Investigating Changes in Water Balance Components Using the SWAT+ Model and Determining the Role of Factors Affecting Them

Detailed water resource balance reports are very important in promoting sustainable watershed management programs. The main purpose of this study is to evaluate and use the SWAT+ model as a new version of the SWAT model in simulating hydrological processes and determining the role of effective factors in the changes of water balance components in Tashk Bakhtegan basin. In this regard, after preparing the required information and entering them into the SWAT+ model, the steps of calibration and validation of the model for river flow in 10 hydrometric stations, base flow in three upstream stations, as well as calibration of the underground water level, evapotranspiration and yield of major cultivated crops was done at the basin level during the period (1980-2014). The evaluation of the model calibration results in most of the stations shows the index values of (R2>0.5) and (NSE>0.2) that the results were favorable and acceptable. Comparing the results of the water balance obtained from SWAT+ with the results of other studies shows that the final values of the evaporation-transpiration parameter were higher and the surface currents were lower than the results of other researches, so that only compared to the results of the SWAT model, the three mentioned components increased by 17. 0, 1.13 and showed a decrease of 0.06 billion cubic meters per year. Examining the spatial and temporal changes of the balance components also showed that the changes in precipitation and evapotranspiration decrease from north to south of the basin, and in areas with high rainfall, their fluctuations are more proportionate and congruent. The most important factor in the decrease of water flow (74.7 percent) and the increase of evaporation-transpiration (80.3 percent) of the basin is related to non-climatic and human factors such as land use change, construction of dams, etc. and the effect of climatic factors on water flow changes and evaporation-transpiration is 25.3 percent and 19.7 percent respectively. Evaluation of SWAT+ model calibration results and comparison of its results with other studies conducted in the basin indicate the acceptable performance of this model in simulating and separating the contribution of different climatic and human factors in the hydrological conditions of the studied basin. Therefore, according to the new capabilities of this model and the improvement of the simulation processes of underground water and its exchange with the river compared to the SWAT model, it is recommended to use this model in order to estimate and verify the water balance components of basins.