Finding Eroding Areas and Patterns with GIS, Caesium-137 Tracers, and Community Knowledge in the Ethiopian Highlands

While soil erosion has been reducing agricultural productivity in developing countries for decades, its effects and seriousness are gaining more attention, especially with respect to shortening the life of reservoirs for hydroelectric power generation. This has urged policy makers to develop comprehensive sustainable strategies for environmental conservation. To develop such strategies, however, assumptions made within the different areas of knowledge that concurrently study soil conservation must be challenged and verified. A central aim of this study on soil erosion and conservation is to bridge between social science and engineering disciplines by comparing and contrasting estimates and identification of eroding areas and patterns. In Debre Mewi, Ethiopia, we examine spatial and temporal variations in erosion from an agricultural watershed and its subsequent soil nutrient changes using GIS-based erosion prediction tools, the soil-adsorbed radionuclide caesium-137 as a sediment tracer, and community knowledge. We plan to reconcile differences between theoretical formulations, actual measurements, and community perceptions and insight. Sixteen sites are monitored in this small watershed 50 km south of Lake Tana, with characteristic semi-monsoonal rains, during the long (kremt) rainy season for topsoil depth change, water table height, soil nutrients (N, P, K, Mg, Ca), and ceasium-137 inventory. While focus group discussions and transect walks with the community describe spring flow paths, saturated areas, degraded areas and active gullies as primary zones of erosion, GIS-based prediction tools treat steep slopes and cropped land as areas vulnerable to erosion. Cs-137 tracers map the upland mildly sloping areas to be eroding at a greater average rate than any of the mid-slope or toe-slope areas. Finally, from these comparisons, we draw information helpful in understanding why and how sediment concentration and erosion decreases and what impact we can expect to see on soil nutrients. By reconciling these different detection methods and perceptions of erosion patterns we hope to enable the development of more effective and sustainable, community-backed conservation practices.