طول دوره آماري مناسب براي برآورد ميانگين سالانه رسوب و رابطه آن با مساحت ، تغيير رسوبدهي سالانه ، خصوصيات اقليمي ، زمين شناسي و پوشش گياهي حوزه آبخيز

Evaluation of the proper length of record for estimation of mean annual sediment yield and its relation with area, variation of annual sediment yield, climate, geology and vegetation cover

The aim of this research is evaluation of proper length of record for estimation of mean annual sediment yield, its relation with climate, geology as well as vegetation cover and finding the optimum statistical period for suspended sediment sampling. In this research, it is tried to define actual (or best) mean annual sediment yield. For this study, 15 watersheds with sediment gauging station were selected around the country (Iran). Suspended sediment yields were calculated from 1 to n years (n is the most recent available data) for each station by suitable equations and water- discharge data. For determination of the proper lengths of record for each station and the length of time after which mean annual sediment yield stabilizes in each station, mean annual sediment yields were compared with the most recent annual mean (n year) as the most valid mean. Then, the variations from mean (standard deviation of the best mean) were calculated and each year that its variation was less than 10% was chosen as the optimum statistical period of data for that station. After that, the relationship between optimum statistical period and climate, geology and vegetation cover of the watershed was studied. The results have shown that the optimum statistical period for 13 stations is between 9 to 25 years. With increase of the area, percentage of annual sediment yield variation, percentage of sensitive and semi-sensitive geological formations and decrease of annual rainfall and De martin climatologic indes, optimum statistical period also increases.