Comment on 'Modelling the effect of soil and water conservation practices in Tigray, Ethiopia' [Agric. Ecosyst. Environ. 105 (2005) 29-40]

摘要

Ongoing land degradation in Tigray (Ethiopia) requires urgent action at different levels of society. Soil and water conservation activities are now widespread, integrating local knowledge, farmers' initiatives and introduced technologies. Hengsdijk etal. [Hengsdijk, H., Meijerink, G., Mosugu, M., 2005. Modelling the effect of three soil and water conservation practices in Tigray, Ethiopia. Agric. Ecosyst. Environ. 105, 29-40.] evaluate this through the application of a suite of models, the results of which show that after stone bund building, productivity would decrease. Furthermore, their simulations suggest that crop residues used as mulch would hardly contribute to lower soil nitrogen depletion at farm level. Nearly complete forestation of the catchment would reduce erosion by only 14%, which is deemed insignificant compared to the sacrifice in cultivated land that needs to be forested. Overall, the results of the model simulations lead Hengsdijk et al. to suggest that conservation efforts in Tigray are inefficient and absorbing a disproportionate amount of resources, which could have been spent differently and more efficiently if model simulations would have been used as an ex-ante evaluation. Here, we compare the results of the model simulations by Hengsdijk et al. with field data that we collected over the last decade in the Tigray area. Based on the results of this comparison, we question the validity of the conclusions by Hengsdijk et al. regarding the efficiency of soil conservation measures in Tigray. We believe this discussion illustrates, at a more general level, the difficulties in transposing environmental models from one region to another. Extensive fieldwork remains necessary for site-specific calibration and validation. Neglecting to do so may result in improper understanding of the issues at hand and consequently in ill-targeted and costly remediation schemes.