An Evaluation of the Optimal Change Scenarios of Land Use and their Impacts on the Hydrological Parameters: A Case Study of Dinevar Watershed

EXTENDED ABSTRACT      The land use change of watersheds has entailed many impacts on the processes of erosion and sediment yield over the time, and has caused the loss of soil quality and fertility. On the other hand, an increase in the sediment generation and its accumulation would reduce the capacity of the reservoirs. In this respect, the purpose of this study was to evaluate the effects of the implementation of land-use change scenarios on runoff and suspended sediment from the outlet of the watershed. The simulation of runoff and sediment was investigated according to the defined scenario for the Dinevar watershed with a focus on SWAT model based on the statistical indicators. The results of NS and R2 indicators above 50 and 60 percent for both calibration and validation steps, respectively, imply the model efficiency regarding the hydrological data simulating in the watershed. The results of the coefficient of determination and Nash-Sutcliffe were 0.55 and 0.6, respectively, in the calibration, and were 0.58 and 0.62, respectively in the validation, implying the model efficiency in the watershed. The suggested scenario was applied to improve the lands. This means that the bare land, which increased during this period, was planted by the evergreens and deciduous. The results of this scenario showed that the implementation of this land use change in the scenario reduced the suspended sediment and runoff into 52 and 32 percent, respectively. 1- INTRODUCTION      Land use with inappropriate vegetation patterns can cause severe water, soil and food losses and land degradation. Land use change is a large and fast process and assessing such changes and their impacts on the development, and management of the water resources in the future is very important. In order to better understand the process and the fate of the pollution and also the hydrologic regime, the SWAT model was used to simulate the runoff and suspended sediment under the influence of land use change over time during a given period based on the hypothetical scenarios in Dinewar watershed. 2- METHODOLOGY      Dinewar watershed with 214577 hectares is located in the northeast of Kermanshah. The average of the annual precipitation of this area during a 25-year period is 549.1 mm. The types of visible erosion in this watershed include surface erosion, rill erosion, water way erosion, gully and bank river erosion. The existing land use in the watershed included 34% agriculture, 1% garden, 10% free, 52% rangeland, 1% stone, 2.5% roads and residential areas. One of the models used to investigate the effects of land use change and climate change on runoff and sediment in the watershed scale was SWAT model. In the SWAT model, there is a possibility to assess the effects of different natural or managerial changes on runoff, sediment, nutrients and chemical balance in the watershed. With regard to runoff and sediment simulation, the data pertained to the synoptic station of Kermanshah and the meteorology stations of Sahne, Bisotun, Sonqor and Hasan Abad Sofla were utilized. Also, with regard to calibration, the validation and uncertainty analysis of results, i.e. the SUFI-2, was applied. To evaluate the simulated results, the performance criteria of the model including "P factor" and "R factor" as well as the two parameters of determination coefficient (R2) and Nash-Sutcliff were used.The new scenario was defined based on the previous and current proceeding land uses and the results of the simulation were compared to the current situation related to 2010. 3- RESULTS      Flood and suspended sediment data of 1998 to 2006 were used for calibration, and 2007-2010 were used for validation. The results for flow data showed that determination coefficients of both the calibration and validation steps were higher than 0.77, and the Nash-Sutcliff coefficient was about 0.6 and higher. The results of the calibration step for sedimentation data were also equal to 0.55 in the determination coefficient and 0.66 in the Nash-Sutcliff coefficient, and the results of the statistical indices during the validation period increased 0.1 percent. The P factor is closer to the one that implied a higher percentage of observation data in the uncertainty band and the R factor is closer to zero that implied better simulation at this step. In the new scenario, degraded and no vegetation areas were replaced by the evergreen and shrubbery trees with moderate density, and the simulations were carried out based on a new scenario. The land use map for 2010 was taken as a control map. The results of the simulation based on the new land use scenario showed that the daily average runoff is 4.86 m3 / s (32.72%), and the maximum discharge rate during the studied period decreased to 74.13 m3/s (36/69 %). The proposed scenario can also reduce the daily average suspended sediment in the study period by 252.98 mg / s (52.83%) and decrease the maximum suspended sediment rate by 9050 mg / sec (71.54%). 4- DISCUSSION and CONLLUSIONS In the defined scenario in this study, a change was made whose implementation is possible in reality. Many changes, such as reducing the area under the cultivation, residential and industrial areas, are not applicable in most of the watersheds. The new scenario, which aims at improving the vegetation cover and soil conservation, reduces sediment yield from the watershed, and this effect is especially noticeable at the peak of the simulation curves, and little effect is observed in places where the amount of sediment is lower. In addition, the results of the statistical indicators implied the high performance of the SWAT model in simulating hydrological processes which demonstrates the impact of various management measures and land use on these processes in the studied watershed.