# Projections of Climate Change Impact on Stream Temperature: A National-Scale Assessment for Poland
## 3 Results
### Projected Stream Temperature Changes  
The validated stream temperature model was applied to simulate climate change scenarios, aiming to evaluate the prospective impact of climate change on stream temperature regimes in Poland. The study focused on seasonal average stream temperatures at the reach scale, considering two future periods across six distinct climate models under RCP4.5 and 8.5 scenarios. Comparative analyses with the baseline period were conducted to discern the climatic alterations. Figure 5 visually represents the outcomes, revealing that all reaches in Poland are anticipated to exhibit elevated stream temperatures during the summer season compared to the baseline conditions. In the baseline scenario, the country-averaged stream temperature, initially at 16.1 \({}^{\circ}\)C, is projected to rise by 0.5 \({}^{\circ}\)C in the NF and further increase by 1 \({}^{\circ}\)C in the FF under the RCP4.5 scenario. The impact is notably more pronounced under the RCP8.5 scenario, with increases reaching 1 \({}^{\circ}\)C in the NF and a substantial 2.6 \({}^{\circ}\)C increase in the FF. In the winter season, the projected increases in stream temperatures are also illustrated in Figure 6. Under the RCP4.5 scenario, the anticipated rise is 0.6 \({}^{\circ}\)C in the NF and a further elevation of 1.3 \({}^{\circ}\)C in the FF. Comparatively, the RCP8.5 scenario exhibits more substantial increases, reaching 1.1 \({}^{\circ}\)C in the NF and a notable 2.8 \({}^{\circ}\)C increase in the FF. The analysis revealed that climate-induced changes in stream temperatures across Poland do not exhibit distinct spatial patterns. Instead, the projections suggest a uniform warming tendency, encompassing the entire territory of the country. These projections emphasize the persistent tendency of escalating stream temperatures during winter across Poland, underscoring the differentiated impacts foreseen under the distinct RCP scenarios. The temporal variability of mean seasonal stream temperature is presented in Figures 7 and 8. The Mann-Kendall test revealed no statistically significant trends in the data within the multi-year periods considered (ACT, NF, FF). However, when comparing the analyzed time frames (ACT-NF and ACT-FF), all changes in stream temperature were found to be statistically significant based on the \(t\)-test, indicating notable shifts in temperature between the baseline and future projections under the given climate scenarios. Figure 4: Box plots showing the model performance expressed by Kling–Gupta efficiency (KGE), coefficient of determination (R2) (**A**), and percent bias (PBIAS) (**B**) values in 369 water quality monitoring points. Figure 5: Spatial distribution of multi-annual summer season mean stream temperature at the reach level for Representative Concentration Pathways (RCPs) 4.5 and 8.5 at baseline (ACT), in the near future (NF), and in the far future (FF). Figure 6: Spatial distribution of multi-annual winter season mean stream temperature at the reach level for Representative Concentration Pathways (RCPs) 4.5 and 8.5 at baseline (ACT), in the near future (NF), and in the far future (FF). The analysis of stream temperature projections across seasons and RCP scenarios reveals distinct changes in the low (20 th percentile) and high (80 th percentile) temperature values. For the summer season under the RCP4.5 scenario, the 20 th percentile temperature is projected to increase from 13.2 \({}^{\circ}\)C in the baseline period (ACT) to 13.7 \({}^{\circ}\)C in the near  
Figure 8: Projections of the average daily air temperature during the winter period over multiple years for RCP4.5 (**A**) and RCP8.5 (**B**). The bands indicate extreme values (min and max) from nine climate models, the solid line represents the median. The green colour represents the historical period. The intensity of the blue and orange colours denotes the time horizon, where lighter shades indicate the near future, while darker shades indicate the far future.