Evaluating impacts of climate change on hydrology and total nitrogen loads using coupled APEX-paddy and SWAT models

Climate change is currently one of the most critical issues in watershed management, and typical paddy systems should be addressed by watershed modeling approach in paddy-dominant landscapes. This study is designed to evaluate and enhance the watershed modeling approach currently used to characterize the impacts of climate change on hydrologic and water quality responses while considering a paddy environment. APEX-paddy, which is a newly developed and modified APEX (Agricultural Policy/Environmental eXtender) model for paddy ecosystems, was coupled with SWAT (Soil and Water Assessment Tool) model to take advantage of the strengths of the two models. The resulting hybrid model, SWAPX, was calibrated and validated using observed data from 2008 to 2017 for two sites in the study watershed. Compared to SWAT, the accuracy of SWAPX was improved, showing statistically better results in the downstream including more paddy field areas. Ten GCMs were selected, and the characteristics of these GCMs were evaluated to assess the impacts of climate change scenarios. When applying the climate change scenarios to the SWAPX model, the results indicated that the future streamflow would increase due to increased rainfall. The results also showed that total nitrogen (T-N) loads would increase rapidly in the near future, then decrease gradually through the 2090s (2091–2100). T-N load was affected by the characteristics of rainfall patterns (e.g., daily maximum rainfall and rainfall intensity) occurring in various GCMs. This approach will be helpful for decision-makers in adapting to climate change and evaluating Best management practices (BMP) for paddy-dominant watersheds.