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UBC Theses and Dissertations

Evaporation from a Canadian west coast douglas-fir forest : seasonal patterns and controls Humphreys, Elyn Robin


In order to understand how forest functioning will respond to global climate change, longterm and direct measurements of water vapour and energy exchange over forested ecosystems are needed. Between October 15, 1997 and May 31, 1999, eddy covariance measurements of latent and sensible heat flux were made above a 50-year-old, 33-m tall Douglas-fir forest, part of the seasonal temperate rainforest located on Vancouver Island, B.C, on the west coast of Canada. Winters at this site are wet and mild while summers are warm and dry. Evaporation rates and energy exchange varied widely on annual, seasonal and diurnal time scales and were used to discern the processes involved in the evaporation of water from this forest canopy. Ultimately, these variations were integrally linked to the canopy wetness and the surface conductance. Spectral analysis and turbulence characteristics were used to ensure measurement quality during inclement weather. The Gill Model 1012R2A sonic anemometer-thermometer used in this study performed well up to rainfall intensities of 4.6 mm h⁻¹. However, signal attenuation of the water vapour mixing ratio was observed during high humidity conditions due to water vapour adsorption/desorption effects on the sampling tube walls leading to the infrared gas analyzer. Total latent heat flux losses of up to 38% could be corrected using the sensible heat cospectra. Significant evaporation rates occurred during the winter months at this site. The wet canopy acted as a sink for energy characterized by downward fluxes of sensible heat supporting evaporation of intercepted rainfall. Latent heat fluxes often exceeded available energy and equilibrium evaporation rates on a half hour basis throughout the day and night. Wet canopy evaporation required turbulent conditions and only a small non-zero saturation deficit. Evaporation occurring between April and September, inclusively, accounted for 74% of the total annual evaporation of 402 mm for 1998. Summer evaporation rates were on average 1.57 mm day⁻¹ with a maximum of 3.56 mm day⁻¹. Sensible heat flux dominated energy exchange throughout the summer with β values ranging from 1.3 to 3.5. Uncorrected midday Priestly-Taylor α values ranged from 0.20 to 0.55. Physiological control was important in limiting dry canopy evaporation or transpiration rates. Mean daytime canopy conductance was generally less than 6 mm s⁻¹ and was closely related to saturation deficit, soil water potential, and photosynthetic photon flux density. However, canopy conductance was found to have a weak negative linear relationship with transpiration rates only during late morning hours.

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