Buttle, James M

Understanding snowpack variability is important as it plays an imperative role in environmental, hydrologic, and atmospheric systems. Research questions related to three linked areas were investigated in this thesis: 1) scaling issues in snow hydrology, 2) forest-snowpack relationships, and 3) methods of integrating snow water equivalent (SWE) into a hydrologic model for a large, forested drainage basin in central Ontario. The first study evaluated differences in SWE across process, measurement, and model scales. Point scale snowpack measurements could be bias corrected using scaling factors derived from a limited number of transect measurements and appropriately stratified point scale measurements may be suitable for replacing transect scale mean SWE when transect data are not possible to collect. Comparison of modelled products to measurements highlighted the importance of understanding the spatial representativeness of in-situ measurements and the processes those measurements represent when validating snow products or assimilating data into models.The second study investigated the efficacy of field-based, and remotely sensed datasets to describe forest structure and resolve forest-snowpack relationships. Canopy cover was highly correlated with melt rate and timing at the site scale however, significant correlations were present in 2016 but not 2017, which was attributed to interannual differences in climate. Peak SWE metrics did not correlate well with forest metrics. This was likely due to mid-winter melt events throughout both study years, where a mix of accumulation and melt processes confounded forest-snowpack relationships. The third study evaluated the accuracy of the Copernicus SWE product and assessed the impact of calibrating and assimilating SWE data on model performance. The bias corrected Copernicus product agreed with measured data and provided a good estimate of mean basin SWE. Calibration of a hydrologic model to subbasin SWE substantially improved modelled SWE performance. Modelled SWE skill was not improved by assimilating SWE into the calibrated model. All models evaluated had similar streamflow performance, indicating streamflow in the study basin can be accurately estimated using a model with a poor representation of SWE. The findings from this work improved knowledge and understanding of snow processes in the hydrologically significant Great Lakes-St Lawrence Forest region of central Ontario.

Author Keywords: data assimilation, hydrologic model, multi-objective calibration, remote sensing, scale, snow

Long-term avian population declines, particularly for the avian insectivore guild, are a conservation concern. With widespread and continuing population trends, climate change and its negative effects on avian food resources is a plausible cross-species driver. My goal was to evaluate whether bottom-up trophic effects of climate change could be influencing avian populations. I used a space-for-time approach to assess the influence of snowpack and soil moisture variability on arthropods and subsequent effects on nest survival. In the 2010 and 2011 growing seasons, I sampled arthropods, soil moisture (soil volumetric water content; VWC), snowpack (snow water equivalent; SWE), forest floor depth (L, F, H layers) and soil texture in conifer plantations and mixed deciduous forest in Southern Ontario's Ganaraska Forest (~4, 400 ha). I used additive linear mixed effects models to assess the responses of arthropod groups' (e.g., order or class) relative biomass (g/day) and abundance (count/day) to those variables. Influences for each arthropod group's biomass and abundance were typically in the same direction. Maximum annual SWE significantly positively influenced most arthropod groups and annual relative difference in VWC positively influenced one quarter. In mixed directions, forest type influenced half of the groups and soil texture and forest floor depth each affected less than one quarter. I then used structural equation models to evaluate relationships between SWE, VWC, the biomass of three arthropod functional guilds, and logistic-exposure model calculated daily nest survival rates for American Robin (Turdus migratorius), Eastern Wood-Pewee (Contopus virens), Least Flycatcher (Empidonax minimus), Ovenbird (Seiurus aurocapilla), and Red-eyed Vireo (Vireo olivaceus). Arthropod guilds included diet-based food, predaceous arthropods and soil-dwelling bioindicators. SWE significantly positively influenced food biomass in all five models and negatively influenced predaceous arthropods in three models. Soil moisture had a mix of positive, negative, and null effects. Eastern Wood-Pewee and Red-eyed Vireo nest survival positively related to food and negatively related to predaceous arthropod biomass. American Robin, Least Flycatcher and Ovenbird nest survival did not appear to be related to arthropod biomasses. Through bottom-up relationships, predicted climate change-induced reductions in snowpack may cause food resource declines and negatively affect some forest breeding bird populations.

Author Keywords: Arthropod biomass, Bottom-up, Forest birds, Nest survival, Path analysis, Precipitation

Water balance models calculate water storage and movement within drainage basins, a primary concern for many hydrologists. A Thornthwaite water balance model (H2OBAAS) has shown poor accuracy in predicting low flows in the Petawawa River basin in Ontario, so lake storage and winter snow processes were investigated to improve the accuracy of the model. Lake storage coefficients, represented by the slopes of lake stage vs. lake runoff relationships, were estimated for 19 lakes in the Petawawa River basin and compared on a seasonal and inter-lake basis to determine the factors controlling lake runoff behaviour. Storage coefficients varied between seasons, with spring having the highest coefficients, summer and fall having equal magnitude, and winter having the lowest coefficients. Storage coefficients showed positive correlation with lake watershed area, and negative correlation with lake surface area during summer, fall, and winter. Lake storage was integrated into the H2OBAAS and improved model accuracy, especially in late summer, with large increases in LogNSE, a statistical measure sensitive to low flows. However, varying storage coefficients with respect to seasonal lake storage, watershed area, and surface area did not improve runoff predictions in the model. Modified precipitation partitioning and snowmelt methods using monthly minimum and maximum temperatures were incorporated into the H2OBAAS and compared to the original methods, which used only average temperatures. Methods using temperature extremes greatly improved simulations of winter runoff and snow water equivalent, with the precipitation partitioning threshold being the most important model parameter. This study provides methods for improving low flow accuracy in a monthly water balance model through the incorporation of simple snow processes and lake storages.

