Half-Moon (Demi-lune)

From the Sahel: origin and results

Unlike the zaï pit, the half-moon is not an old peasant technique. The FAO manual expressly notes that it was not traditionally used. According to the account by Chris Reij, Gray Tappan and Melinda Smale, an agroforestry project funded by Oxfam tested half-moons in Burkina Faso's Yatenga region in the 1980s, where they did not catch on at first. Only in the decade before the study appeared in 2009, when a project funded by the International Fund for Agricultural Development promoted them, did they spread more widely, and by then, the authors write, farmers were using them more and more.

The yield effect is well documented and at the same time instructive. Hamado Sawadogo, a Burkinabè researcher working on water harvesting, reported sorghum yield increases of 100 to 300 percent over untreated plots at an FAO workshop in 2017. The range itself is the lesson: where compost or manure goes into the half-moon and the year is not too dry, the effect is large; without organic matter it is much smaller. On exhausted soil the water only becomes yield when there is something in the ground that can use it. The constraints named in the same presentation are familiar from all hand-built earthworks: a great deal of labour, a lack of manure, and the need for simple instruments to find the contour.

What carries over to the Mediterranean

The physics carries over, the calendar does not. In the Sahel the half-moon serves millet and sorghum in the summer rainy season. In the Mediterranean the rain comes in autumn and winter, and the crop that is to profit is rarely an annual grain. Here the half-moon makes sense above all for trees and shrubs in their first years: it fills the soil in winter so that the roots find a reserve when the dry season begins.

Three limits should be named honestly. First, slope: the FAO's design limits of a few percent are quickly exceeded on Mediterranean hillsides, and on steep ground the arcs must become small and close together, or a different form is better. Second, terraces: where old terraces exist, they already hold water on every step, and a half-moon adds little. Third, organic matter: the Sahel results show that without compost the water effect remains modest, and that applies just as much on poor Mediterranean soils. Spanish restoration research, such as the work on small catchment basins for afforestation in Spain by Andrés Martínez de Azagra and colleagues, shows how such forms can be sized and adapted for dry slopes in Europe.

How Terra Humana understands it

For Terra Humana the half-moon is less a model to copy than an example of how farmers in one of the driest farming regions in the world turned degraded land back into fields: with their hands, with a spade and with what the land and the animals provided. That is close to our own approach on Sardinia, where rainwater harvesting and the protection of the soil come before every planting.

In practice the half-moon is a tool for particular places: a slope section without terraces, a single tree in an exposed spot, a gully edge where water should be slowed before it gathers. It works best together with mulch inside the arc, which keeps the collected water from evaporating, and with plants on the bund that hold it in place.