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

Abstract

The understanding of hydrologic processes in Central and Northern Ontario's mesoscale watersheds, located within the Precambrian Shield region, remains limited, posing challenges for accurate hydrological modeling and assessment of climate change impacts on water resources. This study focuses on Central and Northeastern Ontario, typically characterized by granitic bedrock, small depressions, and shallow acidic soils, where annual precipitation exceeds evapotranspiration, resulting in abundant surface waters. Changes in hydrological processes in this region can have significant consequences for the local ecosystem of mesoscale watersheds. Therefore, investigating the effects of climate change on water quantity is crucial. This research utilizes stable water isotopes (SWIs) as cost-effective tools to improve our understanding of hydrologic processes and flowpaths in mesoscale Precambrian Shield watersheds. By analyzing long-term meteorological, hydrometric, and SWI data from the Sturgeon River, French River, and Muskoka River watersheds, valuable insights are gained regarding the impacts of climate change on hydrological processes in these regions. The study employs a new isotope-enabled distributed hydrologic model, isoWATFLOOD, which provides a good representation of fluxes, storages, and their changes due to climate change in mesoscale and large- scale watersheds. The research objectives include exploring the key controls and importance of surface water storage (lakes and wetlands) on hydrologic function in the Sturgeon River-Lake Nipissing-French River (SNF) and Muskoka watersheds, evaluating isoWATFLOOD hydrologic model's performance in simulating streamflow and isotope values in the Sturgeon River-Lake Nipissing (SN) watershed, evaluating the importance of wetland connectivity representation in isoWATFLOOD performance across the SN watershed, and assessing the impacts of climate change on streamflow and hydrologic partitioning in the SN watershed using the isoWATFLOOD hydrologic model. PCA and HCPC approaches are used to identify variation in controls on hydrologic function in SNF and Muskoka watersheds using combination of hydrometric, geology, landscape and isotopic metrics. The findings reveal greater evaporative enrichment impacts in Muskoka compared to the SNF catchments, with Muskoka exhibiting less variability in streamflow isotopes. The study identifies a positive correlation between wetland area and damping ratio (coefficient of variation of isotopes in streamflow to coefficient of variation of isotopes in precipitation), suggesting that wetland connection/disconnection and varying evaporation impacts contribute to isotopic value variability in catchments with higher wetland coverage. Muskoka and SNF catchments generally fall into separate clusters, primarily influenced by wetland and lake area percentages, mean slope, and the extent of glacialacustrine and glaciofluvial outwash deposits. The combination of catchment classification analyses and stable isotopes (δ 18O and δ 2H) proved effective in studying how different catchment characteristics influence variations in hydrometric response. An application of isoWATFLOOD was set up for Sturgeon River-Lake Nipissing (SN) watershed. Five separate models with varied connected wetland (CW) ratios between 10% to 50% are set up to evaluate the importance of CW ratio in model performance. The SN isoWATFLOOD model, calibrated using isotope and streamflow data, successfully simulates streamflow and isotope values (KGE > 0.6) across 11 catchments. Wetland connectivity percentage significantly influences streamflow and isotope simulations, particularly during the calibration period. The most accurate streamflow simulations occur with 40% wetland connectivity, improving baseflow representation. This study advances isotope-enabled hydrologic simulations using isoWATFLOOD and provides insights into wetland connectivity representation, a critical landscape aspect of Precambrian Shield watersheds. Stable isotopes prove valuable in addressing the challenge of equifinality. Using the SN isoWATFLOOD model and considering 16 global climate model (GCM)- emission (RCP) models, findings project a future characterized by warmer and wetter climatic conditions (2020-2082) compared to the baseline period (1990-2019). On average, the study predicts an annual discharge increase ranging from 4.8% to 11.5%, with elevated winter and fall streamflow across the watershed. These changes result from warmer fall and winter seasons, reduced freezing days, increased annual precipitation, and more frequent extreme precipitation events. Additionally, the simulations indicate an earlier spring freshet peakflow, accompanied by a reduced peak flow rate. Furthermore, climate change will impact hydrological partitioning, leading to alterations in the contributions of annual average daily baseflow to streamflow. Moreover, there will be a rise in average annual daily direct runoff due to intensified annual precipitation, more frequent extreme precipitation events, and rain-on-snow occurrences within the watershed. The results highlight the significance of integrating climate change impacts into water resources management planning, specifically concerning peak flow timing, seasonality, and changes in flow volume during different seasons.