Extended abstract
1- INTRODUCTION
Landuse change due to human activities is one of the important issues in regional planning. Considering the advantages and capabilities of the distributed hydrological models, they are appropriate for the survey of landuse changes as well as their quantitative estimates. Land evaluation methods are used to determine the compatibility of the land according to land capability, identification of effective factors on the floodwaters potential of the basins and zoning of the basins which are essential for runoff capability. The land is used according to their potential. For this purpose, the remote sensing data and geographic information system (GIS) were used to identify the factors affecting the flood potential and zoning of the basin in terms of flood potential.
2- THEORETICAL FRAMEWORK
In this research, a hydrological distribution model of WetSpa is used to simulate the hydrological components. This study examines the Simulation of optimal scenarios of urbanization impacts on flow hydrograph and sediment concentration using WetSpa model in Ziarat watershed in Iran.  In this study, TOPSIS method and GIS were used to model the suitability of land for urbanization, providing optimal scenarios.
3- METHODOLOGY
In this study, the basic WetSpa model inputs were the maps of a digital elevation model (DEM), land use and soil type in GIS raster format, and hydrometeorological data including hourly precipitation, evapotranspiration, temperature and sediment. In ArcGIS software, the distributed required maps for the model were extracted using the model input maps. Then, the model was implemented using these maps and data on rainfall, evapotranspiration, temperature, flow rate, sediment. With the help of the reference tables in the ArcView software environment, the spatial parameters of the model were determined in each cellular network.
After modeling the suitability of land for residential development through TOPSIS and GIS methods, the effects of the optimal scenario of residential development on the hydrograph and sediment were assessed. According to expert opinions, three scenarios including the current state of the residential areas of the basin, the development of residential areas with alternative low forest cover, and the residential development with alternative crops were considered, and also WetSpa model was implemented through mapping the three scenarios.
4- RESULTS
The accuracy of flow simulations based on the Nash-Sutcliffe model efficiency was 0.67.The evaluation of suspended sediment simulations for the calibration period based on the Nash–Sutcliffe criteria was 0.63 for the suspended sediment concentration. According to the assessment criteria considered in this study and using TOPSIS, 37 hectares of  Ziarat watershed area conditions are to create residential area. After running WetSpa model with the optimal scenarios, increased roughness coefficient reduced the flow velocity and runoff coefficient as well as the peak discharge watershed. Also, the amount of suspended sediment concentration and sediment transport was reduced to the current land use scenario. Thus, by applying the optimal scenarios, urbanization, flow hydrograph and sediment density showed better conditions in comparison to the current land use scenario.
5- CONCLUSIONS & SUGGESTIONS
Changes in the flow hydrograph parameters are insignificant among the scenarios. After applying optimal scenarios, the extent of residential areas has decreased, which has led to a slight decrease in surface runoff and peak discharge in Ziarat watershed. By applying the optimal scenario, the amount of runoff coefficient has decreased slightly, which is a slight decrease in runoff coefficient due to a slight increase in penetration and evaporation. Also, the coefficient of roughness has increased slightly with the application of optimal scenarios, which is due to the reduction of the size of residential areas in the optimal scenarios. After applying the optimal scenarios, the flow velocity has been reduced slightly, which is a slight decrease in flow velocity due to a small decrease in the runoff coefficient and a slight increase in roughness coefficient. So, with the implementation of the optimal scenarios for the development of residential areas, hydrograph and sediment had better conditions than the scenario of the current state of residential areas. According to the results of this study and similar studies in Ziarat watershed, it is hoped that the authorities pay more attention to the crisis in the watershed and the increase in the indiscriminate and unprincipled residential areas in the watershed so as to prevent the unexpected consequences of the change before it can be prevented.