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Rheinischen Friedrich-Wilhelms-Universität (2011)

Soil erosion modeling and soil quality evaluation for catchment management strategies in northern Ethiopia

Gebreyesus Brhane Tesfahunegn

Titre : Soil erosion modeling and soil quality evaluation for catchment management strategies in northern Ethiopia

Auteur : Gebreyesus Brhane Tesfahunegn

Université de soutenance : Rheinischen Friedrich-Wilhelms-Universität zu Bonn

Grade : Doktor der Agrarwissenschaften (Dr. agr.) 2011

Résumé
About 85% of the Ethiopian population is engaged primarily in agriculture. However, changing environmental factors have led to soil quality (SQ) degradation that poses a critical risk for food security. But, despite some alarming figures, there is no consistent information on the rate and extent of soil degradation in the country. This is due to the fact that the results of research on SQ degradation are more generalized to the country with its different environments and also based on empirical models or on runoff plot studies. It is problematic to extrapolate results from such case studies to other areas, and the resulting reports are thus inadequate to guide policy action on a large scale. Appropriate approaches that address such research gaps are thus needed for the country. This study employs a participatory survey and scientific soil measurements, geostatistics and erosion modeling to concurrently evaluate SQ degradation that can facilitate development of appropriate management strategies for the Mai-Negus catchment conditions in the northern Ethiopian highlands. A participatory SQ survey and group discussions with local farmers were conducted to identify SQ diagnosis indicators as well as the severity and determinants of SQ degradation. Soil samples were collected for analysis from the different SQ categories, land-use and soil management systems and erosion-status sites identified in the catchment. Data were subjected to statistical analysis. A soil erosion model (Soil and Water Assessment Tool ; SWAT) interfaced in a GIS environment was evaluated and then applied to identify and prioritize erosion-hotspot sub-catchments. Finally, potential management strategies (scenarios) were simulated targeting prioritized areas to identify scenarios that can better reduce soil degradation caused by erosion. The results of this study show that farmers used indicators such as crop yield, soil depth, soil color, soil erosion risk, sedimentation, for categorizing the catchment soils into high, medium and low SQ status (categories). The scientifically measured soil attributes were significantly different (P ≤ 0.05) among these SQ categories. Using the soil attributes (cation exchange capacity, porosity, sand, total phosphorus, and Ca:Mg) retained in four component factors that explain about 88% of the SQ variability, discriminant analysis correctly classified the soils in the different SQ categories. Such SQ variability shows that farmer evaluation of SQ agrees well with the measured soil attributes. The maps of the interpolated soil properties show a well-defined trend of higher contents of fine soil particles and soil nutrients in the toe-slope and foot-slope areas in the catchment and those with better vegetation cover and soil management practices. The results of the soil erosion model show that > 45% of the catchment area has experienced soil losses through erosion of over 30 t ha-1 y-1, which is higher than the soil loss tolerance for Ethiopia (18 t ha-1 y-1). About 91% of the catchment experienced a soil erosion rate over 15 t ha-1 y-1, which is higher than the average African soil loss (10 t ha-1 y-1). Land management scenarios that involve land-use redesign, terracing, grassed waterways and gully stabilization structures can reduce runoff, sediment yield and nutrient losses by up to 75% at catchment level and up to 90% in the hotspot sub-catchments (soil loss over 18 t ha-1 y-1) as compared to the baseline scenario. Generally, the results of this study confirm that the use of farmers` knowledge to evaluate SQ status and prioritize areas for implementing management intervention is useful as it is rapid, less expensive, has high reproducibility and is reasonably accurate as compared to scientific soil measurements and erosion modeling. This can thus support informed decision-making about SQ degradation in areas where professional experts and resources are limited, and where extrapolation of measured soil data is difficult. However, further research on catchments with contrasting environment is necessary to account for the heterogeneity of farmer knowledge of SQ degradation on a regional and national scale.

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