38 SOIL EROSION RISK MAPPING USING USLE/GIS METHODOLOGY IN ROZE-CHAY CATCHMENT, NORTHWEST IRAN Farrokh ASADZADEH, Maryam RAHMATI, Hossein ASGARZADEH Urmia University, Department of Soil Science, Urmia, Iran Corresponding author email: f.asadzadeh@urmia.ac.ir Abstract Soil erosion by water is a serious environmental problem all over the world, particularly in semi-arid regions like Iran, where agricultural water quality and productivity potential of soils have been decreased as a direct consequence of this process. Therefore, the development of a model assessing soil erosion potential and proportional risk maps, would help to protect soil and water resources. Among the empirical models, Universal Soil Loss Equation (USLE) is a widely accepted model developed for soil erosion prediction. The aim of this study was to assess erosion risk by integrating the USLE model and GIS techniques in Roze-Chay catchment, northwest Iran. The map of the USLE factors was produced as follow: rainfall erosivity factor (R) map was interpolated from data point taken from the meteorological stations; Soil erodibility factor (K) map was created by determining the particle size distribution and organic carbon of the 98 soil samples from the study area; Topographic factor (LS) was derived from digital elevation model (DEM); Cropping- management factor (C) was computed from NDVI values derived from Landsat-TM data. Assuming no erosion control practices in the catchment, Conservation practice factor (P) was set to be a unit, in calculations. The results showed that the annual average soil loss for the Roze-Chay catchment was 57 Mg.ha −1 .ya −1 in 2015. The erosion risk map showed that, 33.6% of the total area has a low erosion risk with annual average soil loss of less than 10 Mg.ha −1 .ya −1 , but the area of 265 km 2 (≈ 59%) in the catchment showed high to highly severe erosion risk, which should be protect by appropriate conservation practices. The high erosion risk of the catchment can be related to the topography and low levels of the vegetation cover. The developed soil erosion risk map can be used to highlight the erosion risk areas and therefore, assist the farmers and decision makers in implementing suitable conservation program to control soil erosion. Key words: soil loss, Geographical information system, USLE, Roze-chay catchment. INTRODUCTION Soil erosion is considered as one of the most important environmental problems leading to significant reduction of soil fertility and crop yields (Olivares et al., 2011). In the northwest of Iran, water erosion affects negatively agricultural productivity and reduces water and soil quality (Vaezi and Bahrami, 2014). Therefore, the assessment of erosion risk at the watershed scale can be very useful to establish conservation measures and soil and water management plans. Universal soil loss equation (USLE) is the most widely used erosion model throughout the world to estimate average annual soil loss resulting from rill and sheet erosion (Erdoghan et al., 2007; Belasri and Lakhouili, 2016). USLE model can be integrated with geographic information systems (GIS) which allow preparing erosion risk maps in larger areas. In a GIS environment, the USLE can be applied to determine soil erosion risk quantitatively and spatially (Lufafa et al., 2003). The aim of this study was to assess erosion risk by integrating the USLE model and GIS techniques in Roze-Chay catchment, northwest Iran MATERIALS AND METHODS Study area. Roze-Chay catchment is located in the northwest of Iran and approximately 10 km south of Urmia city, west Azarbayjan province. Roze-Chay catchment covers an area of 453 km 2 (Figure 1). The climate of the region is characterized as semiarid with average annual precipitation of 313 mm. The minimum and maximum monthly temperature are -2 o C and 24 o C, respectively. USLE model. USLE (Wischmeier & Smith, 1978) was used to estimate annual soil loss. Scientific Papers. Series A. Agronomy, Vol. LXI, No. 1, 2018 ISSN 2285-5785; ISSN CD-ROM 2285-5793; ISSN Online 2285-5807; ISSN-L 2285-5785