Author Keywords: Lake Storage, Model Calibration, Monthly Water Balance, Petawawa River, Precipitation Partitioning, Snow Melt

The Oak Ridges Moraine (ORM) is a key hydrogeologic feature in southern Ontario. Previous work has emphasized the importance of depression-focused recharge (DFR) for the timing and location of groundwater recharge to the ORM's aquifers. However, the significance of DFR has not been empirically demonstrated and the relative control of land cover, topography, and surficial geology on DFR is unclear. The potential for DFR was examined for topographic depressions under forested and open, agricultural land covers with similar soils and surficial geology. Recharge (R) was estimated at the crest and base of each depression during the 2012-13 and 2013-14 winter-spring periods (~December – May) using both a 1-dimensional water balance approach and a surface-applied Br- tracer. At each depression, air temperatures, precipitation, snow depth and water equivalent, soil water contents, soil freezing, and depression surface-water levels were monitored and soil properties (texture, bulk density, porosity, and hydraulic conductivity) were measured. Both forested and agricultural land covers experienced soil freezing; however, concrete frost did not develop in the more porous and conductive forest soils. Concrete frost in agricultural depressions resulted in overland flow, episodic ponding and drainage of rain-on-snow and snowmelt inputs. Recharge was an order-of-magnitude greater at the base of open depressions. Observations of ponding (as evidence of DFR) were made at an additional 14 depressions with varying land cover, geometry, and soil type during the 2014 snowmelt period and measurements of pond depth, pond volume, land cover (i.e., percentage of agricultural vs. forested cover), depression geometry (i.e., contributing area, average slope, relief ratio) and soil texture were made. Ponding was restricted to depressions under mostly agricultural cover and a positive, non-linear relationship between pond volume and average slope was shown for sites with similar land cover and soil texture, but neither pond depth nor volume were related to any other depression characteristics. Results suggest that DFR is a significant hydrologic process during winter and spring under agricultural land cover on the ORM. Topographic depressions under agricultural land cover on the ORM crest may serve as critical recharge "hot spots" during winter and spring, and the ability of the unsaturated zone beneath these depressions to modify the chemistry of recharging water deserves further attention.

Author Keywords: Concrete frost, Depression-focused groundwater recharge, Oak Ridges Moraine, Ponding, Topographic depressions, Water balance

Groundwater recharge was estimated and compared in two open grasslands, three mixed deciduous forest stands (100+ years in age), three young red pine plantations (27 ¨C 29 years in age) and two old red pine plantations (62 ¨C 63 years in age) on the Oak Ridges Moraine, southern Ontario, Canada. Recharge was estimated using a 1-d water balance with measured precipitation, throughfall, stemflow, snowpack water equivalent and soil water storage, and modelled evapotranspiration. Throughfall distribution beneath red pine canopies showed no consistent variation with distance from the tree boles. Old red pines were not major stemflow producers and although the young red pines showed a slight tendency to focus stemflow (focussing ratio > 1), the inclusion of focussed stemflow when calculating recharge at the stand scale made little difference. Conversely, sugar maple (the predominant species in the mixed deciduous stands) showed a strong tendency to focus throughfall proximal to tree boles and produce large quantities of stemflow, resulting in relatively high soil moisture contents and enhanced opportunities for recharge within ~ 0.5 m of tree boles. Inclusion of these focussed inputs resulted in a ~ 11 ¨C 18 % increase in stand scale recharge estimates. The interpretation of land cover control on recharge was complicated by variations in soil texture between sites. Soil texture and its influence on soil water storage capacity resulted in temporal variations in recharge, with sites exhibiting large storage capacities producing less recharge in the fall and greater recharge in the spring than sites with limited storage capacities. Recharge estimates for the entire study period or seasonal values for sites grouped on the basis of soil water storage capacities showed a general trend of increasing recharge in the order: old red pine ¡Ö young red pine ¡ú mixed deciduous forest ¡Ö open grasslands. The disparity between the red pine plantations and the other sites was driven in large part by greater modelled evapotranspiration in the red pine plantations. The similarity in recharge between mixed deciduous forests and open grasslands was the result of focused inputs and less soil evaporation and transpiration in the mixed deciduous forests compared to the open grasslands. The results of this study suggest planting red pine on grasslands on the Oak Ridges Moraine will initially decrease recharge and this decrease will continue as the red pines mature. However, as the red pine plantations are succeeded by mixed hardwood stands recharge will recover to that of the initial grasslands.

Author Keywords: Groundwater Recharge, Land Cover Type, Oak Ridges Moraine, Stemflow, Throughfall, Water Balance