Critical Zone Response Times and Water Age Relationships Under Variable Catchment Wetness States: Insights Using a Tracer-Aided Ecohydrological Model

The dynamic relationships between water flux and storage, together with the associated water ages and speed of hydrological responses (as proxies for velocity and celerity respectively) are fundamental to understanding how catchments react to hydroclimate perturbations, such as floods and droughts. Using results from a calibrated, tracer-aided ecohydrological model (EcH₂O-iso) we analyzed the dynamics of storage-flux-age-response time (RT) interactions at scales that resolve the internal heterogeneity of these non-stationary relationships. EcH₂O-iso has previously shown an adequate representation of ecohydrological flux partitioning and storage dynamics (celerity), and water ages (velocity) over 11-year at Demnitzer Millcreek catchment (DMC, 66 km²), a drought-sensitive, lowland catchment in Germany. The 11-year period had marked hydroclimatic contrasts facilitating the evaluation of flux-storage-age-RT dynamics under different wetness anomalies. Our results show that the spatio–temporal variability of soil moisture and ecohydrological partitioning dynamics reflect both land use (especially forest cover) and distinct soil units (i.e., brown earth vs. podzolic soils). Spatial differences in RTs of storage were driven by rapid soil evaporation and transpiration responses to rainfall, which revealed a divergence of transpiration ages from RTs. RTs of groundwater and streamflow were fast (days), but mediation by soil water storage dynamics caused marked separation from water ages (years-decades) of deeper flow paths. Analysis of RTs and ages revealed a degradation of process representation with coarsening model spatial resolution. This study uses novel analysis of the spatio-temporal interactions of flux-storage-age-RT from a model to understand the sensitivity and resilience of catchment functionality to hydroclimatic perturbations.