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AbstractDynamic nature of soil eorsion and its dependent to climatic, topographic, geologic conditions, soil, vegetation cover, land use, land management and etc. has spatial and temporal effects on soil genesis and erosion. Computation and preparing soil erosion and sediment yield maps would present important information that could be used in planning of dams, reservoirs, channels, soil conservation practices, evaluation of onsite and offsite erosion damages, land use planning and land capability. Soil erosion not only affects on soil degradation and loosing agricultural lands, but also, sediments cause water pulloution, fills irrigation canals and dam reservoirs. The rate of soil erosion depends on land use and climatic conditions, so that, dense forests have the least erosion rate, while constructed lands and excavated lands have higher rate of soil erosion. Different type of land uses, different land management, the kind of rock and soil cause different amount and form of erosion. Erosion map is a kind of fundamental, scientific and practical maps in executive, research and educational sections. On the other hand, based on climatic conditions, soil generation proceses is not ablr to replace eroded soil and almost non-renewable soil resources are wasted. Unfortunately, there is no information about the rate of soil degradation in the country which is a seriouse problem in infrastructure planning. According to importance of measuring the amount of soil erosion and sediment yield in whole country, this project was defined and implemented by Soil Conservation and Watershed Management Research Institute. Therefore, 7th order watersheds were classified in seven homogenous areas based on hydrometric stations and their long-term sedimen data were analyzed. After determination of suspended load by logged mean load within discharge classes method, the average soil erosion rate of the country was calclated in watersheds with hydrometric stations. According to the amount of river bedload rates and sediment delivery ratio of the watershed, the average sediment yield and wheigted average of soil erosion were 2.02 and 15.54 t.ha-1.yr-1 respectively and finally national soil erosion map was prepared.Keywords: National soil erosion map, Sediment yield, 7th order watershed, Soil erosion, Watershed landscape
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