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Rain Water: The Untapped Resource

by Harvest Money Editor
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After what seemed to be a long wait, the second rains are finally here. Farmers in some parts of the country had already planted beans, groundnuts and sweet potatoes, among others.

What these farmers did — planting before the rains, is called dry planting, and it is one of the ways farmers can adapt to changing weather patterns.

By planting early, farmers can take advantage of the first rains to help their crops to grow before the onset of the next dry spell. But while such techniques can be useful for seasonal crops like beans and ground nuts, perennial crops such as bananas, coffee and fruit trees need a different approach.

Farmers, therefore, have to think of low cost technologies for water harvesting. Water harvesting involves collecting and storing water during a period of abundance for future use. One can collect and store water during the rainy season, for use later in the dry season.

The main principle of water harvesting is to work with nature to harness the available water resources.

A farmer has to understand the water cycle, consider all available water sources, options for storage and take the availability of local materials and equipment for use in harvesting.

Any option the farmer settles for should address all these, as well as put into consideration the legal, environmental, economic, and social-cultural and management implications.

Water harvesting techniques

Rain water harvesting techniques are defined based on the distance between the catchment areas (where the water runoff is trapped for collection) and where it will be utilised (where the crops are).

Water conservation techniques such as contour farming and ridging, deep tillage, mulching and the use of farm yard manure which improve the soil’s water retention ability are referred to as in-situ (on location) water harvesting techniques.

Systems with separate catchments and utilisation areas adjacent to each other are referred to as internal or micro. In all these techniques, the catchments and utilisation areas are the same.

These include: Pitting These are small semi-circular pits (about 30 cm in diameter and 20 cm deep) dug to break the crusted soil surface. Farm yard manure is added in the pits thus permitting the concentration of water and nutrients. Seeds are planted in the middle of the pits.

Contour bunds

This system consists of small trash, earth or stone embankments, constructed along contour lines. The embankments trap water flow allowing deeper infiltration into the soil.

The amount of water stored in the soil profile depends on the height of the bund and it is recommended that contour bunds are used where ground slope is not more than 5% and soil depth is at least one metre.

The most popular water harvesting techniques are the external or macro catchments systems. They involve collecting runoff from large areas, including rooftops which are located a distance from where the water will be utilised.

The catchment areas usually have slopes ranging from 5-50%. Harvested water can be used on cropped areas which are either terraced or flat. When the catchment area is large and far from the cropped area, the runoff water is conveyed through structures of diversion and distribution networks.

Systems of water harvest

Hillside sheet runoff utilisation: In this system, surface runoff which occurs on hill-tops naturally collects on low lying flat areas.

Such run-off can be diverted along the way into areas where bunds have been constructed in the garden and allowed to form earth basins which assist in holding water and increasing infiltration into the soil.

The bunds are important if the farming area is not at the bottom of the landscape.

The water may also be directed through long deep canals made diagonally across the slope and placed with flat stones down both sides to help the water pass easily.

A farmer could place more flat stones on top as well and cover them with soil forming canals which deliver the water into collection pits. The water storage pits can be lined with sheets of black kaveera (polythene bag) to increase retention.

Harvesting flood water

This is a system that uses barriers such as watertight stone dams to block the water flow and spread it on the adjacent plain and enhance infiltration.

The watered area is then used for crop production.

Stream diversion

This involves diverting water from its natural stream and directing it to an arable piece of land.

Benefits

Rain water harvesting for agriculture has been practiced in semi-arid regions for thousands of years.

As the climate continues to change and seasons become less predictable, rain water harvesting is gaining in prominence in areas previously considered to have ample rainfall.

Seasonal variation in prices for agricultural produce is one indicator of production spikes during extended dry spells.

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