International Rainwater Cistern Systems Conference
Khon Kaen, Thailand
- January, 1987
Section C: Design
An Assessment of Roof and Ground Catchment Systems in
John E. Gould
35, Chaucer Court
28, New Dover Rd
Kent CT1 3AU
Botswana is a semi-arid country with mean annual rainfall
varying from less than 250mm in the southwest to more than 650mm in the north.
The most common domestic water sources consist of traditional hand-dug pits in
sand rivers and modern piped borehole schemes based on deep groundwater
supplies. The low rainfall and its highly seasonal nature make Botswana a
marginal area with respect to rainwater catchment systems' development.
Nevertheless, the low population density and multiple dwelling places resulting
from Botswana's unique triangular migration system, make the provision of
alternative improved water sources in rural areas difficult and expensive.
Using information gathered from an extensive questionnaire
and technical field survey the current and future potential development of
rainwater catchment systems was assessed. The analysis of mean monthly rainfall
data from 10 stations using a computer modeling technique revealed that for all
the stations the most effective storage tank capacity should have a volume
equivalent to approximately 40%(0.4) of the collectable annual roof runoff.
This tank size would maximize the supply while minimizing the costs and could
supply about 70%(0.7) of the annual roof runoff with 95% reliability. Owing to
the small size and scarcity of corrugated iron roofs in rural Botswana roof
catchment systems are generally only feasible as a supplementary supply.
Bacteriological examination of rainwater samples showed that while roof tanks,
if properly maintained, provide high quality water, ground tanks were
susceptible to considerable contamination.
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