In one week, Nairobi wasted 150 million litres of water

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

05 March 2006

Our prayers have been answered! The rains have started and hopefully, the water rationing in Nairobi will be suspended as the reservoirs begin to fill up. But all the same, it is painful to watch all this rainwater going to waste on the streets. If it was harvested, would it have alleviated the current shortage?

First, let’s find out how much water the rains have poured. When it is reported that there was, say, 10 millimetres of rain, it means that if the water was collected in any container with vertical sides and a flat bottom, the level would rise to 10mm. This height does not depend on the size of the container. Large and small containers would fill to the same height even though the larger one will have more water.

If that is hard to understand, do this simple experiment. Fill a glass with water and measure its height. It will be about 10 centimetres or so. Now pour the water into a regular sized sufuria and check the level. Is it still 10cm? No! It is probably less that one centimetre. But the volume of water is the same.

Now, this last week Nairobi has received about 50mm of rain. How much water was this? Consider the Central Business District, that is, the area bounded by Uhuru highway, Haile Selassie avenue, River road, and University way. This area measures about three square kilometres.

One square kilometre is equivalent to an area measuring 1,000 metres by 1,000m. That is, one million square metres. Now if we have an area of three million square metres and filled it with just 50mm (5 centimetres, or about two inches) of water, what volume would we have?

The answer is 3,000,000 multiplied by 0.05, equals 150,000 cubic metres. That is, 150 million litres – in less than one week! And all this water went down the drain, literally. Let’s put that figure in perspective: every day, Nairobi residents consume about 480 million litres of water. 150 million in five days is 30 million litres per day, or 6.25 percent of the daily supply. Before you say that this is a small portion, remember that it was collected in just a small part of the city centre.

Let’s bring this down to a more practical level. After all, no one is going to be fetching rainwater from the dirty streets. The medium size house has a roof area of about 40 square metres. Nairobi gets about 900mm of rain annually. Therefore the house collects about 36,000 litres of rainwater every year. This is equivalent to two to three months of average consumption. That is, 30 percent of the water requirement in the house.

It makes you wonder, should the building standards be changed to state that every house in Nairobi must have a rainwater harvesting system? I think it we should give it serious consideration.