WORLD OF FIGURES: In Nairobi, your roof cannot collect adequate rain water

Two weeks ago, we found that an urban household uses about 100L of water per person per day when there are no restrictions. Unfortunately, none of our cities and towns are able to supply continuous, unrestricted flow. They all operate on a rationing programme.

So, the question arises: is it possible to collect enough rain water to satisfy the needs of an urban household? Well; it depends on the size of the house, the number of people living in it and the amount of rain falling in that region annually.

Suppose there are five people in the house; they need a total 500L daily. That comes to 182,500L per year. Can their house collect this much rainwater?

Nairobi gets about 1,000mm of rainfall in a year. This means that every square meter of horizontal surface collects about 1,000L. Therefore, to accumulate 180,000L, the house should have a surface area of 180 square metres.

Now, this is a very large area: it would have a foot print measuring about 10m-by-18m (that is, 33 feet by 60 feet). To be clear: that’s the area of the house — not the land on which it is built!

These days, the average area of a three bedroomed unit is about 100sq.m. Two- bedroom ones are around 80 sq.m. Therefore, it turns out that the average house in Nairobi cannot collect enough rainwater to serve the unrestricted requirements of the people living in it!

But I know there are very many families in rural areas who rely exclusively on rainwater. Does that mean that the above calculation is wrong?

No. There are two reasons for this: firstly, those areas where people rely on rain water have much more rainfall that Nairobi — some even up to double the downpour.

Secondly, rural folk use water a lot more sparingly that city dwellers. For example, they don’t have flash toilets.

Still; even if the house was large enough, the next challenge that the 1,000mm of rain falling in Nairobi is not evenly distributed throughout the year. About two-thirds falls between March and May and the remaining one-third between September and November. The other six months of the year are dry hence the need for storage.

Therefore, out of the annual requirement 180,000L, two-thirds (120,000L) will be collected between March and May. But, in those three months, a total of 45,000L (i.e. 15,000L per month) will be consumed in the house. That leaves 75,000L in storage.

In the period from June to August, the house will rely on the stored water and consume another 45,000L. That will leave 30,000L in the tank. Then from September to November 60,000L are collected and another 45,000L consumed. That leaves 45,000L — just enough to last from December to March.

It turns out, therefore that the house should be able to store at least 75,000L of rain water. That is the only way it can be completely reliant on rainwater. Is it worth it? That’s a story for another day.

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