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Abstract Lakes are experiencing ice declines and fundamental changes in winter conditions. For Earth's largest lakes that experience seasonal ice cover, the relationship between ice conditions and evaporation is critical to water balance estimates and global freshwater storage. Here, we analyze robust data sets of net basin supplies, satellite‐derived products, and model estimates of surface turbulent heat flux for the Laurentian Great Lakes during the period 1973–2022. We show that ice cover does not have a strong relationship with lake evaporation in winter months and that often the magnitude of the ice effect on moisture flux reduction is within the range of natural variability and the uncertainty of water budget estimates. This suggests that differences in lake evaporation between cold and warm winters is driven by seasonal overlake atmospheric conditions, more broadly, and that ice cover reduces but does not determine the resultant evaporation. 

Abstract Extreme water temperatures impact the ecological and economic value of freshwater systems. They disrupt fisheries habitat, trigger harmful algal blooms, and stress coastal infrastructure. This study examines the spatiotemporal patterns of heatwaves and cold‐spells in the Great Lakes using 82 years of simulated surface temperature data. Significant increasing trends in heatwave duration were observed in Lake Superior and Lake Michigan‐Huron, while cold‐spell duration increased on all lakes except Ontario. Temperature anomalies during these events varied from the climatological mean by as much as ±10C, but did not change significantly over time. Analysis revealed substantial spatial variability in heatwaves and cold‐spells, both within and across lakes, with differences driven by air temperature and ice cover anomalies as well as associated climate teleconnections (i.e., the East Pacific/North Pacific and Atlantic Multidecadal Oscillation). These findings highlight the importance of both climatic and lake processes in shaping extreme temperature events. 

Abstract Winter ice conditions in the Great Lakes play a crucial role in shaping ecological processes, shoreline dynamics, and regional weather patterns. Although atmospheric factors are widely acknowledged as the primary drivers of ice formation and duration, the influence of subsurface groundwater flow remains largely unexplored. In this study, we evaluate how spatially and temporally variable groundwater flux affects ice formation and thermal structure in Lakes Michigan and Huron, using a coupled hydrodynamic‐ice model. Simulations were conducted for the winters of 2014, 2015, and 2016—a period characterized by distinct atmospheric and ice conditions—and were validated against observed ice concentration maps and temperature profiles. Results show that groundwater enhances ice thickness during colder winters by strengthening water column stability, limiting vertical mixing, and insulating the surface layer, thus promoting thicker, longer‐lasting ice. Sensitivity analyses reveal that moderate increases in groundwater flux intensify stratification and prolong ice concentration, while an extreme, high flux (1000x) disrupts stability and reduces ice thickness. Coastal regions display more pronounced effects due to higher groundwater input, whereas offshore zones exhibit comparatively weaker responses. These findings highlight the significant role of groundwater flux in modulating ice dynamics and stratification in large freshwater systems such as the Great Lakes. This research underscores the importance of incorporating subsurface hydrology into coupled modeling frameworks to improve predictions of ice dynamics and water column stratification. Future work should focus on obtaining high‐resolution observational data on groundwater flux and ice thickness, particularly near shorelines, to further refine coupled hydrodynamic‐ice models. 

Abstract Lake surface temperature extremes have shifted over recent decades, leading to significant ecological and economic impacts. Here, we employed a hydrodynamic-ice model, driven by climate data, to reconstruct over 80 years of lake surface temperature data across the world’s largest freshwater bodies. We analyzed lake surface temperature extremes by examining changes in the 10th and 90th percentiles of the detrended lake surface temperature distribution, alongside heatwaves and cold-spells. Our findings reveal a 20–60% increase in the 10 and 90 percentiles detrended lake surface temperature in the last 50 years relative to the first 30 years. Heatwave and cold-spell intensities, measured via annual degree days, showed strong coherence with the Arctic Oscillation (period: 2.5 years), Southern Oscillation Index (4 years), and Pacific Decadal Oscillation (6.5 years), indicating significant links between lake surface temperature extremes and both interannual and decadal climate teleconnections. Notably, heatwave and cold-spell intensities for all lakes surged by over 100% after 1996 or 1976, aligning with the strongest El-Niño and a major shift in the Pacific Decadal Oscillation, respectively, marking potential regional climate tipping points. This emphasizes the long-lasting impacts of climate change on large lake thermodynamics, which cascade through larger ecological and regional climate systems. 

