Ground Rainwater Harvesting – A general description of how it works.
Ground rainwater harvesting can be an excellent alternative to a borehole – especially if the water table is deep or the water is not suitable for human consumption.
For many rural communities a ground rainwater harvesting system with high capacity storage is a cost effective alternative to a drilled borehole – is easy to maintain – and far less likely to break down.
Ground rainwater harvesting involves the capture and storage of rain using the same principle as rooftop rainwater harvesting – except the rain is harvested from the ground instead of from a roof.
A moderately sloping area or hillside of approximately 100 paces X 100 paces is required to catch the rain. If the ground is flat it is possible to create an artificial slope using community labor. Shallow channels – no deeper or wider than a hand-span – are dug into the slope to guide water from the top to a collection point at the bottom of the slope. The channels guide the fallen rain – and all run in towards the collection point.
The collection point is a waist high U-shaped wall 10 paces wide at the open end with a smooth sloping cement floor inside the U. The deep end is at the closed end of the U and should be two hand-spans deeper at the closed end than at the open end.
A one inch hole is drilled approximately one hand-span up from the floor in the center of the closed end of the wall to allow the collected water to escape. The hole is covered with gauze. A pipe is fitted into the hole and directs the escaping collected water into the top of a cement sand and charcoal filtration box with a removable lid. The removable lid allows access to change the sand and charcoal as required. The top of the filtration box must be level with the floor of the collection point
An outlet pipe at the base of the filtration box directs the now filtered water into a properly constructed 200,000 litre capacity cement tank. The top of the tank must be level with the base of the filtration box. The tank should be fully enclosed with a cement manhole to allow access for cleaning or repairs. A secure tap is fitted to the tank to enable water to be accessed
The whole area – including the collection point – is enclosed using 4 strand post and wire fencing to keep animals out of the water collection zone – and trees are planted within the fenced area. Trees will assist in building up good ground cover over time and mean that the water delivered to the collection point will become increasingly cleaner.
This system requires about half an hour of steady to heavy rain to fill the 200,000 litre tank.
Additional tanks can be added over time using monies raised by charging a small fee for water.
Maintenance involves keeping the water collection point clean and free of any debris or settled mud – ensuring that the fence is always in a good state of repair and that animals cannot enter the water collection zone at any stage – keeping the pipes, taps and tanks in a good state of repair – changing the sand and charcoal as required in the filtration box – collecting a small fee for water to pay for maintenance and additional tanks – nurturing, tending and protecting the trees within the fenced area and not allowing them to be harvested for firewood.
This system can be completely maintained locally. Our experience is that the most effective management of these systems occurs when women have responsibility for their care, maintenance, management and development.
The cost of this system – with one tank – is approximately 750,000 KSH. Costs can be reduced by using community labor – providing clean sand and gravel for all cement work and providing stone for the masonry work locally. Technical specifications are available on request from the Africa Water Bank – although each system needs to be professionally surveyed to ensure adjustments are made to suit local conditions. For more information contact the AfricaWaterBank.