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< prev - next > Environment and adaptation to climate change KnO 100025_Runoff rainwater harvesting (Printable PDF)
Run off rainwater harvesting
Practical Action
As an example, the rain fall pattern in Suriyawewa divisional secretariat, in Hambantota
district is shown in figure 1, which clearly highlights that rain is mainly received during the
four months; March - April and October - November.
“Not even a single raindrop should be allowed to flow into the sea without having made use of
it for the benefit of the people” - King Parakramabahu the Great (1153-1186AD),
Pollonnaruwa Era
Sri Lankan ancestors were successful in harvesting rainwater through a cascade system of
massive tanks. The following data (table 1) indicates the scale of ancient irrigation tanks in
which rainwater harvesting was an integral part of their irrigation management.
Name of the Tank
Parakrama Samudraya
Minneriya wewa
Kantale wewa
Nuwara Wewa
Extent (acres)
Table 1: Major constructed tanks in the Dry Zone Sri Lanka tanks
These tanks were built to irrigate vast areas and support a large population. The rainwater
tanks described in this brief are on a domestic scale, that can store about 10,000 litres of
rainwater at a time, and sufficient to irrigate a quarter or half an acre of land, to successfully
grow 35-40 perennial crops such as `Jak` (Artocarpus nobilis), coconut (Cocos nucifera),
pomegranate (Punica granatum), orange (Citrus spp) and mango (Mangifera indica).
What is rainwater harvesting?
Managing, controlling and making use of rainwater in-situ or within the vicinity of rainfall is
termed as rainwater harvesting.
Run off rainwater harvesting
In this method of collecting rainwater for irrigation, water flowing along the ground during the
rains will be collected to a tank below the surface of the ground. The tank is constructed using
bricks, which are coated with cement. During storage, it is important to incorporate efficient
and effective water conservation methods by reducing evaporation and also by adopting
efficient irrigation techniques. It is a very ‘easy to adopt’ technology proven with many
communities in the country that if used properly can be very profitable.
Step-by-step procedure of run off rainwater harvesting
Selecting a location for the construction of a rainwater harvesting tank
Observe the direction of the surface flow of rainwater in the land.
Even though some believe that such tanks should be constructed in the lowest lying
area of the land, this is not essentially so. Due to the seasonal patterns of rainfall and
the high intensity of rains received in Sri Lanka, it is possible to fill a 12,000 litre
capacity tank without much difficulty.
The tank may be subjected to cracks due to the root zone activities (i.e. ramification),
therefore, it is advisable not to construct the tank in close proximity to large trees.
The tank should be close to the area of cultivation to ensure ease of irrigation.