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In recent decades, the Sahel has witnessed a paradoxical\udincrease in surface water despite a general precipitation decline. This\udphenomenon, commonly referred to as <q>the Sahelian paradox</q>, is not\udcompletely understood yet. The role of cropland expansion due to the\udincreasing food demand by a growing population has been often put forward to\udexplain this situation for the cultivated Sahel. However, this hypothesis\uddoes not hold in pastoral areas where the same phenomenon is observed.\udSeveral other processes, such as the degradation of natural vegetation\udfollowing the major droughts of the 1970s and the 1980s, the development of\udcrusted topsoils, the intensification of the rainfall regime and the\uddevelopment of the drainage network, have been suggested to account for this\udsituation.\ud \udIn this paper, a modeling approach is proposed to explore, quantify and rank\uddifferent processes that could be at play in pastoral Sahel. The kinematic\udrunoff and erosion model (KINEROS-2) is applied to the Agoufou watershed\ud(245 km2), in the Gourma region in Mali, which underwent a\udsignificant increase of surface runoff during the last 60 years. Two periods\udare simulated, the <q>past</q> case (1960–1975) preceding the Sahelian\uddrought and the <q>present</q> case (2000–2015). Surface hydrology and land cover characteristics for these two periods are derived by the analysis of\udaerial photographs, available in 1956, and high-resolution remote sensing\udimages in 2011. The major changes identified are (1) a partial crusting of\udisolated dunes, (2) an increase of drainage network density, (3) a marked\uddecrease in vegetation with the nonrecovery of tiger bush and vegetation\udgrowing on shallow sandy soils, and (4) important changes in soil properties\udwith the apparition of impervious soils instead of shallow sandy soil. The\udKINEROS-2 model was parameterized to simulate these changes in combination\udor independently. The results obtained by this model display a significant\udincrease in annual discharge between the <q>past</q> and the <q>present</q> case\ud(p value < 0.001), which is consistent with observations, despite a\udslight overestimation of the past discharge. Mean annual discharges are\udestimated at 0.51  ×  106 m3 (2.1 mm yr−1) and\ud3.29  ×  106 m3 (13.4 mm yr−1) for past and present, respectively.\ud \udChanges in soil properties and vegetation cover (tiger bush thickets and\udgrassland on shallow sandy soil) are found to be the main factors causing\udthis increase of simulated runoff, with the drainage network development\udcontributing to a lesser extent but with a positive feedback. These results\udshed a new light on the Sahelian paradox phenomenon in the absence of land\uduse change and call for further tests in other areas and/or with other\udmodels. The synergetic processes highlighted here could play a role in other\udSahelian watersheds where runoff increase has been also observed

hydrology and earth system sciences

The paradoxical evolution of runoff in the pastoral Sahel: analysis of the hydrological changes over the Agoufou watershed (Mali) using the KINEROS-2 model