Exploring the hydrologic regulation and pollutant release of complex paddy systems in a large-scale agricultural catchment

Abstract

Complex paddy systems (CPSs) are prevalent in humid agricultural regions, presenting a dilemma between restoring them to wetlands or maintaining them for food production. Thus, a comprehensive assessment of CPSs is necessary. However, the current model’s description of CPS is insufficient, leading to incomplete representation of the water and pollutant transfer processes of CPS in the model. Therefore, a new version of the soil and water assessment tool (SWAT-CPS) was proposed, which utilizes physically based rules to measure the horizontal seepage-overflow process within CPSs. Overall, the SWAT-CPS outperforms SWAT in streamflow, water cycle simulation, hydrological event reproduction and pollutant release simulation. Water balance analysis and scenario simulations showed that CPSs increase baseflow and pollutant release throughout the year (about 7.69% and 3.05%, respectively) while affecting quickflow in different ways depending on the streamflow conditions. Changing the area of the CPS may have significant impacts on the hydrological composition. This study highlights the differences between CPSs and wetlands in hydrological regulation and pollutant release. In fact, not all areas are suitable for “returning farmland to wetlands”. Our work providing insights for decision-makers in planning the allocation of CPS and wetland shares.