Daily atmospheric precipitation stable water isotopes help disentangling water flow paths and sources at a long-term limnological research station

A better understanding of the hydrological dynamics of aquatic ecosystems is of vital importance for assessing their ecological functions and predicting their responses to climate change. Hydrology has been shown in several studies to be a major driver of ecosystem processes in the catchment of Lake Lunz, which is part of the Global Lake Ecological Observatory Network. Stable isotopes of the water molecule (δ¹⁸O-H₂O and δ²H-H₂O) are a valuable tool for understanding water flow and temporal dynamics in complex, karstic catchments. Using stable isotopes, we complemented an ongoing monitoring program that samples the lake catchment and initiated a daily precipitation sampling scheme. Our objective is to examine the insights achievable from daily sampling of precipitation regarding stream hydrology. The study site is a subalpine, karstic catchment of 18 km², overlaid with shallow soils. The data that will be discussed in this presentation covers the period from October 2021 to October 2024 spanning three hydrological years. We collected daily and monthly precipitation samples in the proximity to the outlet of the catchment (elevation 604 m.a.s.l.) and stream grab samples within the regular monitoring programme.

The outcomes of the daily stable precipitation isotope analysis revealed that the widest range in precipitation occurs during winter, with values ranging from -17 to -2.8‰ for δ¹⁸O-H₂O. Conversely, the most depleted precipitation daily sample was measured during a major rain event in September 2024 with -21‰ for δ¹⁸O-H₂O. River grab samples reflected the average catchment precipitation ± 1‰ that was calculated based on isoscapes for δ¹⁸O-H₂O. Several heavy rain events were recorded with depleted isotope ratios. A three-component hydrograph separation suggested that recent water contributes between 10 and 50% to the stream flow and high precipitation events shifted the isotopic composition of the river. However, d-excess indicated that these events contributed little to base flow and groundwater recharge. Daily precipitation isotopes improved hydrograph separation based on stable isotopes, providing the opportunity to understand the contribution of different precipitation events to base flow.

Incorporating stable water isotopes into routine monitoring of the Lake Lunz catchment presents a significant potential to understand the water sources and their temporal dynamics. This offers an opportunity to place the ecological studies conducted in Lake Lunz within a hydrological framework and better comprehend how the system might respond to climate change impacts, including river intermittency, extreme rainfall events, decreased winter precipitation, and the thawing of the snow cover during winter.