Hydrological regime shifts in Sahelian watersheds: an investigation with a simple dynamical model driven by annual precipitation

Abstract. The Sahel, the semi-arid fringe south of the Sahara, experienced severe meteorological droughts in the '70s–'80s. Since these droughts, watersheds in the Central Sahel have experienced an increase in the annual runoff coefficient (annual runoff normalized by annual precipitation). We hypothesize that these increases correspond to regime shifts. To investigate the timing of these regime shifts, we introduce a lumped model that represents feedbacks between soil, water and vegetation at the watershed scale and the annual time step. This model relies on runoff coefficient as a constraint for the state variable and precipitation as unique external forcing. Four watersheds (Gorouol, Dargol, Nakanbé and Sirba), with pluri-decennial observations ('50s–2010s), are modeled. For each watershed, one million parameterizations of this model are sampled and run, and an ensemble of one thousand best parameterizations is selected based on observed runoff coefficients. Our results show that this model can reproduce the trend of runoff coefficients. For all watersheds, almost all selected parameterizations from the ensemble are bistable, and can be utilized to define two alternative runoff coefficient regimes: a low and a high regime. Most ensemble members undergo regime shifts: simulated runoff coefficients belong to the low regime in 1965 and to the high regime in 2014. Finally, we find that the year of the regime shift, defined as the first year with more than 50 % of ensemble members in the high regime, was 1968, 1976, 1977, 1987 for the Gorouol, Dargol, Nakanbé and Sirba watershed, respectively. This article proposes several simple ideas toward improving the modelling and characterization of hydrological regime shifts.