Isotope applications in the Critical Zone across Europe: the latest activities of the WATSON COST Action

The COST Action WATSON - Water isotopes in the Critical Zone: from groundwater recharge to plant transpiration (CA19120; www.watson-cost.eu) is an European network of researchers and stakeholders who use the stable isotopes of hydrogen and oxygen to trace water fluxes to better understand hydrological, hydrogeological, and ecological systems. WATSON currently includes more than 250 members from 38 countries. The Action aims to integrate and synthesize current interdisciplinary scientific knowledge on the use of the stable isotopes of water to understand the mixing and partitioning of water in the Earth’s Critical Zone. The network is organized into four working groups (WGs) that focus on major scientific challenges: 1) groundwater recharge and soil water mixing processes; 2) vegetation water uptake and transpiration; and 3) catchment-scale residence time and travel times. A fourth WG is in charge of the communication and dissemination activities. WATSON started in 2020 and is currently in its final year. In this contribution, we synthetize the most recent results and the current and planned activities. WG1 is analyzing different isotope-based methods to calculate groundwater recharge in various environments, preparing training and educational material, and a review paper on isotope methods to assess groundwater recharge and subsurface mixing processes. WG2 is analyzing the data from two Europe-wide isotope sampling campaigns to estimate the sources of water uptake of beech trees and spruce trees. Moreover, WG2 is finalizing a review paper on isotope sampling, extraction, and isotopic analysis methods to study vegetation water use. WG3 is comparing different isotope-based methods to calculate transit times, and preparing training scripts and educational guidelines. WG4 is in charge of many dissemination and communication activities, including the monthly seminar series, updating the website and the social media accounts with the latest information, videos, and technical and scientific material. In addition, all WGs are involved in the preparation of an online, interactive, and open data-map showing locations where isotope samples have been collected. The WATSON activities will conclude with a large, final online conference, where all WATSON members (and beyond) will be invited to share their knowledge, experience, findings, and recommendations in using stable isotopes to advance our understanding of water fluxes in the Critical Zone.