Water stable isotope approaches for estimating water ages in the hydrological cycle

Water stable isotopes provide a tracer signal input into the hydrological cycle with every precipitation event over a certain area. Tracking this signal, its seasonal distribution, and its relative changes since that water fell as snow or rain, can provide information about water flow and transport processes in the critical zone or integrative information about them within catchments. Water stable isotopes combined with other approaches can also be used to estimate water ages, such as the transit or residence time of water. Knowing the distribution of transit or residence times and how they vary over time and space can further inform about flow paths and hydrological processes as well as time scales of solute transport and hydrochemical processes. In this talk, an overview of the importance of water ages in hydrology will be provided, and different methods will be introduced for estimating water transit or residence times based on water stable isotope data. Several examples will be shown where we used experimental data-based methods and hydrological modelling for estimating water ages in soils and in catchments. The importance of high-resolution isotope data will be emphasized for uncovering hydrological processes, their dynamics, and controlling factors.