# Projections of Climate Change Impact on Stream Temperature: A National-Scale Assessment for Poland
## 1 Introduction
The consequences of rising global temperatures on streams are multifaceted, influencing both thermal balance and the distribution of aquatic species. By synergizing modelling methodologies, researchers and resource managers gain tools for pinpointing vulnerable areas, devising effective conservation strategies, and proactively addressing the ecological disruptions precipitated by the warming of stream environments [22]. Mathematical models play a valuable role in the modelling of stream temperature and assessing the impact of climate change. Over the years, numerous stream temperature models have emerged, falling into two main categories, namely mechanistic and statistical models. A mechanistic model operates on the principles of the energy balance of heat fluxes and water mass balance [23]. Statistical models, in particular, have been extensively employed for these purposes, as evidenced by their widespread use [24; 25; 26]. These models rely on empirical regression relationships, establishing connections between stream temperatures and meteorological parameters. More sophisticated approaches, exemplified by the heat source model, consider the interplay of atmospheric, terrestrial, and aquatic heat exchange processes [27]. Several studies have also undertaken the modelling of stream temperature and the evaluation of climate change effects through the utilization of diverse hydrological models. These models take into account the influences of both meteorological and hydrological conditions [22; 28]. [PERSON] et al. [29] underscored the significance of employing multiple models for a comprehensive analysis of climate change impacts on water temperatures and aquatic ecosystems. Their emphasis on using diverse models stems from observed discrepancies among results, recognizing that employing a range of modelling approaches enhances the robustness and reliability of assessments related to the effects of climate change on freshwater environments. In general, the modelling approach not only fortifies our capacity to adaptively manage freshwater ecosystems but also underscores the important role of interdisciplinary collaboration in navigating the complex terrain of climate change impacts. Against this background, the study aims to evaluate the anticipated impact of climate change on stream temperature and its potential ramifications for freshwater ecosystems in Poland. This investigation utilizes an ensemble of six EURO-CORDEX projections, spanning the period from 2006 to 2100, across two Representative Concentration Pathways, namely RCP4.5 and 8.5. Additionally, the Soil and Water Assessment Tool (SWAT), a hydrological model customized for the Polish region, is employed to address this issue. This study represents a first-of-a-kind macro-scale analysis for Poland, providing valuable insights into the projected changes in stream temperature and their implications for the freshwater ecosystems in Poland, thereby contributing to a more profound understanding of climate change impacts in the region. This national-scale assessment fills a gap in the literature, as no prior studies have comprehensively examined the impacts of climate change on stream temperatures across the entire country.