How to convert rainfall data into litres of water

 By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

28 September 2014

For the third time this year, I gave a talk to the Class 8 pupils at the Nairobi Primary School last week. The school is planning to start harvesting rainwater to supplement the supply from Nairobi Water Company. So I thought the children can help in estimating the volume of water that can be collected.

Thus my talk was about converting rainfall data into volume of water. The first step was to understand how the volume of an object is calculated. In the case of a cuboid we get it by multiplying the width, the length, and height.

If we have a block measuring, say, 1.4m wide, 1.5m long and 0.5m high, then its volume is simply 1.4 x 1.5 x 0.5 = 1.05 cubic-metres. We immediately notice that “1.4 x 1.5” is actually the area of the base of the cuboid. Therefore, whatever the shape of a block, as long as the vertical walls are parallel, the volume is simply the base area multiplied by the height.

Now, when the weathermen report that the rainfall on a particular day was, say, 10mm, what they are telling us is that if we had put out a vertical container in the rain, we would have collected 10mm of water. The size of the container does not matter, as long as it has vertical sidewalls.

My talk was in the school hall which measures about 25m by 10m. So we wondered: if the roof was removed and then we got 10mm of rainfall, how many litres of water would be collected? The base area is 25 x 10 = 250 square metres. To get the volume of water we multiply this area by the 10mm height.

But don’t rush for the calculator just yet! We must first convert the millimetres into metres; thus 10mm = 0.01m. Therefore, the volume collected in the hall would be 250 x 0.01 = 2.5 cubic metres. Now one cubic metre is equal to 1,000 litres, so the hall would collect 2,500L under 10mm of rainfall!

You should notice and interesting relationship connecting 250sq-m, 10mm and 2,500L: Can you see it? OK; it is 250 x 10 = 2,500.

In other words, when the weathermen report rainfall data in millimetres, what they are actually telling us is the number of litres of water we got per square meter of ground.

So, all the pupils needed to do was to take measurements to the building that the school wants to collect water from and calculate the base area of the roofed sections. However, it is not a rectangular block so they have to apply some geometrical knowhow.

With the area at hand, the next step is to get the average rainfall data for Nairobi and use this to calculate the month-by-month rainwater collections. I got this information from the Kenya Meteorological Department and gave it to the pupils. As at the time of writing this article, I was still waiting for the results … but my own estimation is that the school can get at about 500,000L per year from that single building.