Applying RS and GIS for Soil Erosion and Sediment Estimation by MPSIAC Model - A case study of Kenesht watershed in Kermanshah, IranJafari (2002) has performed a qualitative and quantitative estimation for the watershed area of Chiklhab by GIS and has estimated the criteria and scales necessary in PSIAC model and has used them in the above area. Nikkami(2003) in a technical-practical and scientific journal about PSIAC in GIS environment has talked about main problems in implementation and usage of effective factors in the model and the correct and effective from this model in Iran. This research and similar researches which were done in Iran show that if we use the model and observing according to its conditions and criteria; the experimental models can give a correct interpretation about erosion and sedimentation in the watershed area. Sadeghi (1991) in Ghezelowzan watershed area of Ghezelowzan showed that MPSIAC model for estimating erosion is a suitable method. Bagherzadeh karimi (1993) in the same area found that this method has the highest compatibility with the field studies. Jalalian (1992) showed that the estimated sediment by MPSIAC method has a good compatibility with the observed amount (Hadiane amri et al. 2006). nowadays, the speed of human beings a technology in gathering information and the need for quicker, more precise and more cost efficient factors decision making has caused the geographical information systems and remote sensing to be effective for productivity. The use of GIS as a powerful tool in analysis and storage of data has greatly helped the increase of speed and accuracy of the studies and with the aid of satellite images and remote sensing techniques, we can have in time and orderly data for hard to reach places(Pilesjo, 1994). Same of the factors effective in erosion like land cover and land use can be extracted from satellite images (Gumel, 1996). v So in this research by the use of remote sensing techniques and geographical information systems and applying MPSIAC as a good model in qualitative and quantitative erosion and sediment estimation the amount of soil erosion and sedimentation in Kenesht watershed in Kermanshah province was evaluated. Material and method The Kenesht watershed is located in the northern of Kermanshah in the west of Iran. It's area is 14333.36 ha. The highest, lowest and average elevation of the area is about 3360, 1325 and 1837 m above mean sea level, respectively. The average annual rainfall of area is 714.5 mm and annual temperature is 9.5?c. The general view of the watershed in its location in the country is depicted in figure 1. Geologically the area under study is part of the metamorphosed zone of sanandaj-sirjan and includes the calsic stoned of Bistoon and the radiolarite stones of Kermanshah. Geomorphologically the mountainous area covers a vast area, around 60% of area and about 15% as small hills and badland and river and alluvial fans and plains including about 25% of the area. There are 9 types of plants in the area which are as fallows: two agricultural types, two rangeland types with Excellent condition and positive tendency, three rangeland types with good condition and fixed tendency, one type of rangeland with medium condition and fixed tendency and two range land types with weak and very weak conditions and negative tendency. The research method 1- Preparing the maps and the necessary information At first step the maps of geology, hydrologic units (fig.2), slope, land use, plant cover, soil and isohyet map was produced. Then these maps were overlayed and sediment homogeneous units map(fig.3) was defined to run MPSIAC model. Then with regard to existing relationship in the model we calculated the score for the nine factors of erosion and sedimentation and by the aid of GIS they were attached to sediment homogeneous units map. Further more the erosion rate map was produced using aerial photographs and satellite images and checked by the field control (fig.4). Finally in order to determine the runoff production potential of the area the hydrologic studies of Kenesht watershed was used. 2- Determining the sediment production in sediment homogeneous units and hydrologic unites After preparing the final map of the potential erosion and sediment production in sediment homogeneous units, the amount of produced sediment in hydrologic units was calculated according to weight score of the sediment homogeneous units. The final aim of this research was the comparison of the derived results from sediment production potential estimation in hydrologic and sediment homogeneous units; so that the amount of specific sediment was calculated to compare the results. The data analysis For the comparison of the estimated sediments the data were analyzed in the following ways: 1- Sediment estimation and classification of produced sediment by sediment homogeneous units score. The results of the model in each sediment homogeneous units analyzed. The results of these analyzes for homogeneous units are shown in table 1. 
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