The Effectiveness of the Quantitative Analysis of Geomorphometric Parameters in Preparation of Soil Erosion Susceptibility Map (Case Study: Monj Watershed)

Geomorphometry is the science of quantitative land-surface analysis. The main purpose in geomorphometry is to extract the parameters of the earth's surface and the shape of the earth from a DEM (digital elevation model), which first refers to the continuous properties such as slope, aspect and so on. It is produced and extracted in the form of raster maps and images. The latter also refers to discrete spatial features such as alluvial fan, drainage network, and watershed border, which are produced as a vector map in the form of lines, maps and, polygons. Soil erosion in the Monj Watershed is damaging; therefore, the main objective of this research is to analyse the geomorphometric parameters and prepare a soil erosion susceptibility map using VIKOR algorithm and CF method in this watershed. Since the hydrological units are based on the analysis of geomorphometric parameters, the use of the stream networks, and the contour lines in the topographic maps at 1: 50000 scale and the digital elevation data derived from Advanced Space borne Thermal Emission and Reflection Radiometer (ASTER) images version 2 in ArcGIS10.4 software along with Arc Hydro and SAGA GIS v.3.0.0 attempted to determine the boundaries of hydrological units and determine the sub-watersheds. Then, the streams were ranked. Digital elevation data derived from Advanced Space borne Thermal Emission and Reflection Radiometer (ASTER) images version 2 are based on the extraction of basic, linear, shaped, and topographic morphometric parameters. The mentioned geomorphometric parameters were prepared using the ArcGIS10.4 software and other data needed for the next step. After extracting and analyzing the geomorphometric parameters in each sub-watershed, using VIKOR algorithm and CF method, sub-watersheds are prioritized. Then, soil erosion susceptibility map of the study area was prepared with both methods and was verified by the PSIAC method. In this research, the quantitative analysis of the Monj Watershed and its 11 sub-watersheds were carried out to evaluate the geomorphometric characteristics of each sub-watershed and to investigate the susceptibility of erosion in each of the sub-watersheds. In this regard, 22 geomorphometric parameters were investigated, and the results indicate the basic, linear, shape, and topographic characteristics of the watershed. Based on the ranking of the streams, the Monj watershed was classified as a five-order watershed with an area of ​​71/70 km2 and a perimeter of 373/123 km. The total number of streams in the watershed is 2204. The total length of the streams in the watershed is 327.393 km. The lengths of the streams in the watershed vary from a minimum of 4.62 km for the sub-watershed 6 to a maximum of 91.62 km for sub-watershed 7. The mean values ​​of the stream length vary from a minimum of 0.1216 for the sub-watershed 6 to a maximum of 0.1187 for the sub-watershed 8. In the next step, the geomorphometric analysis of watershed was carried out to prioritize the sub-watersheds in different scales and sub-watersheds. Finally, all sub-watersheds were categorized into 4 classes in terms of susceptibility to erosion according to the values of VIKOR algorithm and CF model. The determination of the importance of the effective parameters in erosion using AHP method showed that the parameters of drainage density, slope, and infiltration number with scores (0.125, 0.116 and 0.104),had the greatest effect on the erodibility of sub-watersheds. In contrast, the parameters of form factor, elongation ratio, and the length of overland flow with the lowest scores (0.008, 0.011 and 0.023) had the least effect on the erodibility of the sub-watersheds. The classification of sub-watersheds in terms of susceptibility to erosion in the superior method of VIKOR showed that 27.21 km2 (38.48%) was located in the very high susceptibility class, 1.60 km2 (2.27%) was in the high susceptibility class, 37.62 km2 (53.21%) was in the moderate susceptibility class, and 4.26 km2 (6.032%) was in a low susceptibility class from the total area of watershed (70.707 km2). In addition, from the total study area in the CF method, 62.83 km2 (88.87%) was located in the very high susceptibility class, 6.37 km2 (4.50%) was in the high susceptibility class, and 4.75 km2 (3.36 %) was in the moderate susceptibility class.