Environmental Modelling & Software 18 (2003) 861–868 www.elsevier.com/locate/envsoft Application of a spatial decision support system (SDSS) to reduce soil erosion in northern Ethiopia Massimo Dragan a,* , Enrico Feoli a , Michele Fernetti a , Woldu Zerihun b a University of Trieste, Department of Biology, Via Weiss, 2, Trieste 34127 Italy b Addis Ababa University Department of Biology, the National Herbarium, P.O. Box 3434, Addis Ababa, Ethiopia Received 9 July 2002; received in revised form 11 March 2003; accepted 30 March 2003 Abstract A spatial decision support system (SDSS) based on multi-criteria and multi-objective decision analysis is applied in a case study in Ethiopia to reduce soil erosion on the basis of reallocation of crops according to their capacity to protect the soil. The case study is carried out in the Adwa district. The SDSS has been implemented using the widespread GIS software IDRISI 32 (release 2) and with the direct involvement of local stakeholders in defining factors and constraints. These are based on land cover-land use, altitude, potential erosion, proximity to roads, water and the relative soil protective capacity of each crop species. A reduction of soil loss from an average of 4.5 t ha -1 yr -1 to values below the risk threshold of soil degradation (1 t ha -1 yr -1 ) would be achieved through the application of the SDSS results. The biggest impediment to the reallocation exercise, however, is the shortage of cultivable land suitable for cultivation.  2003 Elsevier Ltd. All rights reserved. Keywords: Crops; Decision support system; Multi-criteria decision analysis; Reallocation; Rehabilitation; GIS 1. Introduction Drought, unbridled population growth, declining pro- ductivity, land degradation and abject poverty contribute to the recurrent famines in many parts of tropical and subtropical countries (TSC). Survival needs supersede concerns for resource conservation and problems of biodiversity depletion and large-scale degradation of natural resources are overwhelming. Ethiopia is an emblematic example of a TSC rural country whose econ- omy is mainly based on renewable resources (pastoralism and/or agriculture of subsistence). In most parts of Ethiopia, problems related to maintaining natural resources are critical and alternative livelihood options are limited (Egziabher, 1990). A long history of land clearance and sedentary agriculture has changed the veg- etation cover in the Ethiopian highlands. Esayas (2000) has shown that a quarter of the highlands of Ethiopia are * Corresponding author. Tel.: +39-040-558-2065; fax: +39-040- 558-2011. E-mail address: dragan@units.it (M. Dragan). 1364-8152/$ - see front matter  2003 Elsevier Ltd. All rights reserved. doi:10.1016/S1364-8152(03)00104-X eroded. Over 4% of this area affected to a stage that it will not be economically productive in the foreseeable future. Half of the area of the highland is prone to ero- sion due to the expansion of cultivation to easily erodible areas. The present annual soil erosion rate in the high- lands of Ethiopia can reach peaks of 300 t ha -1 . Decreasing the area of rural land to reduce soil erosion may not be a viable solution under the present economic conditions, since it can result in a severe reduction of the total crop yield. It would therefore be preferable to keep the cropped area relatively constant and to reallo- cate crop types considering their diverse soil protection capacity along with the cropland erodibility. This paper presents an application of a well estab- lished methodology of spatial decision support systems (SDSS) integrated in a GIS software package (IDRISI 32 for Windows, release 2 (Eastman, 2001), hereafter called IDRISI). A case study is developed in a decision frame to reduce erosion rates thus contributing to environmental rehabilitation without significantly lower- ing the crop yield. The study is conducted in Adwa, which is the small- est, but the most populated district in Tigray, a Federal