Rainwater harvesting in the Sahel is a combination of "indigenous and innovative"[1] agricultural strategies that "plant the rain" and reduce evaporation, so that crops have access to soil moisture for the longest possible period of time.

In the resource-poor drylands of the Sahel region of Africa, irrigation systems and chemical fertilizers are often prohibitively expensive and thus uncommon: so increasing or maintaining crop yields in the face of climate change depends on augmenting the region's extant rainfed agriculture systems to "increase water storage within the soil and replenish soil nutrients.

(According to the Concise Oxford Dictionary of World Place Names, "The Arabs considered the Sahara to be a huge ocean with the Sahel as its shore.

"[5]) The Sahel 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.

[9] About 12 million farmers in the region (including people in Niger, Chad, Mauritania, Mali, and probably Burkina Faso and Senegal[10]), are occasionally or "chronically vulnerable to food and nutrition insecurity.

"[9] Climate changes over the next 25 years are predicted to decrease Sahelian cereal production by double-digit percentages, largely due to increased heat.

[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.

[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.

[29][30] The Sahel is dappled with "unproductive crusty patches" found on "old dunes, sandy plains, colluvial slopes, and alluvial terraces.

"[36] Furthermore, soil restoration is possible and would potentially open up more than 700,000,000 acres (280,000,000 ha) of land in Africa for additional cultivation, which could in turn reduce deforestation for agricultural uses.

[39] One study found that appropriately managed Sahelian rainwater-harvesting techniques increased runoff retention up to 87 percent, doubled water infiltration rates, and extended the crop-growing season up to 20 days.

[48][49][50] Half-moons, which are known as demi-lunes [fr] 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.

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.

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.

However, despite the efforts of numerous projects, water scarcity still limits agricultural production of most of the smallholder crop-livestock farmers of the basin and cereal yields are still lying far below their potential.

"[65] One study found that village training programs, "a low-cost policy intervention," were highly effective in increasing uptake of rainwater harvesting techniques.