In Summary
  • Cattle are estimated to consume an average of 50l/day while irrigation will utilise 60, 90 and 120m3/ha/day for drip, overhead and surface systems.
  • The filters are designed to reject the first flush of water from the roof to prevent debris getting into the reservoir.
  • Given the example of a farm with 10 dairy cows, the above harvested water can take about five months to water the animals.
  • To plant napier in the round holes, separate top-soil from sub-soil, mix a debe of top-soil with 1 to 2 debes of farmyard manure and put into the pits. For the rectangular pit, put the top-soil/manure mixture for every 3 feet length.

With the ongoing rains, water is in plenty and one can be forgiven for thinking that it will always be the case.

However, as it rains, this is the time to harvest water. Rainwater harvesting can be ex situ (collecting run-off water into containers and reservoirs) or in situ (involves increasing soil and water-holding capacity technologies).

A farm water audit will assist in estimating how much water to collect. The audit should include water for household use, livestock, crops and fish use where applicable.

Local estimates indicate water requirements for household use stand at 50-150l/head/day for rural and urban areas respectively for people with individual connections and 15-20l/head/day for rural and urban households without connections.

Cattle are estimated to consume an average of 50l/day while irrigation will utilise 60, 90 and 120m3/ha/day for drip, overhead and surface systems.

A dairy farmer with 10 animals for example is expected to directly consume an average of 50l/day/animal and another 50l/day/animal indirectly through parlour cleaning and washing of tools and equipment, translating to 100l/day/animal and 1,000 liters per day for the 10 animals. In a year, such a farmer will require 365,000 litres.

Estimating the amount of water that can be harvested

Four factors to consider when estimating the amount of water one can harvest from a roof include rainfall and the roof (whether flat or pitched; corrugated or not) and surface area; the roof run-off and the filter efficiency.

The filters are designed to reject the first flush of water from the roof to prevent debris getting into the reservoir.

However, a quick estimate especially on the relatively small structures found at the smallholder farm level is carried out by using the rainfall and the roof area.

In this formula, annual rainfall in mm (the rainfall figures for a given geographical area are available from the meteorological department) is multiplied by the roof area in square meters (m2) to give the amount of water in litres that one can harvest from a given structure.

For every roof area of 1m2 and 1mm rainfall, you harvest a litre of water. For example, if you have a roof 20m long and 5 meters wide, the area is 100m2 and the rainfall in the area is 1,500mm, then you expect to harvest 150,000 litres.

Given the example of a farm with 10 dairy cows, the above harvested water can take about five months to water the animals.

This is of course minus the household use. The water harvesting efficiency is improved by clearing and repairing the roof, gutters and storage tanks just before the beginning of the rainy season.

Cleaning of the storage facilities is especially important where no filters have been installed. Further, closing the filters at the start of the rains diverts/discard any dirt thus increasing the water retention capacity of the storage unit. It is also assumed that the roof is of non-pervious material such as iron sheets.

Rainwater harvesting on its own is not enough, efficient use of water coupled with reduced wastage assures one the value for money invested. Wastage channels include leaking pipes, taps and hosepipes; dripping reservoirs, evaporation and seepage from open unlined ground receptacles.

Water storage

Some simple water storage facilities for smallholder farms include plastic, cement and metallic water tanks, drums and jerry cans, ponds similar to fish rearing ponds, installed underground water tanks and pans.

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