Rainwater harvesting
Practical Action
scarcity. As storage is expensive, this should be done carefully to avoid unnecessary expense.
This is a common scenario in many developing countries where monsoon or single wet season
climates prevail.
The example given here is a simple spreadsheet calculation for a site in North Western
Tanzania. The rainfall statistics were collected from a nurse at the local hospital who had
been keeping records for the previous 12 years. Average figures for the rainfall data were
used to simplify the calculation, and no reliability calculation is done. This is a typical field
approach to RWH storage sizing.
The example is taken from a system built at a medical dispensary in the village of Ruganzu,
Biharamulo District, Tanzania.
Demand:
Supply:
Number of staff: 6
Roof area: 190m2
Staff consumption: 25 lpcd*
Runoff coefficient** (for new corrugated GI*** roof):
Patients: 30
0.9
Patient consumption: 10 lpcd
Average annual rainfall: 1056mm per year
Total daily demand: 450 litres
Daily available water (assuming all is collected) =
(190 x 1056 x 0.9)/ 365 = 494.7 litres
*lpcd : litres per capita per day
** Run-off coefficient values vary between 0.3 and 0.9 depending on the material of the catchment
area. It takes into consideration losses due to percolation, evaporation, etc.
***GI : galvanized iron, in some countries, known as tin roof
It is seen that on the average the daily available rainwater is sufficient to meet the demand.
However, we have to remember that rainfall does not occur uniformly throughout the year. We
must collect and store enough water in the rainy season to last the dry months to ensure
water availability throughout the year. The required volume of the storage tank may be found
by examining monthly rainwater collection potential or supply (based on average monthly
rainfall) against the demand. The cumulative shortfall in the dry months has to be met. In
this example, there is very little rainfall in Jun, July and August and moderate rainfall in May
and September in Tanzania. So we need to store about 115 days demand or about 52 cubic
m (450X115=51,750 L or 51.75 cu m). This is a rough calculation for sizing the storage
tank. Plotting actual rainfall data and demand in a cumulative manner can help in further
refining the tank size.
Rainwater quality and health
There are two main issues when looking at the quality and health aspects of DRWH:
Firstly, there is the issue of bacteriological water quality. Rainwater can become
contaminated by faeces entering the tank from the catchment area. It is advised that the
catchment surface always be kept clean. Rainwater tanks should be designed to protect the
water from contamination by leaves, dust, insects, vermin, and other industrial or agricultural
pollutants. Tanks should be sited away from trees, with good-fitting lids and kept in good
condition. Incoming water should be filtered or screened, or allowed to settle to take out
foreign matter (as described in a previous section). Water which is relatively clean on entry to
the tank will usually improve in quality if allowed to sit for some time inside the tank.
Bacteria entering the tank will die off rapidly if the water is clean. Algae will grow inside a
tank if sufficient sunlight is available for photosynthesis. Keeping a tank dark and sited in a
shady spot will prevent algae growth and also keep the water cool. The area surrounding a
RWH should be kept in good sanitary condition, fenced off to prevent animals fouling the area
or children playing around the tank. Any pools of water gathering around the tank should be
drained.
Secondly, there is a need to prevent insect vectors from breeding inside the tank. In areas
where malaria is present, mosquito breeding in the storage tank can cause a major problem.
All tanks should be sealed to prevent insects from entering. Mosquito proof screens should
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