Introduction to the 2023 Canadian Geophysical Union Special Issue of Hydrological Processes

Abstract

Fifteen new research papers from Canadian researchers are published in this special issue of Hydrological Processes. This represents an increase in papers over the 2022 special issue following the resumption of in-person meetings of the Canadian Geophysical Union (CGU) after the COVID-19 pandemic. We are delighted to see the rebound in special issue submissions. The 2023 Annual Meeting of the CGU was held in May 2023 in Banff, Alberta. This special issue includes research presented at that conference and previous CGU conferences by members of the CGU Hydrology Section. Twelve of the 15 papers include graduate students as first authors. Student engagement in the special issue is a point of pride for the CGU Hydrology Section, given our emphasis on providing opportunities for students. We thank Hydrological Processes for their continued partnership.

The CGU comprises four sections (Biogeosciences, Geodesy, Hydrology and Solid Earth), of which Hydrology is the largest. Established in 1993, the Hydrology Section aims to (1) promote hydrology as a geophysical science, (2) initiate research and education programmes in hydrology, (3) develop national and international cooperation among like-minded organisations and (4) disseminate research results and knowledge to the public. This special issue of Hydrological Processes is one such dissemination mechanism. The CGU Hydrology Section also nominates Canadian hydrology representatives for the National Committee of the International Union of Geodesy and Geophysics.

Canadian hydrologists are well-known for their work on hydrological processes in cold regions. As usual, cold regions were a through-line in this year's special issue, with many studies considering cryosphere processes. Three papers centred on snow and ice-related themes. Bertoncini and Pomeroy focused on improving glacio-hydrological modelling by enhancing a model to consider the impacts on albedo caused by recent wildfires and heatwaves. Assimilating remotely sensed albedo data into a glacio-hydrological model of two Rocky Mountain catchments improved the model performance during wildfires but not during heatwaves. The Kling Gupta Efficiency of the model increased by 0.18–0.2 overall. Groff and Pomeroy considered infiltration into seasonally frozen, sloping soils to better understand snowmelt infiltration in mountain environments. They used eight years of soil moisture and meteorological data from their Rocky Mountain study site to characterise the infiltration patterns using a newly developed infiltration equation for use in mountain settings. Warden et al. simulated the occurrence of rain-on-snow events in a subarctic river basin in northwestern Canada under current and future climates. Rain-on-snow events can intensify runoff and exacerbate flooding. They projected an increasing frequency of rain-on-snow events in the summer and fall, particularly at higher elevations, and a decrease and uncertain changes in spring and winter, respectively.

As the climate changes, water temperatures change too, with consequences for aquatic ecosystems. Two papers focused on stream temperature in the special issue. Browning and Moore developed a novel stream shading model that uses a digital elevation model to quantify topographic shading and a LiDAR canopy model to quantify shading by riparian vegetation. The model achieved 92% accuracy in shading for a river reach in a mountain valley floodplain in British Columbia (BC). Next, Callahan and Moore tested a stream temperature model developed to simplify the data requirements of process-based models while maintaining performance. They calibrated the model for 23 streams in BC and tested it during years with extreme summer heat and autumn drought. They found that the model outperformed a statistical model but did have higher errors during the extreme period than in the calibration period.

While climate change features as a topic or motivation throughout the special issue, other (sometimes related) types of disturbances to hydrological systems were also highlighted by three papers. For example, wildfires are intensifying in many parts of Canada. Spencer and Winkler examined long-term streamflow data in four BC catchments affected by, or adjacent to, burned areas from a 2017 wildfire. Their findings show that the annual water yield increased by 30% and 31% in a small and large watershed, respectively. Furthermore, an earlier onset of spring freshet was observed in the years following the fire. The other two papers addressing hydrologic disturbances focused on the impacts of seismic lines. Seismic lines, which are widespread in Alberta's boreal region, are linear disturbances where trees have been cleared for resource development exploration. Bayatvarkeshi et al. explored winter microclimate conditions on and off seismic lines in a boreal peatland and found that the lines had 1.8 times higher photosynthetically active radiation, seven to eight times higher wind speed, denser snowpack and higher snow water equivalent (SWE) compared to the adjacent undisturbed wooded peatland. They also found that, due to the insulation from increased SWE, there were slightly shorter periods of frozen soil in the seismic lines, which could increase microbial activity and associated winter soil carbon emissions. In a similar vein, Weiland et al. investigated how soil and hydrologic properties are altered by seismic lines in upland and lowland settings. They showed that soil bulk density increased, hydraulic conductivity decreased and water table variability increased in the seismic lines compared to the surrounding forest. These altered hydrologic conditions may influence seedling recruitment, and the findings can guide restoration efforts.

