Assessment of soil erosion in cultivated fields using a survey methodology for rills in the Chemoga watershed, Ethiopia

Soil erosion by water is recognized to be a critical economic problem in highland Ethiopia. However, nearly all the available information about its severity and economic costs are extrapolated from plot and micro-watershed level studies which are too few in number to represent the diverse environments of the country. Moreover, plot and watershed level studies do not show actual soil losses from cultivated fields, while understanding the magnitude of soil loss at the field scale is important for practical conservation planning. This paper reports results of field-scale soil erosion assessment that employed a survey methodology for rills and was conducted over two wet seasons (the years 2000 and 2001) at two sites, Kechemo and Erene, located in the upstream and downstream reaches of the Chemoga watershed, northwestern highland Ethiopia. The two wet seasons average rill erosion magnitudes were 13.5 Mg ha-1 in the Kechemo and 61 Mg ha-1 in the Erene. Assuming that interrill erosion contributes 30%, actual soil losses were around 18 Mg ha-1 in the Kechemo and 79 Mg ha-1 in the Erene. These estimates, which are well in agreement with results obtained by measurements in a nearby experimental micro-watershed, reveal that soil erosion is a threat to agricultural production in the study area and conservation measures are needed. Soil erosion showed significant spatial (between and within the two sites) and temporal variations. Hence, soil and water conservation (SWC) measures that fit well into local-scale circumstances will be realistic and acceptable to the farmers. Additionally, the problem of soil erosion should be tackled in the watershed context, because there is a strong physical interdependence between upstream and downstream areas. Finally, the study confirms that the rill survey approach gives good semi-quantitative information on soil erosion in real life situations of diverse farming and land use practices in a fast and inexpensive way; and it is commendable for practical conservation-oriented soil erosion assessment purposes.