Stable Water Isotope Signal of Snow Meltwater: Testing Three Different Measurement Setups in Boreal‐Subarctic Conditions

ABSTRACT Snowmelt plays a vital role in the hydrological cycle of high‐latitude and high‐altitude regions, contributing to spring runoff, groundwater recharge and regional water resources. In snow‐dominated regions, accurate estimates of snowmelt contributions are essential for water resource management, flood risk assessments and ecosystem health. The stable water isotope composition ( δ 2 H and δ 18 O) of snow and snowmelt provides valuable insights into the processes affecting water sources. The choice of field sampling method and the evolution of isotopic signatures over time can introduce considerable uncertainties into hydrological analyses. This study addresses two research questions: (1) How does the isotopic signal evolve between different stages of the snow during the season? and (2) How do different sampling instruments and methodologies influence the estimated isotopic composition of snowmelt? The research focuses on two distinct boreal and subarctic environments in Northern Finland, comparing three meltwater sampling techniques using a snow lysimeter, passive capillary sampler (PCS) and funnel‐bottle setup. These methods offer different temporal resolutions, from high‐resolution daily sampling to bulk seasonal sampling of cumulative meltwater. Comparing data from different snow season stages (snowfall, snowpack and snowmelt) revealed a gradual enrichment in heavy isotopes. The study highlights the importance of selecting optimal snowmelt sampling methods based on the specific environmental conditions of the site. This selection is critical for minimising biases and improving the accuracy of snowmelt contribution in hydrological models. The isotopic data showed variations in snowmelt isotopic signatures, with differences between sampling setups and study sites. These findings underscore the importance of using representative snowmelt samples to enhance the reliability of isotope‐based data analysis and hydrological models. Ultimately, this research will improve water resource assessments in the context of climate change and provide a more nuanced understanding of snowmelt processes. The application of stable isotopes in snowmelt studies provides insights that are relevant for water management.