This thesis develops a methodology for mesoscale model verification and validation that is founded on the rigorous constraint imposed by the need to conserve both water mass and isotopes simultaneously. The isoWATFLOOD model simulates δ¹⁸O in streamflow, which effectively reduces and constrains errors associated with equifinality in streamflow generation by improving internal parameterizations.

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. Stable water isotopes because of their natural abundance and systematic fractionation have the ability to preserve information on water cycling across large domains. Several coordinated research projects have recently focused on integrating stable water isotopes into global and regional circulation models, which now provides the opportunity to isotopically force land-surface and hydrological models. Where traditionally streamflows are the primary validation criteria in hydrological modelling, problems arise in remote and ungauged basins, or large watersheds where streamflows may not be well monitored. By streamflow validation alone, no insight is obtained on the internal apportioning and physical representation of sub-processes contributing to streamflow. The primary goal of this research is to develop alternative measures to parameterize mesoscale hydrological models in a physically-based manner, and to validate such models over large domains.

This research develops improved model parameterizations that facilitate realistic runoff generation process contributions. The examination of runoff generation processes and the subsequent δ¹⁸O of these processes are performed for two mesoscale watersheds:​ Fort Simpson, NWT and the Grand River Basin, ON. The isoWATFLOOD model is shown to reliably predict streamflow and δ¹⁸O of streamflow, and simulates mesoscale isotopic fractionation associated with evaporation. In doing so, a more physically meaningful, robust modelling tool is developed that is practical for operational use. This research also contributes the first continuous record of δ¹⁸O in streamflow that enables the visualization of spatial and temporal variability and dominant hydrologic controls within mesoscale watersheds.