Exploring causal relationships between vegetation coverage and the environmental parameter of Budyko-type models in the Nakanbe nested watersheds in West African Sahel

International audienceBudyko's conceptual framework is recognized in hydrology for its concise and accurate representation of long-term water and energy balances of watersheds. Based on the climate-environment coevolution, Budyko-type models capture the signature of environmental dynamics through climate. Many studies have shown a good correlation between the environmental parameter (u) of Budyko-type models and the vegetation coverage (M), but the analysis of the causal relationships between these two parameters has often received little attention. In this study, Convergent Cross Mapping, a causal discovery method, was applied to identify the causality between u and M from seven nested watersheds (areas ranging between 38 and 21,178 km2) of the Nakanbe River located in West African Sahel. The Budyko-type model developed by Chen and Sivapalan (2020) was forced with the climate data (precipitation, potential evapotranspiration, and actual evapotranspiration) to calculate u values for 11-years moving windows between 1977 and 2018. The vegetation coverage (M) was deduced from the Normalized Difference Vegetation Index. The results showed causal relationships between vegetation coverage and Budyko model parameter (convergence at a positive prediction skill) for all the watersheds. The causal influence detected is reciprocal (M influences u, and u influences M: M⇆u) for four of the seven watersheds studied. These results highlighted the existence and reciprocity of climate-environment interactions at different spatial scales

Soil Erosion across Scales: Assessing Its Sources of Variation in Sahelian Landscapes under Semi-Arid Climate

Soil erosion varies in space and time. As the contributing surface area increases, heterogeneity effects are amplified, inducing scale effects. In the present study, soil erosion processes as affected by the observation scale and the soil surface conditions are assessed. An experimental field scale setup of 18 plots (1–150 m2) with different soil surface conditions (bare and degraded, cultivated) and slopes (0.75–4.2%) are used to monitor soil losses between 2010 to 2018 under natural rainfall. The results showed that soil loss rates range between 2.5 and 19.5 t.ha−1 under cultivated plots and increase to 12–45 t.ha−1 on bare and degraded soils, which outlines the control of soil surface conditions on soil erosion. At a larger scale (38 km2), soil losses are estimated at 2.2–4.5 t.ha−1, highlighting the major contribution of scale. The scale effect is likely caused by the redistribution of sediments in the drainage network. These findings outline the nature and contribution of the emerging and dominant soil erosion processes at larger scales. At the plot scale, however, diffuse erosion remains dominant, since surface runoff is laminar and sediment transport capacity is limited, resulting in lower soil erosion rates

Climate and COVID-19 transmission: a cross-sectional study in Africa

Abstract The role of climate in the Coronavirus disease 2019 (COVID-19) transmission appears to be controversial, as reported in earlier studies. In Africa, the subject is poorly documented. In this study, over the period from January 1st, 2020 to September 31, 2022, the daily variations in cumulative confirmed cases of COVID-19 for each African country (54 countries) are modelled through time-series-based approaches and using meteorological factors as covariates. It is suggested from the findings that climate plays a role in COVID-19 transmission since at least one meteorological factor is found to be significant in 32 countries. In decreasing order, the most often occurring meteorological factors are dewpoint temperature, relative and absolute humidity, average temperature and solar radiation. Most of these factors show a lagged effect with confirmed cases (between 0 and 28 days). Also, some meteorological factors exhibit contrasting effects on COVID-19 transmission, resulting in both positive and negative association with cumulative cases, therefore highlighting the complex nature of the interplay between climate and COVID-19 transmission

Future climate or land use? Attribution of changes in surface runoff in a typical Sahelian landscape

In this study, the Soil and Water Assessment Tool (SWAT) model is used to assess changes in surface runoff between the baseline (1995–2014) and future (2031–2050) periods in the Tougou watershed (37 km2) in Burkina Faso. The study uses a combination of land use maps (for current and future periods) and a bias-corrected ensemble of 9 CMIP6 climate models, under two warming scenarios. An increase in rainfall (13.7% to 18.8%) is projected, which is the major contributor to the increase in surface runoff (24.2% to 34.3%). The land use change narrative (i.e. conversion of bare areas to croplands) is expected to decrease in surface runoff, albeit minor in comparison to the effect of future climate change. Similar findings are observed for annual maximum surface runoff. This study sheds light on the need to consider simultaneously future climate and land use in framing water management policies.Dans cette étude, le modèle agro-éco-hydrologique SWAT est utilisé pour évaluer les changements dans l’écoulement de surface entre la période de référence 1995–2014 et future 2031–2050 sur le bassin versant de Tougou (37 km2) au Burkina Faso. Cette étude utilise une combinaison de cartes d’états de surface (pour la période actuelle et future) et un ensemble corrigé de 9 modèles climatiques issus des simulations CMIP6, sous deux scénarios de réchauffement. Une augmentation des précipitations (de 13,7 % à 18,8 %) est prévue, ce qui est le principal facteur contribuant à l’augmentation des écoulements de surface (24,2 % à 34,3 %). Les changements projetés sur les états de surface (principalement la conversion des surfaces dégradées en sols cultivés) devrait entraîner une diminution des écoulements de surface, toutefois dans des proportions plus faibles en comparaison des effets du climat futur. Des résultats similaires sont observés pour l’écoulement de surface maximal annuel. Cette étude met en lumière la nécessité de prendre en compte simultanément le climat futur et les changements sur les états de surface dans l’élaboration des politiques futures de gestion de l’eau

IAHS2022 -Variability and change across space, time, extremes, and interfaces Comparison of one-site vs. multi-sites calibration schemes for hydrological modelling of nested catchments in the West African Sahel

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