Mesoscale Model Validation using Stable Water Isotopes: The isoWATFLOOD Model

A methodology to improve mesoscale model validation is developed by calibrating simulations of both water and isotope mass simultaneously. The isoWATFLOOD model simulates changes in oxygen-18 of streamflow and hydrological processes contributing to streamflow. The added constraint of simulated to measured delta oxygen-18 in streamflow lowers the models degrees of freedom and generates more physically-based model parameterizations. Modelled results are shown to effectively reduce and constrain errors associated with equifinality in streamflow generation, providing a practical new approach for the assessment of mesoscale modelling. The WATFLOOD model is a conceptually-based distributed hydrological model used for simulating streamflow on mesoscale watersheds. Given the model's intended application to mesoscale hydrology, it remains crucial to ensure conceptualizations are physically representative of the hydrologic cycle and the natural environment. Building upon the existing flowpath-separation module within WATFLOOD, the capability to simulate changes in oxygen-18 through each component of the hydrological cycle is introduced. Masses of heavy-isotope are computed for compartmental storages; compartmental flows transfer flux-weighted portions of isotope mass between storages; and mass outflows from each compartment simultaneously combine to form the resultant channel flow composition. Heavy-isotope compositions are enriched when storages undergo evaporation resulting from the loss of isotopically-depleted vapour described by the well-known Craig & Gordon isotopic fractionation model. The isoWATFLOOD model is forced by oxygen-18 in rain, oxygen-18 in snow, and relative humidity; and requires no additional parameterizations of WATFLOOD. The first mesoscale, continuous simulations of changes in oxygen-18 in streamflow are presented for the remote Fort Simpson basin in Northwest Territories, Canada and for the largely populated Grand River Basin in south western Ontario using the EnSim post-processor software. These simulations shed light on watershed 'hot spots' and the dominant hydrological controls and responses inherent to various regions and dominant hydrological controls.