# Projections of Climate Change Impact on Stream Temperature: A National-Scale Assessment for Poland
## 4 Discussion
### Modelling Limitations  
This study uses a simplified default equation [34] for stream temperature modelling. This equation provided a practical and efficient approach, particularly suited for large-scale assessments and national-level studies. While the default equation does not account for detailed hydrological processes, such as streamflow, snowmelt, and groundwater flow, its simplicity allows for a straightforward implementation and interpretation. In the pursuit of more accurate simulations, more advancements have extended beyond the basic SWAT framework. Various researchers have contributed to advancing and modifying the SWAT model for improved simulations of stream temperatures. These enhancements aimed to capture the complex interplay of hydro-climatological factors influencing stream temperature dynamics. Key modifications include the incorporation of simplified terms to represent the mass transfer process, allowing for a more comprehensive modelling of inflows and outflows into and out of rivers. This involves considering various SWAT model outcomes, such as snowmelt flow, surface runoff, lateral flow, and groundwater flow, each associated with their respective temperatures [61]. Additionally, Noa-Yarasca [62] have introduced explicit energy balance models that consider additional factors, such as the shade factor of riparian vegetation. These modifications contribute to a more nuanced understanding of stream temperature dynamics by incorporating the ecological context and hydrological conditions into the simulation process. While modified versions of the SWAT model incorporating additional complexities for stream temperature simulations exist, it is crucial to acknowledge that their widespread validation across various case studies might be limited. The default stream temperature equation, as employed in this study, benefits from a more extensive validation history and broader application in diverse geographical settings. The modified versions, although potentially offering more detailed representations of specific local conditions, may lack the comprehensive validation required for robust and generalized use. The use of a simplified default equation for stream temperature modelling in this study holds significant value and merits consideration. The model assumes a linear relationship between air and stream temperatures, which may not fully capture thermal dynamics in cold regions. Specifically, in scenarios where air temperatures fall below approximately \(-6.7\)\({}^{\circ}\)C, the model may inaccurately predict stream temperatures below 0 \({}^{\circ}\)C. This limitation highlights the need for enhanced modelling approaches in future studies, particularly those involving ice formation, groundwater influences, and other hydrological processes specific to cold-region rivers. Despite its simplicity, the default equation provides a practical and efficient approach, especially when dealing with large-scale assessments and national-level studies. The streamlined nature of the default equation allows for a straightforward implementation and interpretation, making it accessible to a broad range of users, including researchers, policymakers, and resource managers. Additionally, the default equation can serve as a baseline or reference point, offering a clear comparison against more complex models. In cases where the primary focus is on broader trends and large-scale patterns rather than fine-scale nuances, the simplified equation remains a valuable tool for gaining insights into the potential impacts of climate change on stream temperature regimes. Its ease of use and interpretability make it a practical choice for studies seeking to provide a comprehensive overview of climate change effects on freshwater ecosystems at a national scale. Given the paucity of case studies validating the modified SWAT versions for stream temperature, the default equation stands out as a well-established and validated option.