Overland flow erosion, Experimental Plot, Efficiency, Sediment, ANOVA

Abstract

Introduction
Information about river flow change and subsequent changes in water quality characteristics can help to manage and plan water resources. The environmental and socio-economic impacts of river flow changes are very important in an environmental water management. Climate change is an important challenge that should influence different parts of human life on earth such as rivers and lakes. The Evaluation of the impact of climate change phenomenon on the hydrological processes of rivers can decrease the challenges of managers and planners of water resources in the next period. The selection of suitable models is important for evaluation and prediction of the effects of climate changes on rivers and watershed discharge. Several hydrological models were used to evaluate the effects of climate change on a hydrological cycle. The Soil and Water Assessment Tool (SWAT) has been extensively used, mainly by hydrologists for watershed hydrology related subjects, since 1993. SWAT model should include both a forecasting model and weather generating model. This means that the generated weather data of the future should be presented to SWAT model for forecasting future rainfall and temperature. This is a new possibility for future river and watershed hydrology studies. The main aim of this study was to evaluate the effect of the future climate change on river discharge of the Heruchay River in Ardebil using SWAT model.
Methodology
In this study SWAT2009 model was used to in investigate and predict the quantitative changes of the discharge of the Heruchay River. For the period of 2014-2041, the daily rainfall and temperature data were predicted under three scenarios of A2, B1, and A1B, using LARS_WG climate model. The simulated data was used as the entered information of SWAT model and the model was implemented for 2014-2041 period.
SWAT is a river basin scale model that should work on a daily time-step. It was developed to predict the effect of the management decisions and climate change on the water cycle. In this study, SWAT model was used for its ability to simulate and forecast stream flow and evaluate the effect of climate change on river discharge.
A topographical map (Digital Elevation Model), climate data (daily rainfall, Maximum and minimum temperature), and soil and land use maps were prepared using GIS and measured data. As the precipitation is an important key input that influences flow and mass transport of the rivers, 5 rainfall gauging stations and 2 weather stations located in the study watershed were used.
 
Result and Discussion
The results of this study showed that SWAT model had an acceptable performance for discharge simulation during calibration and validation periods with coefficients of variation of 0.81 and 0.8 respectively for calibration and validation.  Based on the results of A2 and B1 scenarios, the flow rate of the study river increased, whereas a decrease in the flow rate was predicated based on the results of the A1B scenario. The results of the climatic model indicated that the pattern of the rainfall should change in the prediction periods as the rainfall decreases in the winter and spring, while it increases in the summer. 
Conclusion
This study offered a methodology for flow simulation and forecasting of future discharge via SWAT model. The effects of future climate change on flow quantity were examined. In this study, SWAT model was used to predict the impact of the future climate changes on river discharge. Model evaluation was done via Nash and Sutcliffe (NS), coefficients of determination (R2), P-factor, and R-factor. After model calibration, the predicted data under several climatic scenario were presented to the model. The results showed that the average of discharge will increase based on the A2 and B1 scenarios, while it will decrease under the A1B scenario.  Therefore, it can be concluded that SWAT is a suitable model for discharge simulation in semi-arid areas. The results of this study also indicated that the combination of the results of LARS-WG and SWAT model should lead to an acceptable prediction of hydrological behavior of the rivers. It is important to notice that in this study only the effects of climate change on river discharge was evaluated. For a sustainable management strategy, other aspects of the watershed such as population pattern changes, land use change, and industrial development should be considered. The impact of the climate and land use change on water quality and soil erosion should also be investigated in the future studies.