Modeling the effect of three soil and water conservation practices in Tigray, Ethiopia

Severe land degradation affects the livelihood of many farmers in the highlands of Tigray, northern Ethiopia. Various soil and water conservation practices have been proposed to reduce land degradation and to improve the quality of the natural resource base but quantitative information on their agro-ecological effects is often lacking. In this study, effects of three soil and water conservation practices are assessed using a crop growth simulation model (WOFOST), a nutrient monitoring model (NUTMON) and a hydrological erosion model (LISEM), which are applied at field, farm and regional scale, respectively. Evaluated soil and water conservation practices include (i) bunds along field contours to improve water availability for crop production, (ii) mulching of crop residues to improve soil nitrogen stocks and (iii) reforestation to reduce erosion. Data from the watershed Gobo Deguat in northeastern Tigray have been used as a case-study. Bunds slightly increased crop productivity at sowing dates when water-limited yields were low, while productivity decreased at more favorable sowing dates due to the reduction in cropped area required for the construction of bunds. Crop residues used as mulch hardly contributed to lower soil nitrogen depletion at farm level as the total amount of nitrogen in crop residues was limited, while part of the crop residue nitrogen returned to the soil anyway in the form of manure and compost. Reforestation reduced erosion up to 14% but this is insignificant compared to the sacrifice in cultivated land which needs to be reforested. Applied tools allow rapid ex-ante evaluation of soil and water conservation practices and may contribute to improved cost-benefit analysis of proposed measures, and identification of more appropriate means to combat land degradation. The analyses show important trade-offs and the effects of the assessed soil and water conservation practices may partially explain their low rate of adoption in Tigray.