (i.e. 63,90,000 lakhs) divided by 65 lakhs is 98,307 litres of water per person per year. In other words, about 270 litres per day.

This set of statistics show that only about 9 developed countries show a water use of more than 270 litres per day. India is way down the list, at 150 litres or so.

What it all boils down to (pardon the pun), is that if Bangalore can hold on to all the water that falls in its own territory every year, every citizen will have all the water he or she needs for every purpose. I haven't even considered all the water from the Kaveri river schemes and so on, and the much lower population density of non-urban areas in Karnataka.

Just hold on to the rain - using lakes, by letting the earth absorb the water, by helping the water table rise, and you will solve your water problem for a very, very long time.

[This is a rather naive calculation, I know, but the overall logic sounds pretty fair to me. Try it for your own city]