Sustainable Agriculture Extension Manual

The keyline water-harvesting system

The keyline water-harvesting system spreads rainwater and allows it to seep into the soil through the careful design of a whole piece of land. Developed in Australia, this system works best where there are at least two stream lines or water courses, though aspects of it may be used where there is only one.

A keyline water-harvesting system carefully manages water runoff. The main aim is to capture rainfall and runoff, and allow it to sink into the ground. Any runoff that does occur is spread rather than being allowed to follow its natural course downslope.

It is essential that the piece of land is designed as a whole at the beginning of the exercise. It does not work to do things in an unplanned way.

Requirements

A contour map of the area, at as large a scale as possible. This is not essential, but is very helpful. Aerial photographs are also very useful.
An A-frame, spirit level or water pipes to measure contours (see the section on Marking contour lines).
Various earth-moving tools: wheelbarrows, shovels and pickaxes. Oxen (or a tractor) and an earth scoop are very useful for building dams and roads. If you can, hire a bulldozer for major earth-moving work.
The commitment and understanding of all involved are essential. This can be fostered using the process described in the section on Integrated land-use design. Considerable time and labour are usually needed to put the keyline system into practice.

Procedure

Observe the runoff

1. Follow the first two steps (observation and assessment, and holistic goal formation) in the section on Integrated land-use design. This should lead to an intimate understanding of the land in question by those involved.

2. Pay particular attention to assessing the nature of the runoff. Draw up a checklist of questions to help you do this, such as:

What is the slope?
What is the ground cover like?
How easily will water percolate into the soil, and into the subsoil?
How much runoff in a heavy rainstorm is there likely to be from different parts?

There are many more questions. Drawing up this list of questions is a useful exercise in itself.

3. Part of the observation should include marking contour lines using an A-frame or line-level, at regular intervals down the slope. While doing this, try to identify possible dam-sites. It may be necessary to call in an expert for this.

Design the water-harvesting system

4. With a thorough understanding of the piece of land and a common vision of what the various people working on it would like, the detailed siting can begin. In doing this, think "water flow" all the time. If there are enough people, have them work on ideas in separate groups. This will lead to many options being developed. It is at this stage that creativity is most called for. Consider the following in particular:

Runoff sources. How can roads and paths be designed to carry runoff from above them (and off the road or path itself) to a dam, pond or field? Roads are a common cause of erosion if the runoff from them is not carefully controlled. But if they are well designed, they are a good potential source of water. Consider also roofs, sports fields, land higher up in the catchment, rocks, and land with poor ground cover. Where do water-harvesting ditches need to be sited to control the runoff they will produce?
Dams and ponds. Where will they get their water from? How can their spillways be designed to spread water rather than just run it back into the same water course? Can spillways be designed to take water along the contour to the next water course?

5. When a lot of ideas have been generated, you need to start linking them together. The keyline system is a total water management plan for a piece of land. All aspects are interconnected to keep catching, spreading, sinking and storing water. The aim is to prevent the water from rushing downslope.

Principles of water harvesting

Top down. Start work at the highest point of the piece of land. Control water there first, and then work your way down the slope, putting your design into effect.

Spread and sink. Unless you are specifically carrying water to a dam, pond or tank, sinking water (allowing it to seep into the soil) is the aim of all water management. Remember that in the long term it will be much better if the dams are filled from underground water moving downslope, rather than from surface runoff.

Spillways. Pay special attention to all spillways. These are the weak links in any water-harvesting earthwork. You must design them to stand up to the worst storm. This includes spillways from a dam or pond, from ditches, or from a small pit catching water off a roof. Use the principle above on the spillways: spread and sink.

Ground cover. Always aim for maximum ground cover. In the end, ground cover is the best water-harvester of all. You can design sports fields so that they have banks all round to catch the water; but ensure also that the fields are as well covered by grass. In the long term, the grass will mean much more than the banks in terms of sinking water.

Implementation

6. Once everyone agrees to the design for the whole piece of land, you are ready to implement. The plan is the guide. It can, of course, be adapted as you go along. Many people forget this!

7. Follow the four principles of water harvesting (see the box above).

8. Learn and understand as many of the techniques for water harvesting as you can. There are many of them: pits, swales or fanya chini, contour ditches, bunds, fanya juus, infiltration pits, net and pan, tied ridges, stone contour barriers. Many of these are described in the section on Conserving soil and water.

NULC's keyline system

The Nyahode Union Learning Centre (NULC) in the eastern highlands of Zimbabwe has used the keyline system on its 42-ha piece of land. The Centre has a secondary school and technical college; most of the work has been done by teams of secondary students, who have worked during the school holidays. Many have paid their own way through school in this way.

Not only have the students put in hard work; they have also learned about dam-building and the keyline system. An important principle of the programme has been to develop a culture of discussion. Through discussion, the work teams have always known why they are doing what they are doing. They have contributed to decisions, have elected their own team leaders and have controlled their own work rate.

In the process of implementation, NULC has recognized that plans, no matter how good, are fallible. Reality always throws up surprises, plans can be mistaken, budgets may be out, and unexpected expenditure may raise its unwelcome head. NULC has learned, above all, the need for flexibility in implementation.

The design that NULC developed in 1990. All that is indicated has been implemented. Note the relationships between the roads and the water-harvesting structures, and the interconnections between the dams.

A football field, levelled and with banks all around, acts as a giant water harvester. If there is excess water in a heavy storm, the carefully placed and reinforced spillway carries it to the next level of the water-harvesting system.

A close-up of the plan around one of the dams. Notice the road, which is designed also to harvest water for the dam. The spillway carries water away on the contour around the ridge to the next water course. (The dotted lines indicate contours)

Technical features of the NULC system

The sports fields (football/athletics and netball/volleyball) are sited at the top of NULC’s land. They are themselves giant water-harvesters, encircled by large banks. Some of these banks are terraced as seating for spectators. NULC has made great efforts to ensure good ground cover using Kikuyu grass. Spillways have been carefully sited and constructed, as even with the good grass cover, the fields are unable to absorb all the water from a heavy downpour.
Roof water from classrooms is harvested into 1-m circular pits.
The spillway from the top dam snakes around along the contour to the neighbouring water course, carrying surplus water into the next highest dam. The spillway from this second dam in turn snakes back around the same ridge, carrying surplus water back to the first water course. The same happens with the third dam.
The wetland below the second dam is constantly fed with water, ensuring a year-round supply of clear drinking water from the well next to it. There is water behind the dams all year round. This helps run extensive nurseries and gardens.

While this water-harvesting system needs a lot of monitoring and maintenance, it supplies a training and education institution with a population of 600 at any given time. This would not have been possible without this system. Furthermore, people in the community in the valley below talk of springs which were dry for many years now running again all year. —For more information, contact the Director, NULC, Zimbabwe.


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