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Wageningen University (2006)

Modelling water erosion in the Sahel using physically based erosion models

Muluneh Alemayehu

Titre : Modelling water erosion in the Sahel using physically based erosion models

Auteur : Alemayehu Muluneh

Université de soutenance : Wageningen University

Grade : MSc thesis. 2006

Résumé
The Sahelian region of Burkina Faso has serious problems of soil erosion. To tackle this problem, first, assessment of erosion risk, evaluation of soil conservation practices and designing of new control methods is required. Erosion processes are both varied and complex. Physical modelling approaches developed for a range of temporal and spatial scales are required for a thorough understanding of erosion processes. It was believed that erosion could be dominated by only a few events per year. In northern Burkina Faso water erosion occurs during the whole rainy season, but some intense events produce most of total annual erosion. So, EUROSEM in PCRaster was first designed and then adapted for simulating single events that operate for successive short time steps. The objective of this study was calibration and validation of European soil erosion model (EUROSEM) and comparing its results with KINEROS2 model which was calibrated before on the same data for the same catchment. The study was carried out on calibration and validation of individual rainfall events, using calibration parameters for the two main crust types namely erosion crust and sandy crust which constituted about 98% of the study catchment. Observed discharge and transported sediment data used for this study were obtained from the years 1999-2000 from Katchari catchments in northern Burkina Faso. Saturated hydraulic conductivity for hydrology part and the detachability of the soil particles by raindrop impact (K) (gJ -1 ) and cohesion of the soil (COH) (kpa) for sediment transport part were the calibrated parameters. Nash and Sutcliffe, (1970) model efficiency and visual inspection of hydrographs were used for calibration and validation performance evaluation methods. Results indicated that, in most cases, EUROSEM did perform fairly well in predicting discharge with model efficiencies ranging from 0.50-0.91, which is with acceptable accuracy. Most of these events were from medium to large rainfall events ; on the other hand, small and extreme events had given unsatisfactory results, mostly by underestimating the discharge. EUROSEM did not perform well on individual event simulations in terms of predicting soil loss, because of inaccuracies in estimates of sediment concentration. Generally, the result shows that simulation of runoff by EUROSEM is better than that of soil loss. However, EUROSEM did not perform well in all single event simulations in terms of both simulating discharge and sediment loss. The most probable reason is that, EUROSEM might not be able to simulate crust development, particularly the changes in surface characteristics which are a prevalent process on Sahelian situation. KINEROS2 showed slightly better prediction than EUROSEM in discharge simulation provided that KINEROS2 was calibrated with several parameters unlike EUROSEM which was calibrated for a single parameter.

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