EcH<sub>2</sub>O-iso 1.0: water isotopes and age tracking in a process-based, distributed ecohydrological model

Abstract

Abstract. We introduce EcH2O-iso, a new development of the physically based, fully distributed ecohydrological model EcH2O where the tracking of water isotopic tracers (2H and 18O) and age has been incorporated. EcH2O-iso is evaluated at a montane, low-energy experimental catchment in northern Scotland using 16 independent isotope time series from various landscape positions and compartments, encompassing soil water, groundwater, stream water, and plant xylem. The simulation results show consistent isotopic ranges and temporal variability (seasonal and higher frequency) across the soil profile at most sites (especially on hillslopes), broad model–data agreement in heather xylem, and consistent deuterium dynamics in stream water and in groundwater. Since EcH2O-iso was calibrated only using hydrometric and energy flux datasets, tracking water composition provides a truly independent validation of the physical basis of the model for successfully capturing catchment hydrological functioning, both in terms of the celerity in energy propagation shaping the hydrological response (e.g. runoff generation under prevailing hydraulic gradients) and flow velocities of water molecules (e.g. in consistent tracer concentrations at given locations and times). Additionally, we show that the spatially distributed formulation of EcH2O-iso has the potential to quantitatively link water stores and fluxes with spatiotemporal patterns of isotope ratios and water ages. However, our case study also highlights model–data discrepancies in some compartments, such as an over-dampened variability in groundwater and stream water lc-excess, and over-fractionated riparian topsoils. The adopted minimalistic framework, without site-specific parameterisation of isotopes and age tracking, allows us to learn from these mismatches in further model development and benchmarking needs, while taking into account the idiosyncracies of our study catchment. Notably, we suggest that more advanced conceptualisation of soil water mixing and of plant water use would be needed to reproduce some of the observed patterns. Balancing the need for basic hypothesis testing with that of improved simulations of catchment dynamics for a range of applications (e.g. plant water use under changing environmental conditions, water quality issues, and calibration-derived estimates of landscape characteristics), further work could also benefit from including isotope-based calibration.

Water ages in the critical zone of long-term experimental sites in northern latitudes

“…The R water ages at 50 cm depth in this study are obviously younger than catchment-scale runoff water ages Ala-aho et al, 2017b;Benettin et al, 2017;Kuppel et al, 2018;Piovano et al, 2018). Nevertheless, the dynamics from the plot-scale R water ages are similar to the catchment runoff water ages due to hydrometeorological controls leading to generally increasing values throughout spring towards summer (decreasing storage) and the lowest water ages during winter (highest storage).…”

“…Such catchment models cannot account for the soil physical processes in a similar detail to a 1-D model due to computational limitations. However, our results imply that it might be worth adding a dual-porosity representation, similar to the conceptualization in SWIS, to the recently published EcH2O-iso (Kuppel et al, 2018).…”

“…Godsey et al, 2010;Hrachowitz et al, 2010). Generally, the plot-scale soil hydraulic simulations can help to better understand the processes taking place within the catchment and to constrain or benchmark spatially distributed hydrological models (van Huijgevoort et al, 2016;Ala-aho et al, 2017b;Kuppel et al, 2018). Such catchment models cannot account for the soil physical processes in a similar detail to a 1-D model due to computational limitations.…”

“…We further have to assume that all R flux leaving the soil profile will end up in groundwater, but spatially distributed catchment models (Ala-aho et al, 2017b;Kuppel et al, 2018) might reveal that such water could end up in the ET flux from saturated areas in the valley bottom when the groundwater feeds the riparian zone.…”

Ecohydrological modelling with EcH2O‐iso to quantify forest and grassland effects on water partitioning and flux ages

“…This was not possible at the grassland site as concentrations were too low and uncertainties too high in the absence of monitoring of throughfall and the effects of dry and occult deposition on the grass sward. (Kuppel et al, 2018b). In this study, we further adapt the model formulation to simulate other passive tracers, such as chloride, which are not subject to evaporative fractionation but evapoconcentration.…”