The Effect of Optimum Management of Lands on Reducing the Peak Discharge and Water Turbidity (Case Study: Manshad Watershed, Yazd Province, Iran)

One of the main factors to decrease water quality in rivers is land use changes. This study was conducted to choose the best way for land use management in order to increase water quality related to the least turbidity, improving natural landscape quality and attracting ecotourism. So, the effect of land use changes on turbidity was investigated for 2 to 100-year periods using GIS and Hydrologic Engineering Center–Hydrologic Modeling System (HEC-HMS) model in 2012, in Manshad watershed, Yazd province, Iran. The Curve Number (CN) and land use planning model were variables in each simulation stage. After preparing the CN map, HEC-HMS model was calibrated for the observed rainfall-runoff events using CN method, Soil Conservation Service (SCS) model and lag time searching process. Finally, the best model was chosen among linear and non-linear sediment rating curves. The results showed that there was a lot of conformity between present land use and land use planning model. Terrace removing in land use planning model (as a scenario) increased flood volume (6.11%), peak discharge (6.23%) and turbidity in the peak discharge (11.02%) compared to the current land use. Therefore, according to the results, garden terracing in allowable slopes helps to manage water and soil so that there was almost no difference between two scenarios in terms of water turbidity in Watershed. It means that this watershed is managed on the basis of optimum land use.