The purpose of this study was to simulate surface runoff of the Ghareh-Chay River that was modeled using SWAT hydrological model. In this regard, GIS software version 10. 4, DEM and waterways and river network characteristics were first extracted. The statistics used for the 2003-2016 Ghareh-Chay Basin Pole Doab Khondab, Joshiravan, and Sarouq stations were considered. Soil curve number (CN) method was used to calculate basin mortality. Model calibration and validation were performed by SWAT-Cup software with SUFI-2 algorithm. To evaluate the model's ability to simulate runoff discharge, Nash-Sutcliffe, R2, p-factor and r-factor indices were used. Eight years of measured data (2004-2011), precipitation, temperature and daily discharge were used to calibrate the model. And model validation with residual data (2012– 2016). The model validation results with Nash-Sutcliffe, R2, p-factor and r-factor indices for the Khondab station were 0. 65, 0. 66, 0. 35, 0. 07%, and for the Pole Doab station, respectively, the results were 0. 57, 0. 72, 0. 07, 0. 05. The model evaluation confirms the SWAT model's ability to simulate surface runoff of the Ghareh-Chay River. Finally, the comparison of simulated hydrographs and monthly observations showed that the model modeled well the peak discharge and base discharge times but estimated the peak discharge values more than the actual values.

Immense floods are caused by runoff from high intensity precipitation in semi-Arid regions. Nowadays, different methods are used to predict flood volumes for crisis management. SWAT is a comprehensive semi-distributed continuous model for soil and water assessment. This model has the capability to simulate hydrologic procedures in different time scales including daily, monthly and annual using physical data in the basins along with meteorological data. Also, it has capability to link to GIS too. In this research, runoff was simulated in monthly time scale in Hablehrud basin. Hablehrud is located in Semnan province with 3200 Km2 area. For simulation of runoff, primarily basic necessary data were gathered and field controls were done, then, theses data were used in the model. Calibration and sensitivity analysis was performed. Simulation were carried out for a 30 year duration period (1981-2011). Calibration was done using SWATcup extension. A 7 year period was used for calibration and 4 year for verification (1998-2011). Nash Sutcliff coefficient of efficiency was used for assessing the model. Sensitivity analysis were performed and sensitive parameters were determining among 32 parameters. This analysis was done in SWAT-Cup which is an extension for SAWT, also t-stat and p-value were determined. NSE were 0. 74 and 0. 63 for calibration phase and verification phase, respectively. We can conclude sensitive parameters are: SFTMP, SMTMP, GW-DELAY and ESCO in Hablehrud basin.

In recent decades, the amount of runoff has decreased significantly in a few regions of the world. Human activities and climate change are the two main factors in runoff changing. Accordingly, it is very important to analyze the influencing factors and quantify their impact on the runoff. In this research, the analysis of the process of gradual and sudden changes in the QezalOzen River runoff at the Yesaul hydrometric station in Kurdistan province was carried out using the linear regression method and Pettit's test. The results showed that the runoff time series in the upstream watershed of Yesaul hydrometric station has a decreasing trend and the breaking point of the runoff time series is 1998. According to this, the runoff time series was divided into the base period and the evaluation period. Runoff simulation was done using SWAT model. After entering input data of the watershed in the SWAT model, the model was implemented between 1987 and 2010. From 1987 to 1993 is before the change point. Based on this, syear 1987 was used as warmup, 1988 to 1991 was used for calibration, and years 1992 and 1993 were used to validate the model. SUFI-2 algorithm was used in SWATCUP software package to calibrate the model. Since the number of the parameters in SWAT model is large, before calibration, the sensitivity analysis was done and the sensitive parameters were determined. Examining the results of calibration and validation of the model in the base period showed that the SWAT model has a good performance in simulating the runoff in Yesaul hydrometric station. Also, the results of the study showed that the impacts of climate change (76%) in reduction of runoff during the evaluation period is much higher than the impact of human activities (24%). In general, the current research states the necessity of taking necessary measures to adapt to climate change and create guidelines for human activities for sustainable access to water resources.