Synergies entre recuperation des eaux de ruissellement et fertilisations minerales dans les pays saheliens, pour accroitre la securite alimentaire, faire face a la croissance demographique, s’adapter aux changements climatiques et limiter les emissions de ges

Are compared here, in the Sahelian regions of northern Burkina Faso, two main run off water harvesting techniques: zaïs pits [micro-cuvettes of about 0.12 m3, Fig.2, Roose et al. (1993) and 200 m3 reservoirs for complementary irrigation of 0.2 ha of land, Fig. 3 Diarra et al. (2016), and the effect of adding fertilizer on territorial intensities (TIt), (area, in ha, necessary to get 1 ton of cereal), or per supplementary person deriving 65 % of his calories from grain (TISP65%c). Sorghum yield in 1992 increased from 0.063 t.ha-1 for controls, to 0.976 t.ha-1 on zaïs (Fig. 3 and 4) with 65 kg of NPK and 3 t of compost. Yields with fertilizer alone were higher (0.829 t.ha-1) than with only 3 t or 1.5 t of compost per ha (respectively 0.690 t.ha-1 and 0.317 t.ha-1). Yields were however almost divided by 2 in 1993 with only 10.5 % less annual rainfall than in 2012, but with more erratic distribution of rain, in June and August. (Table 1 and 2). In 2012-2014, average maize yield increased from 0.8 t.ha-1 with neither fertilizer nor irrigation to 2.88 t.ha-1 at Bam and 2.215 t.ha-1 at Yatenga, with complementary irrigation and fertilizer (155 units of NPK/ha). With but fertilizer and no complementary irrigation yields reached only 1.98 t.ha-1 at Bam, and 0.955 t.ha-1 at Yatenga. Without complementary irrigation yields decreased dramatically in 2013, in particular at Yatenga where, with only fertilizer, they were below the average yield of maize of that region (0.8 t.ha-1), (Fig 4 and 5 and table 3). This was due to low rainfall in the middle of July and also in the middle of August. Complementary irrigation with reservoirs helped therefore to maintain high average yields on fertilized plots much better than with zaïs with more erratic rainfall distribution. This allows also a better and safer payback of fertilizer inputs. The territorial intensity per additional capita (TISP65%c) with sorghum decreased in 1992 from 2.58 ha for the control to 0.17 ha with fertilizer and compost (table 2). For maize, it decreased from 0.2 ha to 0.07 ha at Yatenga (table 3). Despite fertilizer input, energy and GHG budgets were positive in 1992 with zaïs, and during 2012-2014 with maize getting fertilizer and complementary irrigation. And rainfall efficiency increased from 0.16 kg .m-3 to 0.44 kg .m-3. Possible improvements are suggested, and policies and measures to increase land use efficiency by promoting crop production, with both reservoirs and increased mineral fertilisation (Fig. 6), are discussed.