Rainwater harvesting in the Sahel

Irrigation in the Sahel tends to be reserved for cash crops like groundnuts; these Nigerian women are selling okpa made from groundnuts.

Agriculture contributes between 10 and 70 percent of GDP to the economies of most sub-Saharan countries.[12] The major agricultural systems of the Sahel are oasis, pastoral, and mixed production of cereals and root crops.[13] The root crops are predominantly sweet potato and cassava; cereals are predominantly millet and sorghum, with some maize; the "north-south rainfall gradient defines...a successive north-south dominance of millet, sorghum and maize."[9] Climate changes over the next 25 years are predicted to decrease Sahelian cereal production by double-digit percentages, largely due to increased heat.[14][11] The Intergovernmental Panel on Climate Change also predicts double-digit decreases due to increased rainfall variability.[15]

Homegrown staple crops account for an estimated 90 percent of food consumption in the Sahel, and 90 percent of these crops are grown using exclusively rain-fed agriculture.[16][11] A general African transition to first-world-style irrigation systems is considered unlikely, and the Sahel region has an "especially limited irrigation potential."[17] According to the United Nations Food and Agriculture Organization, no more than 10 percent of African food production is likely to be grown under irrigation over the next 20 years.[18] Mechanized irrigation, where it exists, is typically limited to more lucrative cash crops, rather than subsistence.[19] Therefore, in order to increase or even maintain the Sahel's dryland agriculture production capacity the "most logical strategy...will be improving rainfed productivity for most staples."[18]

Freshly ground millet flour ready for millet couscous or millet porridge; millet, a staple ingredient for cuisines across the arid and semi-arid Sahelian climatic zone, goes by many local names: in Wolof, soungouf, in French, farine de mil.

Precipitation patterns and soil quality are "key constraint[s]" in Sahelian food production.[20] Rainfall levels are both generally low to start with and "highly variable" to complicate matters.[21] This variability is a common cause of crop failure due to unpredictable "onset and distribution" of rainfall;[9] cereal yields are impacted by the start date and duration of the rain as much as by the absolute quantity.[15] The majority of the year is the dry season, which ends with harmattan winds blowing dust south from the Sahara; rain usually falls between one and four months of the year, from June through September.[22][23]

Soils in the Sahel are typically degraded, [21] often "crusty, abandoned agricultural lands"[24] and "particularly poor in organic carbon."[25] In Burkina Faso, one-third of all land is degraded.[26] The human-induced structural damage to soils[27] wrought by intensive 20th-century agriculture methods "is especially evident during droughts when the land is stripped bare of vegetation and erosive winds and water take their toll."[28] In addition to the toll of soil and wind erosion on old fields, the practices of burning or removing crop residues, and a shift to fewer or no fallowing periods due to increased population density (and commensurate increased need to cultivate all accessible land) have contributed to further decreases in natural fertility. [29][30]

The Sahel is dappled with "unproductive crusty patches" found on "old dunes, sandy plains, colluvial slopes, and alluvial terraces."[29] These "glazed" patches are regionally known as glacis and are found, for example, on approximately 60 percent of all degraded land in Niger.[31] "Glacis" describes a slope made particularly slippery, for whatever reason, and is related to the Old French glacier.[32] Glacis patches in the Sahel are more or less impermeable; rainwater runs off or evaporates, further immiserating the soil biome, and thus the plants and the people.[31]

Even before the full impact of climate change is felt in the Sahel, the region struggles with challenges including "unsustainable management strategies, weak economies, weak infrastructure, 'inappropriate resource tenure', inappropriate interventions (such as eucalyptus plantations), [and] ineffective institutions."[33] The future of the Sahel is insecure.[34] Climate change impacts will be variable but there is a "likelihood of negative impacts in most locations from increased temperatures, greater rainfall variability, and more extreme weather events."[35]

In South Sudan, a mother harvests the root crop cassava while her daughter collects firewood.

