Zaï, Tassa and Half-moons: How indigenous rainwater harvesting techniques are protecting the Sahel from certain death.

Introduction

The Sahel is the southern border of the Sahara desert. It crosses Senegal, The Gambia, Mauritania, Mali, Burkina Faso, Ghana, Niger, Nigeria, Cameroon, Chad, Central African Republic, South Sudan, Sudan, and Eritrea in a belt up to 1,000 km (620 mi) wide that spans 5,400 km (3,360 mi) from the Atlantic Ocean in the west to the Red Sea in the east. The area, which has been warming for decades, receives very little precipitation. More recently, it has come under numerous attacks from terrorist organisations following the collapse of the Libyan government and this is bringing a lot of instability to the region.

The Sahel’s hot and humid conditions have been a constant challenge for local communities for decades. When he was president, Burkina Faso’s Thomas Sankara launched an ambitious plan to plant ten million trees. This is probably the single biggest initiative of that nature in the Sahel before the Africa Great Green Wall was conceived. In fact, the Africa Great Green Wall initiative was inspired by Sankara’s project.

Greening the Sahel is an important priority because at least one hundred million people call it home. Estimates show that this population is set to more than double over the next two decades. Feeding the poor is always going to be a challenge for countries that 1) do not get a lot of annual precipitation; and 2) are among some of the poorest countries in the world. So how do they make it work? Some answers have come from the ancestral rainwater harvesting techniques that different communities in the Sahel have used for hundreds of years.

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The Zaï (Burkina Faso) or Tassa (Niger) rainwater harvesting method.

The Zaï system (Burkina Faso) or Tassa (Niger) technique involves digging holes 20 to 40 cm in diameter and 20 to 25 cm deep using a pickaxe or hoe. The soil extracted from the hole is placed in a crescent shape downstream to retain run-off water. A handful of manure or compost is then placed in the trough. The holes are spaced 0.8-1 m apart, giving around 10,000 holes/ha. At the start of the rainy season, the seeds are sown directly into the holes. By conserving moisture, the pits enable the bare soil to be recovered, resulting in good yields. Repeating this process ensures that the soil continues to keep moisture and pretty soon, the entire area becomes much greener than ever before.

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Bouli technique

The Bouli technique is another ancestral innovation from the Sahel. It consists in digging ponds either on the slope or at the bottom of a raised area where there is concentration of rainwater runoff. The surrounding area is then sown or the water can also be transported for use elsewhere. Boulis have a key advantage over half-moons or Tassas in that they also serve as watering holes for livestock. Where they are close to villages, they can be made big enough to serve some household needs.  

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Half-moons or semi-circular bunds

The half-moon is a technique for reclaiming encrusted soils and severely degraded land. It involves digging holes in the shape of a half-moon to plant crops. These holes are oriented against the slope of the ground, generating a small dike in the curved area.

The soil removed during digging forms a protective ridge that keeps the water in the hole. The spaces within the hole where the moisture is trapped is sown with crops, ideally in a mixed-cropping method to produce the density of roots that is needed to hold both the soils and the roots. Spaces are left around the half-moon so that runoff water can flow to other half-moons, thereby improving the general moisture of the entire plot.

This type of micro catchment water harvesting technique is suitable for slopes up to 15%, however bunds made of earth are seldom used in areas with slopes greater than 5% with a precipitation rate higher than 300 mm/y. At lesser slopes, half-moons are also used in areas with higher rainfalls.

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Stone or raffia leaf walls

In many parts of the Sahel, stone walls and walls of other vegetation or even adobe are built in contours against the direction of the wind to trap sand and moisture. The sand forms a wall against the wall and this helps to stop soil erosion. When it rains, the water is also retained in the plot for longer. In countries like Mauretania, palm leaves have been shown to be very effective in trapping sand during wind- or sandstorms. This means that local communities do not need to travel far to get material for such purposes.

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Conclusion

Many African communities have simply abandoned large plots of encrusted land due to low rainfall. Such plots, which are common in the Sahel region, are often regarded as ‘useless’. People avoid working them. Those perceptions are certainly also deeply held due to the shortage of water as well as an inability to mobilise large pools of labour to soften the soil.

Providing techniques like Tassa or Zaï as well as resources to mobilise labour can transform much of the African continent. As much as 300 million hectares of land can be reclaimed using rainwater harvesting techniques. It is urgent to start mobilising resources now to protect the vulnerable communities in the Sahel where people often give up hope and migrate to urban areas in search of a better life. Governments and international organisations can help set up cooperatives, mechanisation units, and rural networks that provide the heavy lifting that people need to roll out these ancestral innovations at scale.

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