EcoTWIN 1.0: A Fully Distributed Tracer-Aided Ecohydrological Model Tracking Water, Isotopes, and Nutrients

Abstract. The value of stable water isotopes in constraining process representation in hydrological models is well acknowledged with numerous tracer-aided hydrological models developed in recent years, yet few have leveraged these benefits for more robust water quality modelling. Therefore, we introduce EcoTWIN, a fully distributed tracer-aided ecohydrological model that simultaneously tracks water, isotopic, and nutrient fluxes in an integrated C++ framework. A thorough validation was conducted by calibrating EcoTWIN against discharge, in-stream isotopes, and NO₃-N concentrations (1980–2024) in 17 large-scale (10³ – 10⁵ km²) European catchments spanning a wide range of geographic and climatic gradients. Furthermore, three reanalysis products (ERA5 snow depth, MODIS evapotranspiration, and GRACE surface water anomaly) were employed to further validate the capacity of EcoTWIN to reproduce associated internal water fluxes without calibration. Results showed good model performance of both calibrated in-stream targets and uncalibrated internal fluxes in most catchments. Therefore, we conclude that EcoTWIN is a flexible, transferable modelling tool for prediction and process inference in terrestrial ecosystems ranging from boreal to subtropic climates. Constrained by tracer simulations, the model not only captures the celerity, but also the velocity of hydrological fluxes, thus providing spatio-temporally-explicit estimations of water ages and travel times. Such information provides opportunities to bridge catchment hydrology and water quality by linking travel times with biogeochemical processing times. We demonstrate this with a proof of concept using Damköhler Number in nitrogen modelling.