The Relationship between Geomorphic Characteristics and Watershed Sediment Yield: A Case of Selected Subwatersheds of Khorasan Razavi

Extended abstract 1- Introduction Soil erosion by water is a dominant geomorphic process which threatens food security in most parts of the world .The geomorphic characteristics of a watershed play an important role in watershed hydrology, soil erosion processes and sediment yield. Geomorphic characteristics can be an indicator of soil erosion and sedimentation of a watershed. Geomorphic characteristics of watersheds are classified into linear, relief, and areal categories. Linear characteristics include stream number, bifurcation ratio, stream length and streams order. Relief characteristics include three-dimensional features of the watershed such as hypsometric integral, ruggedness number, and relative relief. Areal characteristics encompass morphological characteristics such as drainage density, stream frequency and watershed shape parameters. Accessibility to Digital Elevation Models and remote sensing data as sediment yield predictors simplify the calculation of the watershed geomorphic characteristics. The purpose of this study was to use the latest capabilities of geographic information system to extract the watershed geomorphic characteristics and determine their relationship with sedimentation in the subwatersheds of Khorasan Razavi province. 2- Methodology This study was conducted in 22 subwatersheds in Mashhad, Neyshabour and Fariman watersheds in Khorasan Razavi province. In order to select appropriate subwatersheds, the hydrometric and rainfall data for hydrometric and meteorological stations were obtained from Khorasan Razavi Regional Water authority for the selected watersheds. Annual sediment load was calculated using sediment rating curve method. Physiographic and geomorphic characteristics including 30 geomorphic parameters were calculated for each subwatershed using Digital Elevation Model with spatial resolution of 30 m. In order to determine the relationship between geomorphic characteristics and sediment yield of the subwatersheds, a multivariate regression stepwise analysis was used. In the multivariate regression, the important geomorphic characteristics which affected watershed sedimentation were identified and based on those parameters, the best annual sediment yield and geomorphic characteristics equation were presented. 3- Results The subwatershed areas under study vary from 40 square kilometers for the Chakaneh Olya to 9339 square kilometers for the Hossein Abad subwatershed. The average annual sediment yield for the studied subwatersheds during the period of 30 years varied from 1026 tons per year in the Jang subwatershed to 274572 tons per year at Hossein Abad watershed. The subwatersheds of Kalateh Rahman and Jang had the highest and lowest sediment yield, respectively, with 317 and 5 tons per square kilometers. The relationship between geomorphic characteristics and sediment yield of subwatersheds showed that the annual sediment yield had a positive correlation at the 5% confidence level with form factor and annual rainfall. The results of this study showed that the watershed shape parameters including form factor, elongation ratio and shape index had high correlations with sediment yield with the pertaining coefficients of 76.8, 76.5 and 72 percent, respectively. Also, the correlation coefficient of annual rainfall with annual sediment yield was 73.9 percent. After rainfall and form factor, elongation ratio was the third parameter that had a high correlation coefficient (76%) with sediment yield. In addition, watershed shape index which was a function of form factor was correlated with sediment yield at 72%.  Among these characteristics, the annual rainfall and watershed form factor were used in the stepwise regression in the final model and were selected as predictor variables for sediment yield. Study results showed that the annual rainfall and watershed form factor variables could predict 64% of the annual sediment yield of the studied watersheds. 4- Discussion & Conclusions The results of this study indicated that there was a significant relationship between the geomorphic characteristics of the studied watersheds and annual sediment yield. Watershed form factor was a dimensionless index for flood flow and movement, erosion severity and sediment transport capacity of watersheds. This factor was a function of watershed area and length. The runoff and the amount of flood peak in bigger watersheds will increase the sediment yield. Many researches have reported a high correlation between rainfall and sediment yield. Arid climate and poor vegetation coverage in the selected watersheds were the main reasons for the high correlation of rainfall and sediment yield. Soil erosion and sediment yield would increase due to the high intensity and low duration of rainfall along with the scarcity of vegetation coverage and erodible soils in this region. Overall, the study results indicated that with the development of new technologies and the possibility of extracting different physiographic and geomorphic parameters of watersheds from a Digital Elevation Model, it is possible to present regional equations for the prediction of sediment yield using geomorphic characteristics that can be used in sediment control and Watershed Management Programs.