Climate change, water change and the critical role of community resilienceDr. Amanda Shankland, Dr. Carolyn Johns, and Gail Krantzberg, explore climate change resilience, water change, and the critical role of climate-ready communities. Climate change is impacting communities across the globe. For many communities, climate change manifests as water change in the form of floods, droughts, shoreline erosion, water quality degradation, water insecurity, and uncertainty. According to the United Nations, 153 countries have territory and communities within at least one of the 286 transboundary rivers, lake basins, and 592 transboundary aquifer systems (UN Water 2024). Communities worldwide are grappling with water governance challenges induced by climate change – inherently local, complex, and transboundary. 

Abstract Synthetic ensemble forecasts are an important tool for testing the robustness of forecast‐informed reservoir operations (FIRO). These forecasts are statistically generated to mimic the skill of hindcasts derived from operational ensemble forecasting systems, but they can be created for time periods when hindcast data are unavailable, allowing for a more comprehensive evaluation of FIRO policies. Nevertheless, it remains unclear how to determine whether a candidate synthetic ensemble forecasting approach is sufficiently representative of its real‐world counterpart to support FIRO policy evaluation. This highlights a need for formalfit‐for‐purposevalidation frameworks to advance synthetic forecasting as a generalizable risk analysis strategy. We address this research gap by first introducing a novel operations‐based validation framework, where reservoir storage and release simulations under a FIRO policy are compared when forced with a single ensemble hindcast and many different synthetic ensembles. We evaluate the suitability of synthetic forecasts based on formal probabilistic verification of the operational outcomes. Second, we develop a new synthetic ensemble forecasting algorithm and compare it to a previous algorithm using this validation framework across a set of stylized, hydrologically diverse reservoir systems in California. Results reveal clear differences in operational suitability, with the new method consistently outperforming the previous one. These findings demonstrate the promise of the newer synthetic forecasting approach as a generalizable tool for FIRO policy evaluation and robustness testing. They also underscore the value of the proposed validation framework in benchmarking and guiding future improvements in synthetic forecast development. 

Climate and hydrologic change across the Great Lakes region and other transboundary watersScott Steinschneider, M. Altaf Arain, Paulin Coulibaly, Andrew Gronewold, and Gail Krantzberg, explore climate and hydrologic change across the Great Lakes region in North America and other transboundary waters. Hydroclimate extremes are transforming water landscapes in transboundary regions. These systems are particularly susceptible to hydroclimatic variability due to shared governance structures, interconnected ecosystems, and a wide range of water users. The Great Lakes basin – one of the world’s largest freshwater systems, shared by Canada, the United States, and numerous Indigenous sovereign nations – exemplifies how shifting hydroclimatic conditions are challenging conventional approaches to water management across borders. In this region, the impacts of these changes are evident in increased flooding, shoreline erosion, economic disruption, ecosystem stress, and rising uncertainty surrounding water availability and quality. 

Abstract Forests significantly influence regional and global water cycles through transpiration, which is affected by meteorological variables, soil water availability, and stand and site characteristics. Variable retention harvesting (VRH) is a forest management practice in which varying densities of trees, such as 55% and 33%, are retained after thinning or harvesting. These trees can be grouped together or evenly distributed. VRH aims to enhance forest growth, improve biodiversity, preserve ecosystem functions, and generate economic revenue from harvested timber. Application of VRH treatment in forest ecosystems can potentially impact the response of forest transpiration to environmental controls. This study analyzed the impacts of four different VRH treatments on sap flow velocity (SV) in an 83‐year‐old red pine (Pinus resinosa Ait.) plantation forest in the Great Lakes region in Canada. These VRH treatments included 55% aggregated (55A), 55% dispersed (55D), 33% aggregated (33A), and 33% dispersed (33D) basal area retention, and an unharvested control (CN) plot, 1 ha each. Analysis of counterclockwise hysteresis loops between SV and meteorological variables showed larger hysteresis areas between SV and photosynthetically active radiation (PAR) than vapor pressure deficit (VPD) and air temperature (T_air), particularly in clear sky and warm temperatures in the summer. It demonstrated that PAR was the primary control on SV across VRH treatments, followed by VPD andT_air. Larger hysteresis loop areas and higher SV values were observed in the CN and 55D treatments, with lower values found in the 55A, 33D, and 33A plots. This suggests that maintaining dispersed retention of 55% basal area (55D) is the optimal forest management practice that can be utilized to enhance transpiration and forest growth. These findings will assist forest managers and other stakeholders to adopt sustainable forest management practices, thereby enhancing forest growth, water use efficiency, and resilience to climate change. Additionally, these practices will contribute to nature‐based climate solutions. 

