Study objectives

In examining the effects of needleleaf forest-cover on radiation to snow and snowmelt dynamics in the Canadian Rocky Mountains, the following research questions will be addressed:

1. How does needleleaf forest-cover influence shortwave and longwave radiation exchanges to mountain snowcovers? How are these effects influenced by topography (i.e. slope and aspect) and meteorological conditions?

2. How are changes in forest-cover expected to impact the timing and magnitude of mountain snowmelt?

3. What improvements can be made in simulating radiation and snow processes in mountain forest systems?

These questions will be addressed by analysis of meteorological observations collected at field sites of varying forest-cover, elevation, and topographic orientation in an eastern Canadian Rocky Mountain headwater basin. Field observations will be further employed in the development and improvement of physically-based simulation models, the application of which will provide an extension of field-based results over a larger range of spatial and temporal scales. Within this thesis, analysis and discussion of results are presented throughout Chapters 3 – 7, with major findings of the work summarized in Chapter 8. The following provides an outline of the subject matter and objectives of Chapters 3 – 7:

Chapter 3: Observations of forest-cover effects on radiation and snowmelt (pursuant to research questions 1 and 2)

Utilising multi-year field observations collected in low elevation pine forests and high elevation spruce forests, an assessment of needleleaf forest-cover effects on radiation dynamics and snowmelt within a headwater basin is performed. Analysis focuses on how combinations of varying topography and forest-cover density influence shortwave and longwave radiation exchanges to mountain snowcovers, and impact the timing of snowmelt. Results will illustrate how radiation and snowmelt dynamics differ between low-elevation pine, and high-elevation

Introduction and objectives

7 spruce forest stands.

Chapter 4: Simulation of shortwave radiation to snow in mountain needleleaf forests (pursuant to research questions 1, 2, and 3)

A physically-based approach is outlined and evaluated for simulating shortwave radiation to snow in needleleaf forests of varying canopy density/structure and meteorological conditions.

A particular aim of the model is to provide a more realistic account of shortwave transfers in sparse conifer stands consisting of non-transmitting trunks, partially-transmitting crowns, and fully transmitting canopy gaps. Subsequent application of the model examines how changes in needleleaf forest-cover affect shortwave radiation fluxes to snow at sites of differing topographical orientation.

Chapter 5: Sensitivity of radiation to mountain snowcover with varying forest-cover and meteorology (pursuant to research questions 1, 2, and 3)

Using a simplified modelling approach to describe forest-radiation transfers, an assessment is performed investigating the influence of meteorological conditions on radiation to mountain snow. A particular advantage of the approach lies in the description of forest-cover density using a single, intuitive metric: the forest sky view factor. Application of the model illustrates how forest-cover affects radiation to snow on slopes of opposing topography in an eastern Canadian Rocky Mountain location over winter-spring meteorological conditions. Based on observations of canopy temperature and sub-canopy longwave irradiance in forest stands of differing canopy density and topographic orientation, a procedure for approximating forest longwave enhancements from shortwave heating of the canopy is outlined and evaluated.

Further representation of the meteorological influences on longwave fluxes to snow is made by accounting for snow surface cooling effects on longwave exitance from snow. The improved approximation of longwave radiation to sub-canopy snow by these approaches is used to investigate forest-cover effects on radiation to mountain snowcover under observed winter-spring meteorological conditions.

Introduction and objectives

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Chapter 6: Simulation of forest snow accumulation and melt in needleleaf forest environments (pursuant to research question 3)

A model developed from investigations of forest-snow processes in cold regions is described and evaluated for estimating snow accumulation and melt in needleleaf forest environments of varying canopy density and climate. With incorporation into the Cold Regions Hydrological Model (CRHM), model evaluation is completed by comparison of snow accumulation and melt simulations to observations at five paired forest-clearing sites located in Canada, Switzerland, Finland, and the United States. Further demonstration of the physical approach taken by the model in describing snowmelt is made via comparison of simulated energy fluxes to snow to detailed observations collected at forest and clearing sites within an eastern Canadian Rocky Mountain basin.

Chapter 7: Impacts of forest-cover change upon radiation and snowmelt in the eastern Canadian Rocky Mountains (pursuant to research questions 1 and 2)

Drawing upon the physically-based modelling procedures developed and tested within the previous chapters of this work, the impact of forest-cover changes on the magnitude and timing of mountain snowmelt are examined in the context of forest harvesting treatments performed in an eastern Rocky Mountain headwater basin. The influence of forest clear-cut size on radiation to snowcovers of opposing topography is examined using a geometrically-based model describing radiation dynamics in forest clearings. By coupling corrected radiation fluxes to the appropriate snow process modules within CRHM, forest-cover impacts on snowmelt are assessed by application of the model under observed mountain meteorological conditions.

Model results illustrate the potential impacts needleleaf forest-cover changes may have on the magnitude and timing of snowmelt in a mountain headwater basin.

Methodology

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