Re-conceptualising the continental-scale co-evolution of hydrological and nitrogen cycles under water age framework

A key limitation in advancing ecohydrological understanding stems from the long-standing neglect of explicitly representing water velocities in models. Consequently, hydrological and water quality modelling often remains a grey box—capable of reproducing streamflow or solute dynamics, yet often for the wrong reasons. Stable water isotopes can bridge this knowledge gap, as their dynamics reflect integrated effects of transport and mixing along hydrological flow paths. Therefore, we developed EcoTWIN, a tracer-aided, fully distributed, process-based ecohydrological model that simultaneously tracks water, isotopes, and nitrogen fluxes. The model was applied to 3,821 European catchments at 5-km spatial and daily temporal resolution (1980–2024), and validated against discharge, in-stream isotope, and nitrate data from 1,218 sites, as well as remote sensing products and literature reports.

Through isotopic simulation, EcoTWIN provides novel insights into the velocities of the water cycle, complementing previous research focusing primarily on its celerity and magnitude. This allows a re-conceptualisation of the co-evolution of water and nitrogen cycles through the lens of water velocity. Under this water age framework, distinct hydrological–biogeochemical regimes were mapped along multiple geographic and hydroclimatic gradients across Europe, revealing how water velocity governs nitrogen retention versus export since 1980s. By quantifying the variability of transport (soil residence time) and reaction timescales (time required to remove nitrogen storage via denitrification and plant uptake), we identified four co-evolutionary schemes of water and nitrogen cycling over 1980-2024, which were dominated by the magnitudes of hydrological acceleration/deceleration: moderate hydrological shifts mitigated nitrogen leaching, whereas intense acceleration/deceleration of water cycling exacerbated soil nitrogen leaching/accumulation.

Projections towards 2100 further revealed an uncertain future of coupled water–nitrogen dynamics. Under low-emission scenario (SSP1-2.6), moderate hydrological shifts lengthened reaction times and enhanced biological uptake/denitrification, thus alleviating nitrogen leaching. In contrast, intensified droughts under high-emission scenario (SSP5-8.5) may trigger pronounced deceleration of water cycling in Eastern and Southern Europe, leading to moisture-driven suppression on nitrogen uptake and subsequent nitrogen accumulation. Such dual vulnerability of water quantity and quality is likely not confined to Europe but extends to Central and East Asia where water storage decline is ongoing and projected to intensify. This underscores the need to further extend water age frameworks to the global scale to better understand the coupled hydrological–biogeochemical resilience under climate change. Such insights can inform sustainable land management strategies to safeguard water quality and ecosystem resilience in a warming world.