Multi-scale modeling of sediment and nutrient flow dynamics in the Ouémé catchment (Benin) - towards an assessment of global change effects on soil degradation and water quality

Beyond aspects such as the quantification of soil and water degradation, this work investigated impacts of differences in the input boundary conditions (e.g. soil map, land use) on the performance of the Soil and Water Assessment Tool (SWAT). Effects of different local crop management scenarios (e.g. fertilizer input) on the simulation of plant growth and soil nutrient load to surface water and groundwater systems were also evaluated. The study was carried out in the Ouémé catchment (49,256 km²) at the outlet of Bonou in Bénin, where different sub-catchments were extensively investigated. Therefore, three different soil maps have been used: two maps of the hierarchical Soil and Terrain digital database approach (SOTER) and the soil mapping at the reconnaissance level, with one dominant soil type per mapping unit approach (ORSTOM); all three were available at the same resolution. The mapping approach’s impact on the model results was within the same magnitude of that of maps with different resolutions but developed with the same mapping approach. While the latter aspect is often studied, the first one is usually neglected. A land use map was refined for the study area, enabling the evaluation of four management scenarios: fertilizer supplied only to cotton, rice and maize, as is common in Benin (Sc0); crop systems without the use of fertilizer (Sc0a); similar fertilizer inputs to all cropping systems (Sc1); and the original land use map without fertilizer inputs (Sc1a). Compared to the first scenario, the latter two scenarios, commonly used in regional scale modeling, exhibited distinct biases in plant growth parameters, crop yields, water yield, sediment yield and nitrogen load. A regionalization methodology has been developed and applied to derive scale dependent regression-based parameter models, using catchment physical properties depending on spatial scale as explanatory variables for SWAT model parameters obtained from multi-scale investigations, for accurately simulating water-sediment-nutrient fluxes at ungauged and large scale basins. With respect to process representation in the SWAT model, it was found that in the Ouémé catchment, geology appears to be a major driver of hydrological response, correlating significantly with eleven out of fifteen model parameters. Slope appears to be powerful to control the channel conductivity, groundwater threshold for base flow generation and soil evaporation compensation (accounting for capillary, crusting and cracking actions). The soil type lixisol, which is a dominant soil component within the Ouémé catchment, partly explained the surface runoff lag and the maximum retrained sediment. The occurrence of lateritic consolidated soil layer explained the soil susceptibility to erosion and drainage density explained the fraction of aquifer percolation. Parameters such as the curve number, controlling the surface runoff were not consistently explained by the catchment properties, leading to a slightly overestimation of runoff peaks, probably due to the non-uniqueness of the considered calibrated parameter set. These relationships were successfully used to compute daily runoff hydrographs (Model efficiency ranged from 0.61 to 0.67 and coefficient of determination of roughly 0.70) at different catchment scales (from 1179 km2 to 23488 km2). By adopting this methodology two difficulties in model setup in the Ouémé catchment were overcome: parameter scale-effects and associated uncertainty issues for large scale model application and the lack and non-accurateness of boundary condition data (e.g. stream water-sediment-nutrient measurements). Climate and land use change impacts on the ongoing land and water degradation were compared at different spatial scales (Donga-Pont: 586 km2; Ouémé-Bonou: about 49,256 km2). Surface runoff, groundwater flow, sediment and organic nitrogen loads were dominantly affected by land use change of -8 to +50%, while water yield and evapotranspiration were dominantly affected by climate change of -31 to +2%. It was found that variables such as surface runoff, groundwater flow, sediment and transported nutrients, mainly sensitive to land use change were significantly affected by the increasing scale, while variables such as water yield and evapotranspiration, mainly sensitive to climate change, have changed almost similarly for the both scales

Flood risk assessment and mapping in the Hadejia River Basin, Nigeria, using hydro-geomorphic approach and multi-criterion decision-making method

Flood risk management is crucial for climate change resilience. The Hadejia River basin is known for severe and frequent floods, which have destroyed houses and farmlands and claimed many lives. This study developed a GIS-based flood risk and vulnerability mapping assessment using the Analytical Hierarchical Process (AHP) to outline scenarios that reduce risk and vulnerability associated with floods in the Hadejia River basin. The risk mapping of the basin integrated seven hydro-geomorphological indicators influencing extreme events (elevation, mean annual rainfall, slope, distance from rivers, soil type, and drainage density) and six socio-economic vulnerability indicators (population density, female population density, literacy rate, land use, employment rate, and road network) using a multi-criterion analysis. The average annual rainfall data of 36 years (1982–2018) were used for flood plain mapping in this study. Combining the flood hazard and socio-economic vulnerability indices of the basin revealed high-to-very high flood risk in the downstream and central upstream portions of the basin, which cover about 43.4% of the basin area. The local areas of Auyo, Guri, Hadejia, Ringim, Kafin Hausa, and Jahun were identified as zones at a very high flood risk. The study also revealed that flood hazard and vulnerability indicators have different influences on flood risk. The validated results resonate with the records of previous flood distribution studies of the basin. This research study is significantly important for developing strategic measures and policy revision through which the government and relief agencies may reduce the negative impact of floods in the Hadejia River basin

Nouveaux risques dans les bas-fonds des terroirs soudaniens. Une étude de cas au Burkina Faso

Dans la région soudanienne du Burkina Faso riche en bas-fonds, ces derniers représentent une facette du paysage inondable et fertile, ayant une fonction contre-aléatoire à travers des productions diversifiées en milieu humide en saison des pluies, et en contre-saison. Les activités de bas-fond permettent de réduire la vulnérabilité des groupes les plus exposés, comme les petites exploitations, les femmes et les jeunes, mais les arrangements fonciers restent précaires. C'est aussi un paysage particulièrement changeant avec des actions d'aménagement collectif hydraulique, rizicole et maraîcher. Les changements climatiques en cours atténuent l'aléa de la sécheresse sauf en début de saison, et un risque accru d'inondations est apparu. Ces tendances renforcent l'aléa du régime de l'eau, limitant les rendements, ce qui requiert des innovations dans les domaines technique et organisationnel, qui devront prendre en compte explicitement la dimension sociale et environnementale des bas-fonds

Water balance components estimation under scenarios of land cover change in the Vea Catchment, West Africa

The need for a detailed investigation of the Vea catchment water balance components cannot be overemphasized due to its accelerated land cover dynamics and its associated impacts on the hydrological processes. This study assessed the possible consequences of land use change scenarios (i.e. business as usual (BAU) and afforestation, for the year 2025) compared to 2016 baseline, on the Vea catchment’s water balance components using the Soil and Water Assessment Tool (SWAT) model. The data used include daily climate and discharge, soil and land use/land cover maps. The results indicate that the mean annual water yield may increase by 9.1% under BAU scenario, but decrease by 2.7% under afforestation scenario. Actual evapotranspiration would decrease under BAU scenario but increase under afforestation scenario. Groundwater recharge may increase under both scenarios, but more pronounced under the afforestation scenario. These outcomes highlight the significance of land cover dynamics in water resource management and planning at the catchment

L'agriculture de décrue au gré de la variabilité des politiques publiques sénégalaises

Introduction Sur les rives de la moyenne vallée du fleuve Sénégal, la culture du sorgho de décrue a assuré la base de la subsistance des populations pendant quelques millénaires (fig. 1). Or, dès l’indépendance, le gouvernement sénégalais a opté pour une politique de modernisation de l’agriculture qui prévoyait le remplacement de cette agriculture de décrue par une riziculture irriguée intensive. Après la construction de deux grands barrages régulateurs, à Manantali en amont et à Diama en ava..