Using stable water isotopes and isotope-enabled hydrologic modelling to quantify water in Central and Northeastern Ontario

The understanding of hydrologic processes in Central and Northern Ontario's mesoscale
\nwatersheds, located within the Precambrian Shield region, remains limited, posing challenges for
\naccurate hydrological modeling and assessment of climate change impacts on water resources.
\nThis study focuses on Central and Northeastern Ontario, typically characterized by granitic
\nbedrock, small depressions, and shallow acidic soils, where annual precipitation exceeds
\nevapotranspiration, resulting in abundant surface waters. Changes in hydrological processes in this
\nregion can have significant consequences for the local ecosystem of mesoscale watersheds.
\nTherefore, investigating the effects of climate change on water quantity is crucial.
\nThis research utilizes stable water isotopes (SWIs) as cost-effective tools to improve our
\nunderstanding of hydrologic processes and flowpaths in mesoscale Precambrian Shield
\nwatersheds. By analyzing long-term meteorological, hydrometric, and SWI data from the Sturgeon
\nRiver, French River, and Muskoka River watersheds, valuable insights are gained regarding the
\nimpacts of climate change on hydrological processes in these regions. The study employs a new
\nisotope-enabled distributed hydrologic model, isoWATFLOOD, which provides a good
\n
\nrepresentation of fluxes, storages, and their changes due to climate change in mesoscale and large-
\nscale watersheds.
\n
\nThe research objectives include exploring the key controls and importance of surface water
\nstorage (lakes and wetlands) on hydrologic function in the Sturgeon River-Lake Nipissing-French
\nRiver (SNF) and Muskoka watersheds, evaluating isoWATFLOOD hydrologic model's
\nperformance in simulating streamflow and isotope values in the Sturgeon River-Lake Nipissing
\n(SN) watershed, evaluating the importance of wetland connectivity representation in
\nisoWATFLOOD performance across the SN watershed, and assessing the impacts of climate
\nchange on streamflow and hydrologic partitioning in the SN watershed using the isoWATFLOOD
\nhydrologic model.
\nPCA and HCPC approaches are used to identify variation in controls on hydrologic function
\nin SNF and Muskoka watersheds using combination of hydrometric, geology, landscape and
\nisotopic metrics. The findings reveal greater evaporative enrichment impacts in Muskoka
\ncompared to the SNF catchments, with Muskoka exhibiting less variability in streamflow isotopes.
\nThe study identifies a positive correlation between wetland area and damping ratio (coefficient of
\nvariation of isotopes in streamflow to coefficient of variation of isotopes in precipitation), suggesting that wetland connection/disconnection and varying evaporation impacts contribute to
\nisotopic value variability in catchments with higher wetland coverage. Muskoka and SNF
\ncatchments generally fall into separate clusters, primarily influenced by wetland and lake area
\npercentages, mean slope, and the extent of glacialacustrine and glaciofluvial outwash deposits. The
\ncombination of catchment classification analyses and stable isotopes (δ
\n
\n18O and δ
\n
\n2H) proved
\neffective in studying how different catchment characteristics influence variations in hydrometric
\nresponse.
\nAn application of isoWATFLOOD was set up for Sturgeon River-Lake Nipissing (SN)
\nwatershed. Five separate models with varied connected wetland (CW) ratios between 10% to 50%
\nare set up to evaluate the importance of CW ratio in model performance. The SN isoWATFLOOD
\nmodel, calibrated using isotope and streamflow data, successfully simulates streamflow and
\nisotope values (KGE > 0.6) across 11 catchments. Wetland connectivity percentage significantly
\ninfluences streamflow and isotope simulations, particularly during the calibration period. The most
\naccurate streamflow simulations occur with 40% wetland connectivity, improving baseflow
\nrepresentation. This study advances isotope-enabled hydrologic simulations using
\nisoWATFLOOD and provides insights into wetland connectivity representation, a critical
\nlandscape aspect of Precambrian Shield watersheds. Stable isotopes prove valuable in addressing
\nthe challenge of equifinality.
\nUsing the SN isoWATFLOOD model and considering 16 global climate model (GCM)-
\nemission (RCP) models, findings project a future characterized by warmer and wetter climatic
\nconditions (2020-2082) compared to the baseline period (1990-2019). On average, the study
\npredicts an annual discharge increase ranging from 4.8% to 11.5%, with elevated winter and fall
\nstreamflow across the watershed. These changes result from warmer fall and winter seasons,
\nreduced freezing days, increased annual precipitation, and more frequent extreme precipitation
\nevents. Additionally, the simulations indicate an earlier spring freshet peakflow, accompanied by
\na reduced peak flow rate. Furthermore, climate change will impact hydrological partitioning,
\nleading to alterations in the contributions of annual average daily baseflow to streamflow.
\nMoreover, there will be a rise in average annual daily direct runoff due to intensified annual
\nprecipitation, more frequent extreme precipitation events, and rain-on-snow occurrences within
\nthe watershed. The results highlight the significance of integrating climate change impacts into water resources management planning, specifically concerning peak flow timing, seasonality, and
\nchanges in flow volume during different seasons.

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