Extended Abstract
 
In order to improve the management of water resources in basins, it is important to continuously assess the impact of inter-basin transfers. The water balance is an important tool for the management of these resources. Modeling the water balance of complex basins is a process that requires a thorough understanding of hydrologic behavior, geology, groundwater flow, a variety of data, and the effective use of modeling tools. In many watersheds where inter-basin water transfer occurs, especially if part of it is subsurface, there is insufficient information on how this transfer affects the water balance of the basin and its contribution to water resources.
In the current study hydrological modeling and field data analysis was used to assess the water balance in one of the major sub-basins of the Zayandehroud Basin, focusing on the impact of water transfer in this basin. As an innovation, this study utilized an integrated modeling approach. This approach combined the SWAT software, the results of the FEFLOW groundwater model for the adjacent area, and the development of a river water balance model in the Ghaleshahrokh basin. This integrated framework enabled the comprehensive evaluation of all flow components of the basin. Using SWAT software recalibrated with a Nash-Sutcliffe coefficient of 0.85, the atmospheric components of the water balance, including precipitation and evapotranspiration, were estimated and the resulting direct runoff and infiltration from the conversion of precipitation to runoff were derived. By using the results of the FEFLOW model in the aquifer of the study area adjacent to the research basin, it was also possible to estimate the return of water to the groundwater and the exchange with surface water. As a research innovation, a river water balance model was developed that achieves a Nash-Sutcliffe efficiency coefficient of 0.79 and a KGE coefficient of 0.75. Finally, in the water balance with a monthly time step, surface components such as the direct runoff, the base flow, the amounts of surface water transferred through the tunnel and the outflow from the spring, as groundwater transfer, were determined.
The results of this study which was conducted on Ghaleshahrokh basin, the main sub-basin of the Zayandehroud, revealed that regarding long-term average, the basin output is about 1127 and 1037 MCM/year, observed and calculated respectively. Also, 54% of the basin's output is attributed to the transfer of surface water through the first and second Kouhrang tunnels, 9% is due to the natural water outflow through springs ( such as Dimeh, as the major spring), 28% is direct runoff from precipitation and 9% consists of subsurface flows that forms the base flow of the river. The long term average rainfall of the basin is estimated about 1060 mm/year, with 28% of the total rainfall resulting in direct runoff, 22% in infiltration and the rest in evapotranspiration losses. The agricultural use in the basin was calculated to be 130 MCM/year. Considering that about 63% of the outflow from the study area, which supply the most of the volume of Zayandehroud dam, is discharged from outside the basin, both surface and subsurface, managers and policy makers should seriously consider expanding inter-basin cooperation. Since the contribution of water supplied by Kouhrang sub-basin is significant, adequate attention to this sub-basin is of particular importance both in terms of quantity and quality protection. It is also necessary to protect the discharging area of the Dime spring. Changes in rainfall, temperature, snow melting, as well as human activities such as land use change in the Kouhrang sub-basin and the discharging area of the Dime spring can greatly affect the quantity and quality of water entering the dam reservoir. Continuous review of water balance and analysis of inflow and outflow patterns, monitoring the quality of different water sources, planning for sustainable consumption development, managing flood risks or droughts, and generally developing long-term strategies and plans for sustainable use of water resources and environmental protection in the Ghaleshahrokh basin, as the origin basin of the inter-basin water transfer, are necessary. Also additional studies are suggested to understand the hydrological, monitoring and management of the process of inter-basin water transfer (naturally and artificially), and analyze their effects to ensure the sustainability of local habitats as well as supply the future water, and manage water resources regarding all environmental and social aspects.