A.K.A. Zai.

2.1 Micro-catchment rainwater harvesting systems

Perhaps the most successful of these techniques is the zai ("water pocket") in Burkina Faso Zai is an ancestral planting pit developed in the Yatenga province, North Western part of Burkina Faso (where average rainfall is about 600 mm, with recurrent droughts and where soils are heavily encrusted. . .

the objective of the Zai practice is to regenerate the most degraded part of the field. It consists of digging holes or 'basins' of around 20-20 cm in diameter and 10-15 cm in depth. (Bandre and batta, 2002). The holes store rainwater, for plant growth. Generally the density is about 10,000-15,000 holes/ha depending on the crop chosen and the spacing between holes. Farmers use stone contour bunds to reduce the speed of runoff allowing infiltration into the zai which collect and concentrate the runoff. The larger the planting pits, and the bigger the spacing, the more water can be harvested from the uncultivated micro-catchments. Organic manure is put in the holes at a rate of about 3-4 t/ha. . .

. . . the quality of the amendment in Zai played a significant role. Low TDM as well as grain yield was produced with crop residue and compost of low quality. He observed for instance at the three study sites in Niger, that TDM produced on average with crop residue application was 756 at Sadore; 925 at Damari and 2185 kh ha-1 at Kakassi in 1999, compared to 3957, 4600 and 5030 kg ha-1 respectively with same rate of manure application. The grain yield was 151 kg ha-1 at Damari and 393 kg ha-1 at Kakassi with crop residue application, while it was 987 and 778 kg ha with manure application.

Ftondji (2002) observed that the Zai planting technique induced a higher water use efficiency than flat planting at three sites in Niger. Combination of Zai with manure improved considerably water use efficiency in three different sites. Therefore it is imperative to promote technologies that can on one hand help increase potential water availability and on the other hand consequently help rehabilitate degraded lands. "Zai" enhanced soil water storage and increased plant water availability, though most of this water could be drained out in soil with low water holding capacity as in Sadore and Damari in Niger. Nevertheless, the use of good quality organic amendment (manure) promoted rapid and deep root growth and helped limit water loss by drainage.

Also mentioned are bunds: earthen or stone embankments, sometimes built along the land's contours, which impede surface flows and so increase infiltration while decreasing erosion. A similar concept mentioned here before, and which I use in my work, is the Keyline system.

The first book on Keyline book was published in 1954. In it, P.A. Yeomans exploded the myth that it takes 1,000 years to produce an inch of topsoil. Yeomans pioneered, among other things, the use of on farm irrigation dams in Australia, as well as chisel plows and subsoil aerating rippers. Yeomans perfected a system of amplified contour ripping that controlled rainfall run off and enabled the fast flood irrigation of undulating land with out the need for terracing. . .

Keyline Designs features large earth walled irrigation dams that are all equipped with through-the-wall Lockpipe systems to gravity feed irrigation stock water and yard water. Across the landscape the dams may be interlinked by channels. Graded earth channels broaden the catchment areas of high dams, conserve the height of water and transfer rainfall run-off into the most efficient high dam sites. Flat land flood irrigation at up to sixteen hectares per hour and flood irrigation of undulating grass land. The road systems of Keyline follow both ridge lines and the graded contour water channels. Contour roads and paths, being almost flat, provide easy low energy movement across even very undulating land.

Avenues of trees are left or planted parallel to the roads and channels and as well as in tiered contoured forests. Better building sites become revealed and then fences and gateways can be positioned more strategically. Within this harmonious design, soil regeneration can occur by deepening the bio-fertility of the topsoil and converting subsoil into living topsoil. This is achieved partially by soil aeration, which increases water absorption, also by an amplified contour cultivation pattern, which prevents soil wash and by using an enhanced cell grazing design for "rational" grazing. Incidental results are the healing of soil erosion and salinity and better stock health.

In a rural setting Keyline is far more than a unique combination of water conservation and farming with nature. Keyline completely supersedes the widely imposed but misconceived concept of Soil Conservation. One of the typical benefits of Keyline is the rapid development of living soil.

There are other benefits for these various water cachement systems worth emphasizing. They not only reduce erosion and build soil fertility, they replenish aquifers and reduce seasonal instream flows so that wells don't go dry and downstream flooding in wet seasons is reduced. Erosion is not just loss of "dirt", it is also loss of nutrients that leach away from the land where they are valuable, to concentrate in water bodies where they are " pollutants". The longer that rain water lingers on the land before rushing to the sea the better. The Keyline system addresses both wet and dry season issues since it improves the results of dry season irrigation as well as wet season surface flow control.

There are issues on the other side too. Improving soil so that water infiltration increases not only helps avoid loss of water, it also helps soils drain when there is too much water. The increase in soil carbon that results from such systems also helps hold water in the root zone. It's a triple benefit: catch water, drain water, hold water.

This is all very old tech but not always well understood or practiced. One of the unfortunate parts of agronomic changes with development has been the lack of sophisticated understanding by remote bureaucracies. Looking on such systems from the commanding heights the small but crucial behaviors were invisible. Big think blunders such as massive irrigation projects that dammed water courses and doled out water during the year; cleaned, dredged and straightened water courses for improved navigation; and drilled deep wells to pump out aquifers have resulted in monotonic degradation of land while increasing both drought and seasonal flooding.

What makes this worth discussing (again) is the potential benefit of marrying char usage to such water systems. As noted above the benefit of zais increases with the quality of the material used to fill them. A mix of manure and char seems like a best practice, better than manure alone. Char helps hold water and nutrients, and is not consumed in the process. Top dressing with char in Keyline and bund systems should increase benefits for the same reasons as with zais, and won't just erode away as it might if top dressed on land where surface flows are poorly managed.