Three papers focused on spatial and temporal patterns of streamflow variability and water quality. Grewal and Carey investigated spatial and temporal patterns in streamflow and water chemistry (ions, dissolved organic carbon and isotopes) in a subarctic mountain catchment in the Yukon, which is subject to pronounced seasonal hydrologic variability. They found that the spatial patterns in streamflow and water chemistry across 34 subcatchments remained similar throughout seven synoptic sampling campaigns despite pronounced seasonal variations. The findings suggest that the biophysical heterogeneity among catchments is therefore critical to understanding flow and water chemistry in subarctic mountain watersheds. Leach et al. examined streamflow records in 13 temporary headwater streams in the Canadian Shield to probe what controls the frequency of zero-flow days. They found a wide range in the number of zero-flow days (0–166 days) despite the proximity of the catchments. The variability was most related to May–November precipitation and evapotranspiration (ET). Total snowfall was not a strong predictor of zero-flow days. Between-catchment variability was most related to catchment area and groundwater influence. Moving on from low-flow periods, the study by Ross et al. considers rainfall-runoff response patterns. Rainfall–runoff responses are often mediated by thresholds which are not well captured in common runoff models. In this study, the authors took inspiration from ecological threshold metrics. They examined thresholds in runoff response which are controlled by rainfall amount and intensity across a nested watershed in Ontario with diverse land use. The strength (abruptness of the threshold) and diagonality (simultaneous influence of both rainfall amount and intensity) varied across catchments, with slope, imperviousness and orientation of the weather patterns influencing the thresholds.

Groundwater and surface water are closely linked systems. Three papers in the special issue focus on groundwater. A ‘geological weighing lysimeter’ is a relatively novel approach that uses wells in confined aquifers to quantify the integrated water storage within and above a soil profile based on groundwater level dynamics. In a two-part paper, Braaten and Ireson first performed a critical assessment of a geological weighing lysimeter by integrating shallow groundwater and SWE data from a field site in Saskatchewan. They found that the geological weighing lysimeter can accurately quantify total water storage at the field scale. In Part 2, Braaten et al. then used their observations from the geological weighing lysimeters to evaluate the performance of two land surface models. Both land surface models performed reasonably well when compared to the surface and subsurface water storage observed in the field. However, some limitations were identified in the individual hydrological processes simulated (e.g., timing of snowmelt, total ET). In the third paper on groundwater, Samways et al. quantified temporal trends in groundwater levels in 171 observation wells within mountain regions of Canada and the United States. They found that 54% of the wells had statistically significant trends over the period of record (20 years or longer) and that 69% of the significant trends were negative (indicating reductions in recharge and storage). Warmer, lower-elevation wells were more likely to show declining groundwater levels. This study provides an initial assessment of how mountain groundwater resources are changing over time in a setting where we typically lack groundwater data.

Finally, one paper focused on hydrometeorology. Sobral and Dery examined how much atmospheric rivers contribute to the water budget of a large, regulated watershed in north-central BC. Using a meteorological reanalysis dataset, they found that 21% of precipitation falling in the basin was associated with atmospheric rivers and that the highest proportional contribution to precipitation occurs in October.

Collectively, these studies advance our understanding of hydrological processes in diverse geographic settings across Canada with applications to the global hydrologic community. We thank the authors of these studies as well as the many peer reviewers (Canadian and otherwise) without whom the special issue would not be possible. Finally, we thank Editor-in-Chief Dörthe Tetzlaff and the Hydrological Processes staff for once more providing a venue through which Canadian hydrologists can disseminate research to the global hydrology community. The next CGU special issue in Hydrological Processes is being guest-edited by Drs. Lauren Somers, Barret Kurylyk and Jason Leach, and is already welcoming submissions for research presented at the 2024 Canadian Geophysical Union conference held in Ottawa, Ontario.