Impacts of Land use Changes Scenarios on Water Balance Components using WetSpa Model (Case Study: Ziarat Watershed of Golestan Province)

Every year, due to population growth and economy development, land uses and land covers are changing. The various types of land uses have significant roles on components of water balance of a watershed. The simulation modeling has the capability to predict floods, to evaluate land use impacts on floods, and to make decision in watershed management. In this research, the effects of land use change scenarios such as current state, deforestation, 100-percent growth of urbanization and the combination of two scenarios (i.e. deforestation+ urbanization) have been evaluated on the water balance using the WetSpa model in the Ziarat watershed, which covers an area of 95.15 km2 and located in Golestan province. In order to execute the WetSpa model, the hourly hydro meteorological data including rainfall, evapotranspiration, temperature, and discharge are used as inputs for a four-year period (from 2007 to 2011). The result of the simulation shows a good agreement between the simulated hydrograph and the observed one. Then, the scenarios of deforestation, urbanization development and the combined scenario of deforestation with urbanization have been designed in GIS environment, and the simulation was performed using the calibrated model. The finding showed there were increases in surface runoff and subsurface flow, groundwater and total flow rates; the maximum change was for runoff rate that increased 5.2 in first scenario to 18 mm in fourth scenario. The decrease of land surface roughness and vegetation canopy lead to decrease in evapotranspiration and leaf interception in the study area.