A house in Nairobi cannot collect enough rain water to sustain itself
By MUNGAI KIHANYA
The Sunday Nation
Nairobi,
03 March 2019
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 60feet). 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 is 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 pe 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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