WISE: A spatially explicit carbon cycling model at the watershed scale

Watershed-scale carbon cycling models characterize both vertical and lateral carbon fluxes and enable a comprehensive assessment of carbon budgets. However, existing models have limitations in representing hydrological flow pathways, which limits the accuracy of simulation results. In this study, we developed a spatially explicit watershed-scale carbon cycling model, named WISE (Watershed-based Integrated Simulator for the Environment), which incorporated detailed representation of hydrological connectivity across landscapes to examine the effects of hydrological flow pathways on watershed carbon cycling. First, a spatially explicit framework was designed to ensure that simulation units have clearly defined spatial locations and explicit flow relationships among them, including the interception of water and carbon by impoundments. Subsequently, four types of processes—hydrology, soil erosion, terrestrial carbon cycling, and aquatic carbon cycling—were represented and integrated based on this framework. Two representative case studies in an upstream watershed of the Ammersee lake in southern Germany and a well-measured catchment in northern Sweden were conducted to demonstrate the capacity of the developed model. Sensitivity analysis revealed that considering wetland interception improved KGE and NSE values by 0.28 and 0.54, respectively. By accurately simulating the processes of carbon transport and transformation in soil and inland waters, the WISE model provides a valuable tool for simulating and understanding watershed carbon cycling processes under global climate change scenarios.