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< prev - next > Agriculture Irrigation KnO 100426_Underground rainwater harvesting (Printable PDF)
Rainwater storage tanks are more important in semi arid areas than any other areas because
they are useful in storing limited water supplies for agricultural purposes. In semi-arid
regions, where precipitation is low or infrequent during the dry season, it is necessary to store
the maximum amount of rainwater during the wet season for use at a later time, especially for
agricultural supply. One of the methods frequently used is the underground storage tanks.
The in situ technology consists of making storage available in areas where the water is going
to be utilised. All rainfall-harvesting systems have three components: a collection area, a
conveyance system, and a storage area. In this application, collection and storage is provided
within the landscape.
Tank design criteria
Consider five main factors when designing your underground tank.
1. Seepage
It is very important to store rainwater and not to lose it! The tank should have a durable,
watertight, opaque exterior and a clean, smooth interior. A common method used in rural
areas is puddling as shown in picture 2. Below ground tanks must also be plastered well and
correctly installed, otherwise they can collapse.
2. Evaporation
All storage tanks should have a roof made from locally available materials as indicated in the
material list. A tight fitting top cover prevents evaporation, mosquito breeding and keeps
insects, rodents, birds and children out of the tank. A suitable overflow outlet(s) and access
for cleaning are also important.
3. Length of the dry spell
This is important in determining the size and number of tanks to be constructed. The longer
the dry spells the more water you would need. Sometimes sophisticated calculations are
involved, but these tend not to take into account human behaviour and the willingness to use
water if it is available and not to conserve it for future use, in the hope that the dry spell will
soon be over.
4. Daily usage
This is related to the above factor but ensure that the crops are hardened to withstand some
stress during the dry spell. Determine how much area you would want to save in case of a very
long dry spell lasting more than three weeks.
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