Bouli in Rambo-Wottionma, Burkina Faso

A bouli is a small-scale artificial pond dug "where there is convergence of runoff" at the midpoint or bottom of a slope. This water tends to last for two[41] or even three months into the dry period after the monsoon.[42][43] In addition to supplying additional water for livestock and vegetable gardens the bouli "can recreate an ecosystem favourable to the life of the fauna and the local flora, boosting recharge of water tables during droughts and allowing vegetation to grow even during the dry period."[44] Bouli may be the most poorly studied of the rainwater harvesting techniques appropriate for the Sahel, as there are relatively few studies about the mechanics and benefits of this system.[44]

Barrier to slow down the advance of sand dunes in the outskirts of Chinguetti, Adrar, Mauritania

Mauritian farmers build weirs to trap windblown sand during the dry season and during the "infrequent rains" these weirs serve to minimize water runoff and maximize groundwater recharge; the stone rows of Burkina Faso, Mali, and Niger function by similar principles.[45] Stone rows, typically called bunds, are a traditional and widely used means of land improvement in the Sahel. Laid out on contour, stone rows minimize soil erosion but also minimize rainwater runoff and offer favorable microclimates.[46][1][42] Bunds not laid out in parallel with the natural contours of the land may result in "some gully formation during rainy periods."[47] Bunds can also be made of earth, which was the original practice that preceded the use of stone.[46] Bunds may be laid out up to 30 meters apart and may themselves be planted with indigenous vegetation such as Andropogon gayanus or Piliostigma reticulatum.[46]

Both earth and stone bunds are prone to material deterioration over time and demand periodic maintenance; as a general rule, the more stones used the more stable the row.[46][44] Projet d’aménagement des terroirs et conservation des eaux (PATECORE) popularized the three-stone system for building more durable, animal-disturbance-resistant stone rows, in which one large stone is placed atop three smaller stones.[48][49][50]

Stone bunds and demi-lunes constructed on a farm in Ethiopia

Half-moons, which are known as demi-lunes through much of the Sahel because of the French colonial influence on regional languages, are a widely used traditional form of semi-circular planting pit. Half-moons are formed by digging a hole up to four meters across but somewhat shallower in depth,[51] and "placing the removed earth on the downhill side." [42] Half-moons are particularly useful for remediating the more or less impermeable glacis soils.[31] These edged planting pits capture and hold organic matter and moisture. The accumulated detritus in turn attracts termites and other invertebrates whose actions create passages and pores in the organic matter, building humus, and permitting better water infiltration.[1][26] Half-moons have been shown to reduce the risk of crop failure and increase agricultural productivity, especially with the use of "complementary inputs" such as animal manures.[52]

Half-moons, however, are extremely labor-intensive: "constructing just one takes several hours"[30] and the preparation of the planting areas must be done during the dry season when the ground is very hard and the heat may be severe. According to one account based on interviews with Sahelian farm families, "preparation of [one hectare of demi-lunes] amounts to two to four person-months of work, and yearly maintenance of approximately one-person month is required."[1]

Zaï pit

A zaï is a "water pocket"[42] and is another indigenous planting method, developed in the Yatenga.[53] The word comes from the Moré language, and means something like "getting up early and hurrying out to prepare the soil" or even "breaking and fragmenting the soil crust before sowing."[15] Tassa is the Hausa language word for this concept.[54] A similar practice in the Yako region is called guendo.[55] Similar to half-moons, but smaller, zaï are usually 24 to 40 cm wide, 10 to 25 cm deep, spaced about 40 cm apart in a grid across the field. Zaï are usually established with "two handfuls" of organic matter in the form of animal manure, crop residues, or a composted combination of the two.[44] These pits were traditionally used on a small scale to remediate degraded zipélé lands but are now being used on much larger plots.[47] Zaï are best-suited for use in areas that see "isohyets of 300 and 800 mm rainfall."[56] Zaï have been shows to increase yields between 2.5 and 20 times normal, "depending on the crop."[57]

As with half-moons, the major drawback of zaï is in the hundreds of man-hours that are necessary to build them.[58] Families must either have a large number of fit and able-bodied workers, or "pay for the services of the young people's association."[59]

Other beneficial and successful practices in the Sahel include:

• Living hedgerows[60]
• Straw mulching[60]
• Coppicing/pollarding rather than cutting down trees wholesale, ideally leaving two or three shoots for regrowth[15]
• Paddock systems for grazing animals[60]
• Tied ridges, a planting system that looks a Belgian waffle[46]