| 12 Sep 2025
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.
This manuscript describes a novel spatially distributed ecohydrologic model ECOTWIN. This model links hydrology with nutrient and isotopic modules. The capability has great potential to improve our understand the terrestrial and aquifer ecosystem interactions. Although similar capability has been attempted in several other studies with different focus such as dissolved organic carbon (DOC), the inclusion of lateral flow with nutrient and carbon cycle is still a challenge in most spatially distributed (eco)hydrological models.
There are some missing information in the current manuscript, such as (1) groundwater level; (2) resolution for evaluation; (3) chemical reaction. See details below. The presentation of some information can be improved, especially some figures are too small to be read.
Distributed, do you mean spatially distributed?
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Sentence stoped abruptly without giving reasons.
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What is “somehow”
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Where is 81% come from? Reference needed
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On disk/memory change to online and offline coupling
Figure 1. Is soil zone part of vadose zone?
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What if the water table is above the last soil layer?
Equation 16:
If the pond is not next to the river channel, why or how it contribute to the channel flow?
Similarly, in equation 18, if the terrestrial grid cell is far away from the river channel, how does the vadose zone contribute to the channel flow?
Figure 2:
Does not have coordinates. Such as longitude and latitude information.
What are the particular reason why these 17 watersheds were chosen.
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Does the model need spin-up to reach steady state?
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How do you consider the upstream - downstream dependency in discharge, some watersheds are connected based on Figure 2.
Figure 4:
Show line/color meaning in the Figure as well. Also missing time step information.
The subfigures are too small to read. Maybe move some into SI and only show a few here.
Figure 5. Is the time series watershed averaged? If so, the caption should mention that.
Figure 6:
The GRACE datasets resolution might be too coarse for comparison; there need to be some justification somewhere.
Check other spatial products as well, maybe a table listing all produce with original and resample resolution for reference.
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Integrated C++ framework: C++ is a programming language, not a framework.