Multi-Scale Modeling of Sediment and Nutrient Flow Dynamics in The
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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 Dissertation zur Erlangung des Doktorgrades (Dr. rer. nat.) der Mathematisch-Naturwissenschaftlichen Fakultät der Rheinischen Friedrich-Wilhelms-Universität Bonn vorgelegt von Yaovi Aymar Bossa aus Kpinnou, Benin Bonn, Juni 2012 Angefertigt mit Genehmigung der Mathematisch-Naturwissenschaftlichen Fakultät der Rheinischen Friedrich-Wilhelms-Universität Bonn 1. Gutachter: Prof. Dr. B. Diekkrüger 2. Gutachter: Prof. Dr. Ir. E. K. Agbossou Tag der mündlichen Prüfung: 07.09.2012 Erscheinungsjahr: 2012 Dedication To all my family, first to my wife Hélyette Arielle Bossa, who supported me with faith and love. My Children, Heysenam and Helynam Bossa for whom I did not often have enough time, but who gave me courage and motivation. To my parents, Antoine, Josephine, Reine, Valerie, Odile, Constance, Ines, Brigitte, Benoît, Bernard, my parents and brothers in-laws (Théophile, Clotilde, Harsys and Théo-wyls), and my nephews (Rosine, Octave, Rose,..) this is my most emotional way of saying thank you for giving me support and tenderness. i Acknowledgements This work has been supported by the Federal German Ministry of Education and Research (BMBF, Grant No. 01 LW 06001B) as well as the Ministry of Innovation, Science, Research and Technology (MIWFT) of the federal state of Northrhine-Westfalia (Grant No. 313-21200200) for the funding of the IMPETUS project in the framework of the GLOWA program. The German Academic Exchange Service has supported this work through a three year scholarship. I am very grateful to Mr. Bernd Diekkrüger, Professor at the Department of Geography of the University of Bonn, responsible of the Hydrology Research Group and supervisor of this work, who has really spent time for me, giving me daily advises, and has early gave me unconditional support in my orientation. He also gave me many opportunities to visit international conferences. His patience to listen to my questions sometimes not clear enough and his commitment to a well done work were my main motivation. I am really so proud for having belonged to his working group. I express my gratitude to all belonging to the Hydrology Research Group, especially Dr Simone Giertz, Gero Steup, Dr Luc Sintondji, Dr Claudia Hiepe, Dr Anna Klose, Henning Busche, Britta Höllermann, Jakob Sorge, Guido Scuito, Mareike, Asfaw Kebede Kassa, Derib Sisay, Daniela Dowersteg, Dr Oliver Schulz, Ambros Waidosch, Aurelien Tossa, Andreas Enders, Thomas Cornelissen, Dr. Constanze Leemhuis, whose contributions in various forms were very valuable. Scientific rigor, commitment, passion and intellectual generosity made the exchanges with them interesting and enjoyable. I express my gratitude to Dr Igué and Dr Gaiser, who took an active part in all related to soil investigation, leading to the publication of the sixth chapter of this thesis in the Geoderma Journal. Many thanks to IMPETUS in all its hierarchy and its participants at the both sides Germany and Benin, since they have taken an active part in my background construction, through series of integrated trainings in soil hydrodynamics, hydrological process modeling coupled with Geographic Information Systems and Remote Sensing, within the capacity building rubric of the IMPETUS project. As for the administrative side, I cannot forget Prof. Dr Reichert, Dr Michael Christoph, Andreas Preu, and all of you whose names are not mentioned here. I am grateful to Mr. Euloge Agbossou, Professor at the Faculty of Agricultural Sciences (FSA) at the University of Abomey-Calavi (Bénin). Few months before completing my master degree, he gave me the opportunity to meet Prof. Diekkrüger, Dr Giertz and Mr Steup, who accepted me to join their Research Group just after the defense of the Master degree. He believed soon in my potential, he has forged my personnal confidence face to a completely new professional environment, and seized spontaneously every opportunity that can be relevant to my education and my professional profile. I couldn’t forget his closed assistants, Dr Luc Sintondji and Dr Bernard Ahamidé, who have played remarkable roles for achieving the objectives of this work. Bearing in mind that I was originally compartmentalized to the scientific field of fundamental Physics, thraiting only few aspects of Hydrology Sciences (e.g. continuum mechanics), it is the time to thank Professor Brice Sinsin, currently Rector of the University of Abomey-Calavi, who motivated me and gave me the opportunity (through a master program in Natural Resources Management at the faculty of Agricultural Sciences, co-piloted with Belgium Universities) to glimpse the transdisciplinarity of Environmental Sciences. I am really grateful to him. I have often resorted to various contributions from Pierre Adisso, Head of the Coordination Service of Integrated Water Resources Management of the Benin General Directorate of Water. He has shown a strong attachment to the success of this work, and has created good working environment for me inside his institution. To Mr. Antoine Gohoungossou, responsible of the Hydrology Service, Dr Arnaud Zannou, responsible of the Ouémé 2025 Project, Dr Félix Azonsi, Director of the Water Service of the General Directorate of Water of Benin, I say thank you for having doing your best to help me. Thanks to Martial Dossou, Farouk, Lamidi, Doussi, Comfort, Florence, Dr. Vincent Orékan from IMPETUS, Maxime Wubda, Afouda Simon, Theodore Ouani-Yossidé from IRD. I do not forget Mr Ahlonssou of DMN, Dr Valens Mulindabigwi, Mr Pofadji of CePEG, and my friends Ulrich Demagbo of CCA. ii Abstract 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