What do numbers mean in the flow of water? An explanation from S. Vishwanath
The way China has approached its urban water supply projects is something to learn from. They have adopted the PPP model (Public-Private Partnership), with tariff setting and the resource in control with the government. One of the ways for India to pursue its urban water supply projects is to learn from the Chinese example: follow the good and discard the bad. This article is however on the peculiarities shown by various ‘water using communities’ on the way they use measures of water. China regularly uses tonnes as a measure for water. A tonne of water is of course 1,000 litres or as we would say in India a cubic metre of water or sometimes a kilo-litre of water. That is because a litre of water weighs 1 kg.
The KRS dam on the Cauvery had only 1.03 tmc ft of water. This is sufficient for Mysore, Bangalore and Mandya for only 10 days. This was one report in the press. 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. Now we have 60 tmc ft. of water in KRS and the water supply to Bangalore is assured for some time to come.
The neighbour drilled a borewell for two days. “I struck water at 750 ft. and got 2 inches of water,” he says. The rain gauge at 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 tmc ft is one thousand million cubic feet of water. This is 28316.85 million litres of water. If there was 1 tmc ft 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 would be available to the city.
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 into 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 1,500 cusecs of water was released in 2 days.
A common way to describe the yield of a new borewell 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.
Litres per hour flow
Using a rather complex formula, calculations are made. Here is an approximation of the litres per hour flow:
1/2 inch - 95 litres per hour
1 inch - 600 litres per hour
2 inch - 3,400 litres per hour
3 inch - 300 litres per hour
4 inch – 19,100 litres per hour
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 = 1,000 litres of water or as the Chinese would say 1 tonne of water.
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.