Water harvesting is the collection and storage of rainwater for production or domestic use.
All water harvesting systems comprise a catchment area and a storage component. Storage can be either short-term or long-term.
Short-term storage techniques are usually for crop, fodder, pasture and tree production, whereas long-term storage techniques are for domestic and livestock water supplies.
Most of the materials used in water harvesting systems are readily available.
The oval tank
One can get a gigantic 100,000-litre tank depending on site soil profile. It takes four days of rains under a reasonable size roof to fill this tank.
It is enough to sustain an 8x15m greenhouse for three months, which, in most cases, is the estimated dry period, making a smooth transition through seasons.
Unlike the common ferroconcrete underground water tanks that are prone to irreparable frequent cracks, developing as a result of inevitable geodynamics, the oval water tanks are durable, are of low cost and rarely crack as they have no straight walls.
In an event that oval tanks develop a crack, they are easy to repair. The tanks are shaped like an egg, the most stable and strong shape with low surface area requirement.
That is why they can be installed underneath the driveways, paved compounds, in the parking lots, backyards, school compounds, poultry farms and any other places where space is limited.
To keep out the dirt from the roof after a dry spell, the tank can be fitted with a first-flush device to flush the first rain water that washes the dirty roof for about five minutes and later allows the clean water to remain in the tank.
This makes it suitable for domestic use. The tanks can also be used for collecting runoff water for irrigation.
A homestead can set up an oval tank of at least 1,000 litres. A water tank is buried into the ground and gutters connected from the roof of the house to it.
Rock catchments
These are reservoirs located on bare rock surfaces with sufficient catchment areas to capture rainwater. The reservoirs are constructed using brick walls.
Then gutters are constructed on the rock surface at a gradient to direct runoff to the reservoirs.
Rock catchments can be used in parts of Karamoja or any other region with rocky hills. The cost of setting this up depends on the size and materials of the tank, plus the distance from the rock catchment.
Rooftop rainwater harvesting
This technique involves collecting rainwater from the housetop to storage tanks, using gutters located at the edges of the roof. The cost for setting up a 2,000-litre system is about sh2m.
Surface run-off harvesting
This method is mainly used in collecting water for irrigation. Water flowing along the ground during the rains is collected into a tank below the surface. The tank is constructed using bricks, which are plastered with cement.
During storage, it is important to incorporate effective water conservation methods by reducing evaporation and adopting water efficient irrigation techniques.
It is an easy to-adopt and profitable technology. The average cost of the system is sh1m, excluding a pedal water pump, which costs sh500,000.
How to harvest surface run-off
The tank should be close to the area of cultivation to ensure ease of irrigation.
The tank should not be near the house or paths to prevent people from drowning in it. Construct a fence around the tank.
The opening of the tank should be to the direction of the flow of rainfall.
It is not advisable to obstruct patterns of natural flow of water as there is a possibility of mud and other waste getting into the tank. (The mud filters help only when the water flows directly through them).
Clear the selected land thoroughly. Flattening the land is important for ease in taking measurements.
Determine the quantity of water required for irrigation purposes
The tank should not be more than 1.75m deep in order to withstand the pressure of the water. Low depth makes cleaning and use of the tank easier.
The tank should have a slope of about one foot from the periphery to the middle of the tank. This helps to absorb the water pressure.
There are usually various waste items in flowing water. Mud, sand and gravel deposits in the tank will lessen the quantity of water that can be stored in the tank. Therefore, mud-filters are used to reduce the flow of waste items into the tank. This system can last 10 or more years.
Open surface rainwater tanks
In areas where good water quality is needed for domestic and animal consumption, but the roof catchment is too small to collect sufficient water, open surface water tanks can be constructed.
These can, however, only be constructed in areas that receive good rainfall amounts to ensure collection of enough water.
The design is based on the precept that for every 1mm of rainfall, 1m3 of water is collected in 1m2 of surface. Therefore, a water tank constructed with a surface area of 24m2 will collect 24m3 of water from a storm of 1mm rainfall.
These tanks provide clean water for a variety of purposes, including domestic use.
Keeping the tank clean
To keep out the dirt from the roof the oval tank can be fitted with a first-flush device to flush out the first water for about five minutes and later allow the clean water to remain in the tank
