Soil erosion hazard Zonation using SLEMSA model in the Ziarat catchment

Soil erosion is a natural process in the geographical cycle that occurs as a result of water or wind intrusion in the transport of soil particles. In addition, human intrusion causes an increase in erosion rates. Ziarat catchment has faced a drastic change in land use, from forest to agriculture fields and road/villa construction. These have caused severe soil erosion, increase in the sediment load and considerable change in river route. The average slope of this catchment is 35.5. Such a great slope plays an influential role in many occurrences such as sedimentation, different types of erosion and mass movements. These are all significant in studying and zoning a catchment in terms of erosion.

In this study, SLEMSA model was utilized which can be used to estimate erosion in a regional scale with splash erosion. The catchment under investigation is Ziarat catchment which is a mountainous catchment in the south of Gorgan. SLEMSA model intermingles simple and fundamental data and emphasizes on some important environmental factors, vegetation and soil erosion. The numeral output of this model indicates the erosion risks and is reported as ton per hectare per year. In this model topographic, soil erosion, and vegetation coefficients are noteworthy. They are calculated using the amount of slope, slope length, erodibility, rain energy and free energy.

In order to prepare the topographic coefficient, soil erosion map and vegetation map ARC GIS was used. The map of region slope was prepared with DEM and consequently, the length of hillside map was obtained using the amounts of m. Finally, the topographic coefficient map was generated using the amount of slope and the length of hillside in ARC GIS.  Rain kinetic energy and soil erodibility are needed to obtain soil erosion map. Initially, gradient map was obtained via calculating rain gradient and its application on region DEM. Then, the resulted map was used to prepare rain kinetic energy map. In the next stage, the soil erodibility map was prepared using the resultant geology and soil maps of the catchment. Then, climate factor was calculated using the rain kinetic energy. In the third stage, in order to prepare the vegetation coefficient map, the vegetation quantity (i) was extracted. Finally, the maps of all three coefficients were entered into Z=C.K.X and soil erosion zoning for Ziarat catchment was prepared.

Results show that the western parts and a small area of the eastern part of Ziarat catchment have high soil erosion rate due to being located on Shemshak formation which is sensitive to erosion. These two areas, and the western area specifically, have high slopes due to the process of placing the rate curves, which are highly compacted. This factor can accelerate water flow velocity on the ground and intensify aggravation of soil erosion. In addition, slope amount in the eastern and western areas of the catchment is increasing due to elevations. Maximum topographic coefficients are located in western and eastern elevations of the catchment which might be the result of high slope amounts and slope lengths in these elevations.
The climate factor is derived from rain kinetic energy amounts (E) and soil erodibility (F). The maximum of this factor is observed in the western and eastern areas of the catchment and also in the path of the river. The reason for such an amount is the existence of Shemshak formation. Also, the soil material of this area is formed by shale, sandstone, and alluvium and are very erodible.  In general, considering erosion, the catchment under investigation includes the erosion range of at least 1.65 to utmost 131 ton per hectare annually. Taking the consequent high erosion rate of the catchment into account, optimization of land use is one of the appropriate strategies in order to achieve sustainable development and decrease wasting resources in Ziarat catchment.