Dynamique spatio-temporelle des états de surface et influence sur le ruissellement sur un bassin de type sahélien: cas du bassin de Tougou (Nord Burkina Faso)

Résumé

Land use/land cover (LULC) changes is a major factor explaining the hydrological response of a watershed. Such observation stands more acutely in the case of Sahelian hydrosystems, where it acknowledged that surface runoff generation processes are typically dependent on soil surface conditions. Hydrological modelling requires a LULC map of the simulation space among the input data. This information is of particular importance as it indicates to the model the organization of the landscape and allows seamless integration of spatial variability in the simulation of hydrological processes. However, in practice, a single LULC map is usually provided to the model and remains static for a simulation that is often conducted over a long period (up several tens of years). Eventually, due to the equifinality of the modelling parameters, the model always manages to represent the observed data. However, this results in biases on the temporal patterns of the simulated processes, which can be significant. This study aims to contribute to the quantification of the impact of LULC changes on surface runoff in the context of Sahelian hydrosystems. The Tougou watershed (37 km²), located in the province of Yatenga (in the Northern Region, Burkina Faso) is selected as a case study. LULC maps of the watershed are produced from 1952 to 2017 through remote sensing. LULC change analysis revealed the intensity of the landscape transformation, which is related to the evolution of rainfall and anthropic pressure over the same period. It mainly appears that from 1952 to 2017 in the Tougou watershed, the natural vegetation significantly decreased (from 46 to 2.5%) while bare/degraded and cultivated soils expanded (from 3 to 33.5% and 51 to 64%, respectively). The spatial development of the basin is projected for 2050, using an artificial neural network and demographic perspectives, predicting an increase in cultivated soils to 90.2% of the watershed area. The hydrological implications of these LULC dynamics are assessed through hydrological modelling. To this end, the SWAT (Soil & Water Assessment Tool) model is calibrated and validated on observed runoff data, collected over the period 2004-2018, at the watershed outlet. The simulations are carried out under two scenarios: a scenario using a static LULC map (in 1999), corresponding to the classical modelling approach; a second scenario, integrating dynamically the observed LULC trends in the watershed from 1999 to 2017. This second scenario allows a more precise simulation of the observed runoff (at daily, monthly, annual time scales), but also a better estimate of other processes such as actual evapotranspiration and deep recharge. Using calibrated parameters in the dynamic scenario, surface runoff over the Tougou watershed were reconstructed over the historical period 1952-2005. At this stage, it is shown once again that only the integration of the LULC dynamics helps in reproducing the Sahelian hydrological paradox, well known to hydrologists. Therefore, by analysing the isolated contributions of climate variability and LULC changes on the increase in runoff between the sub-periods 1952-1968 and 1986-2005, it appears that the impact of LULC changes is dominant over the climate variability. Regarding the future evolution of the Tougou basin in the face of climate change, an ensemble of five (5) regional models (from the CORDEX-Africa package) was constructed as part of this study. This ensemble projects, by 2050, a non-significant decrease in rainfall (2.3 to 3%) and a significant increase in potential evapotranspiration (5 to 6%). Considering the projected increase for cultivated soils, the average annual runoff is projected to decrease in the RCP4.5 scenario (by 11.7%), mainly caused by the increase in cultivated soils (LULC change). In opposite, in the RCP8.5 scenario, a slight increase in the mean annual runoff (by 0.6%) is projected, attributable to the intensification of rainfall, which tends to dominate the effect of the increase in cultivated soils (LULC change). The results of this study call to hydrologists and, to a greater extent, planners and managers of water resources. This study stands a pledge in favour of the monitoring of LULC trends and change at the watershed scale, but also the coupling of this information in hydrological modelling as a new practice. Also, research should be directed towards the development of integrated modelling tools and platforms to this end, to allow better forecasting of water resources, in particular in the case of Sahelian hydrosystems.

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