assessing the impact of land use changes and rangeland and forest degradation on flooding using watershed modeling system

extensive flood damages all over the world necessitate its control and operation. hydrologic impacts of land use change appear in many ways such as total runoff, and flood peak flow. this study was performed in 2014 and aimed to investigate the impacts of land use changes on the occurrence of floods in the catchment of boostan dam in golestan province, iran. for this purpose, watershed modeling system (wms) was used to compare land use areas in 1996 with those in 2006 using the corresponding maps. after the calibration and validation of model in each period, rangeland and forest degradation and its effect on the flooding of catchment were evaluated using two representative parameters of peak flow and volume of flood. land use maps of both time periods were compared and the achieved results revealed that the total area of rangeland was increased whereas good rangeland areas were decreased, fair rangelands were increased and poor rangeland areas were remained relatively constant that mean a decrease in high quality rangelands in the catchment. also, the forest areas that decreased intensified flood. but peak flow and flood volume of the whole catchment have been mitigated. in spite of negligible change in total curve number (cn) of the catchment, rangelands in downstream and near residential areas converted to the agricultural lands and upstream agricultural lands converted to high and medium density rangeland. this means that distribution of land use changes was in such a way that influential upstream watersheds in flooding were associated with the reduced cns. so, the implemented biological measures have reduced the flooding potential of the catchment. sensitivity analysis of the model showed that 5% decrease in cn can cause 40% decrease in peak flow of the catchment and in contrast, 5% increase in cn can enhance flood peak flow up to 60%.