Understanding water units

The KRS dam on the Cauvery has only 1.03 tmcft of water. This is sufficient for Mysore, Bangalore and Mandya for only 10 days. This is one report in the press today. We need 600 cusecs of water to flow in the Shiva Anicut for us to tap water into the Netkal balancing reservoir. We will then pump 1110 MLD into the city says the BWSSB according to another statement.

The neighbour drilled a bore-well for two days. I struck water at 750 feet and got 2 inches of water he says. The rain gauge on our office has measured 300 mm of rainfall so far since April. What do these numbers mean? How do we understand them in a common language? Here is an attempt to explain some of them.

1 Tmcft is One thousand million cubic feet of water. This is 28316.85 million litres of water. If there was 1 Tmcft of water in the dam and this was to be pumped equally, every day for 365 days in a year to the city, 77.58 million litres per day (MLD) would be available to the city.

By the way 1 cubic metre of water is 1000 litres of water

Cusec is a measure of the rate of flow still commonly used by the irrigation department. 1 cusec is one cubic foot of water flow per second.  It translates to 28.32 litres of water per second. If 1 cusec of water is released from a dam for the whole day 2.45 million litres of water would have flown in the river in the 24 hours. 

A common mistake made by many is to add up cusecs. If 600 cusecs of water was released for one day and 900 cusecs the next day it does not mean that 1500 cusecs of water was released in 2 days.

Borewell:  A common way to describe the yield of a new bore-well is to say that 2 inches of water was struck.  What does this ‘2 inches’ of water mean? It is actually the free, unrestrained flow of water from a Borewell over a 90 degree V notch.

Using a complex formula, calculations are made. Here is an approximation of the litres per hour flow

Height               Flow in litres per hour

½ inch                95 litres per hour

1 inch                600 litres per hour

2 inch              3400 litres per hour

3 inch              9300 litres per hour

4inch              19100 litres per hour

Rainfall: Rainfall is measured in typical standard rain gauge as prescribed by the Indian Meteorological Department. Typical measurements are taken at 8.30 a.m. in the morning and reported as mm of rain.

For those harvesting rainfall, here is a good way to calculate the volume of water falling on a roof. Suppose you have a roof area of 100 square metres and the rainfall on a particular day was 10 mm. The total volume of water that fell as rain on your rooftop would be 100 x 10 = 1000 litres of water.

Conclusion:  Knowing the numbers is a very important step in water literacy. Water literacy leads to water knowledge and better management of the resource. With years perhaps water wisdom will also come.