Extensive model evaluation for seasonal and spatial dynamics of dissolved oxygen in SWAT+

Dissolved oxygen (DO) is a critical indicator of aquatic ecosystem health, but multidecadal trends show a general decline in DO concentrations in riverine ecosystems, highlighting the need to better understand the driving dynamics and improve predictions. This study presents the first implementation of dissolved oxygen (DO) dynamics in SWAT+ (SWAT+DO). A rich dataset of spatially high resolution observed DO data is used to investigate modelled and observed differences in seasonal and spatial DO dynamics related to stream temperature and hydrological conditions. The spatial model performance was improved by introducing a spatially distributed formulation of the parameter and achieved a KGE of 0.73 on a daily resolution without further calibration. The influence of streamflow and stream temperature on DO has strong seasonal and spatial differences. With these findings our study highlights the strengths and limitations of a process-based ecohydrological model, depending on the scale that it is applied on. • Spatial model improvement with SWAT+DO using dynamic reaeration rate • Climatic effects on modelled DO concentrations • Interactions of DO, stream temperature and streamflow depend on scale and season