Towards Haudenosaunee research sovereignty: Investing in local research and training to support community developmentThe article emphasizes the importance of Indigenous Research Governance in Six Nations of the Grand River, addressing the harmful historical effects of academic research on Indigenous Peoples and advocating for structural changes that promote Indigenous data sovereignty and community ownership of research. In both Canada and the United States, academic research has long been part of the colonial project (Hodge, 2012; Williams et al., 2020). The impact research has had on Indigenous Peoples has resulted in a legacy of deep mistrust and negative perception of research by many Indigenous communities (Garrison et al., 2023). Indigenous scholars and leaders who have advocated for repairing this relationship have led major transformations away from the way in which research has traditionally been approached and administered. Most recent paradigm and policy shifts seek to support the establishment of self-determined Indigenous Research Governance (Garba et al., 2023; Morton et al., 2017), which encapsulates many interconnected key concepts, including Indigenous data sovereignty (Schnarch, 2004; Kukutai & Taylor, 2016; Cannon et al., 2024), Indigenous research ethics (Castellano, 2004; Kuhn et al., 2020; Fournier et al., 2023), Indigenous/ decolonizing methodologies (Kovach, 2009; Smith, 2021), and Indigenous epistemologies (McGregor et al., 2010; Karanja, 2019). 

ABSTRACT Evapotranspiration (ET) from temperate forests plays a significant role in the regional and global water cycles. However, extreme weather events such as heat and drought are affecting the water use and water use efficiency (WUE) of these forests. Climate change impacts may be more severe in plantation forests where the age of the forest plays a significant role, causing differences in their responses to environmental stresses. This study presents 14 years (2008–2021) of water flux data measured using the eddy covariance technique in an age sequence (83, 48 and 20 years as of 2021) of eastern white pine (Pinus strobusL.) forests in the Great Lakes region in southern Ontario, Canada. The mean annual ET was 465 ± 41, 466 ± 32 and 403 ± 21 mm year⁻¹in the 83‐, 48‐ and 20‐year‐old stands, respectively, with the highest annual water flux observed in the 83‐year‐ old stand, which was similar to that of the 48‐year‐old stand. Mean annual gross ecosystem productivity (GEP) was 1585 ± 100, 1660 ± 115 and 1634 ± 331 g C m⁻² year⁻¹in the 83‐, 48‐ and 20‐year‐old stands, respectively, while mean annual WUE was 3.4 ± 0.4, 3.6 ± 0.4 and 4.0 ± 0.8 g C kg H₂O year⁻¹in the respective stands. Lower ET and relatively higher GEP resulted in the highest WUE in the youngest stand, even though the highest GEP was observed in the middle‐aged stand. Air temperature (Tair) was the dominant control on ET, GEP and WUE in all three different‐aged stands, while drought, characterised as the relative extractable water (REW) in the soil, had a significant impact on ET in the late summer. The results of this study further showed that forest age significantly influenced how forests responded to drought stresses. The younger stand was more efficient in carbon sequestration and water use despite exhibiting greater sensitivity to water stress and higher drought coupling. The long‐term eddy covariance measurements analysed in this study have helped to enhance our understanding of water exchange processes in the temperate conifer forest ecosystems in Eastern North America. Specifically, this work contributes to a better understanding of how different‐aged forests respond to extreme weather events, aiding in the development of new strategies for managing water resources and ensuring water security in the region under a changing climate.