Bossa, Yaovi Aymar: 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. - Bonn, 2012. - Dissertation, Rheinische Friedrich-Wilhelms-Universität Bonn.
Online-Ausgabe in bonndoc: https://nbn-resolving.org/urn:nbn:de:hbz:5n-29832
@phdthesis{handle:20.500.11811/5386,
urn: https://nbn-resolving.org/urn:nbn:de:hbz:5n-29832,
author = {{Yaovi Aymar Bossa}},
title = {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},
school = {Rheinische Friedrich-Wilhelms-Universität Bonn},
year = 2012,
month = sep,

note = {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.},

url = {https://hdl.handle.net/20.500.11811/5386}
}

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