Quantifying the Relative Impacts of Climatic and Anthropogenic Factors on the Interacting Surface and Groundwater Resources

• Quantifying climatic and anthropogenic impacts on interconnected water resources • Integrated modeling of surface and groundwater resources using SWAT-MODFLOW • Employing a fixing-changing method to assess the share of climatic and human effects • Applying the proposed methodology to a large-scale real-world case study • Human activities are responsible for 83% of the available water variations This study presents an integrated modeling framework to quantify the relative impacts of climate change and anthropogenic activities (CCAA) on coupled surface and groundwater systems. The approach combines an Integrated SWAT-MODFLOW (ISM) model with the fixing-changing (F-C) method to separate climate-driven and human-induced contributions to observed hydrological changes under historical conditions. The framework is applied to the Zarrinehroud basin and the Miandoab aquifer in northwestern Iran using observed hydro-meteorological, streamflow, and groundwater-level datasets for the period 1993-2017. Change-point detection based on Buishand, Pettitt, and SNHT tests identifies 2002 as a statistically significant shift, dividing the study period into a reference period (1993-2001) and a change period (2002-2017). The ISM model is calibrated and validated for the reference period and subsequently applied to the change period to quantify climate and anthropogenic impacts on surface runoff, groundwater levels, and their integrated response. Results indicate that anthropogenic activities, primarily groundwater abstraction, account for approximately 83.4% of the combined surface and groundwater variability, while climate change contributes about 16.6%. Surface runoff and groundwater levels both exhibit significant post-2002 reductions, accompanied by weakened surface-groundwater interactions, despite a slight increase in mean annual precipitation. Rising temperature and evaporation intensify climate-driven stress but play a secondary role relative to human water abstraction. The findings highlight the dominant role of human water abstraction in altering both surface and subsurface components of the hydrological system and demonstrate the value of integrated surface-groundwater modeling for attribution analysis. The proposed framework provides a transferable tool for diagnosing historical drivers of water-resource change and informing sustainable water management in heavily managed basins.