Technical Information Online

Dead-Level Contours

Technical Briefs and Manuals: English

Introduction

The semi-arid regions experience long dry spells and erratic rainfall less than 500mm per annum. This is often poorly distributed over three months. In addition the areas experience high temperatures of about 38° Celsius resulting in high evaporation rates. Where soils are poor there is also deep percolation. Water received therefore quickly disappears within a few hours of a rainstorm. Farmers in Manama Communal Area in Gwanda district of Zimbabwe have modified the standard contour ridges and the infiltration pits to suit these conditions and named the technique - Manama Infield Rainwater Harvesting Storage Facility. This was after the realisation that farmers were losing water after harvesting it, resulting in total crop failures especially during the dry spells.

What is the Manama In-field Rainwater Harvesting Storage Facility?

This consists of a dead-level contour with storage underground tanks made from local materials. The capacity range from 5m3 to 20m3 depending on farmer preference. A dead-level contour is a channel dug in the field with a zero gradient or slopes, so that water does not flow but is harnessed and stored in the channel. The underground storage tanks are constructed in such a way that they are covered to reduce evaporation and rammed to reduce deep percolation.

Material requirements to make the facility

An A-Frame to peg and grade the contour channel, 1 rammer locally made from dry wood, Ant-hill soil (termitaria), water, 5 x 1,5-2m long dry poles, pick, shovel (depending on labour available), 10 farm/earth bricks, Tape measure or ruler, string 2m, knife/hacksaw/small axe, 50 wooden pegs, old Hessian or plastic bags.

To find out how to make an A-Frame, read our Technical Brief on A Frames.

How to construct the Manama in-field rainwater harvesting storage facility

Marking the contour

• Use the A-Frame to mark contours.
• Choose a place on the slope to begin. Stand the A- frame up and mark where the first leg stands with a peg or large stone.
• Keeping the A-frame upright, and without moving the first leg, swing the second leg up or down the slope until the string crosses the cross bar exactly at the centre mark.
• Mark where the second leg stands with another peg.
• Keeping the second leg in the same place, lift the first leg up and pivot it on the ground. Move it up and down the slope until you find the place where the string crosses the crossbar at the centre mark.
• Continue in this manner to the end of the field.
• The line of pegs or stones will mark a contour line. THEY WILL BE ON THE SAME HEIGHT ON THE SLOPE. The pegs are usually not in a straight line. If necessary make a smooth curve by moving them a little up or down.
• You can then dig channel and use the A-Frame to grade the channel to a zero gradient.

References and further reading

• A-frame, Practical Action Technical Brief
• Underground Rainwater Tanks, Practical Action Technical Briefs
• Simple Methods of Preventing Soil Erosion and Conserving Water: On a Technical Note, Nigel Walsh, Baobab, Issue No 1 1988

This article was added to our catalogue on Wednesday 06 September, 2006.

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