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Be smart, harvest rainwater now and enjoy during tough times

Friday April 13 2018

A farmer fetches rainwater collected into a tank from the roof of his house.

A farmer fetches rainwater collected into a tank from the roof of his house. Four factors to consider when estimating the amount of water one can harvest from a roof include rainfall, the roof and surface area; the roof run-off and the filter efficiency. FILE PHOTO | NMG 

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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.

The latter are structures developed through excavation or natural depressions that retain water below the ground level. The excavated ground is also compacted to minimise seepage.

With good planning, these facilities are affordable to smallholder farmers.

Water quality
Harvested rainwater is relatively clean for household, livestock and agricultural use. However, there are issues that can make such water unfit for the above uses or might reduce the water quality.

These include the roofing material, the surrounding environment plus the presence of filters. As for roofing materials, asbestos and some tiles makes the water unfit for farm use due to corrosion and contamination of the water with the toxic roofing material.

As for the environment, water collection in the vicinity of industries or heavy traffic poses a problem known as acid rain.

The acid rain comes about when fumes containing carbon dioxide or sulphur dioxide from nearby factories/heavy traffic are mixed with rainwater to form carbonic or sulfuric acids.

This produces water that is too acidic for household or farm use. While water from the first two sources might not be used at all, water that is contaminated by animal and vegetable material might not be toxic for farm use.

However, purification of the same especially through filtration or boiling for purposes of human consumption is recommended.

Water conservation technologies

One water and soil conservation technology is the tumbukiza method of planting napier grass. This is a non-conventional farmer initiative that is practised in low and medium altitude water stress areas.

It involves digging round or rectangular planting holes. The round pits should be 2ft in diameter and 2ft deep and the spacing between the rows should be 2ft apart.

The rectangular pits should be 2ft deep by 2-3 ft wide. The lengths of the pit can vary depending on available land. The pits should be 3 feet apart.

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.

Leave about 6 inches unfilled space at the top of each pit. Plant 5-10 cane cuttings or single root splits in round pits. In the rectangular pits, plant 5 – 10 cuttings or single root splits for every 3 feet length.

One acre will require about 11,000 canes. Regularly uproot weeds from the holes and plant other fodders such as sweet potato vines in the spaces between the holes to maximize on land utilisation.

One acre of land can produce enough napier to effectively feed at least two cows for a whole year. This forms the basic feed for dairy cows with a crude protein (CP) 7-9, dry matter (DM) 22, and crude fiber (CF) 30.

Feed the dairy animal at 3 percent DM of its body weight if napier is the only feed being used or at 50-60 per cent of the feeds while the balance is made of concentrates.

Mixing the napier with other roughage such as grass, oats, lucerne and common vetch hay will yield better results.

Reach Dr Mwirigi at [email protected]