Interannual and seasonal variability of latent and sensible heat fluxes above a coastal Douglas-fir forest

This presentation reviews the past two and a half years of tower-based eddy covariance measurements of latent and sensible heat flux made above a 50-year-old, 33-m tall Douglas-fir forest near Campbell River, Vancouver Island. This stand is part of the Pacific Northwest’s seasonal temperate rainforest where the climate is characterized by wet, mild winters and warm, dry summers. The influence of the 1997/98 El Niño/La Niña events resulted in highly contrasting seasonal weather patterns and interannual variability in the region’s weather. As a result, energy exchange patterns varied markedly throughout the study period and an investigation into the processes which control evaporation and flux partitioning for this ecosystem was possible. In general, the eddy covariance instrumentation was found to give reliable year-round measurements despite challenging weather conditions. However, signal attenuation of the water vapor mixing ratio was observed during high humidity conditions due to water vapor adsoption/desorption effects on sampling tube walls leading to the closed-path infra-red gas analyzer. A correction procedure dealing with the resultant flux underestimation was applied.

Energy partitioning was strongly influenced by changes in surface conductance, primarily through the seasonal changes in canopy wetness and soil moisture. On an annual basis, convective exchange was dominated by sensible heat flux during the summer months. The average Bowen ratio for the summer was 1.5, with daily values ranging from 1 to 4 for dry canopy conditions. Approximately 75% of the annual water loss to the atmosphere occurred between April and September, primarily through canopy transpiration, with rates ranging from 27 to 66 mm per month. A rapid and dramatic transition to negative Bowen ratios occurred in the fall with the arrival of winter rains. Throughout these winter months, the canopy remained wet 65% of the time. Sustained downward fluxes of sensible heat towards the canopy supported evaporation of intercepted rainfall under conditions associated with a non-zero saturation deficit and sufficient atmospheric turbulence. Resultant winter evaporation rates were between 10 to 20 mm per month.

Between 1998 and 1999, there was little interannual variability in total evaporation with both years' totals amounting to 22 - 25% of annual precipitation. However, differences in the timing of peak evaporation rates and flux partitioning patterns indicated considerable seasonal variability between the years. Drought was associated with a rapid decline in latent heat fluxes and rise in Bowen ratios in the late summer of 1998, while greater solar radiation, increased temperature and saturation deficit maintained greater evaporation rates through the early summer when compared with 1999. Bowen ratios were closer to one and evaporation rates were moderate through the cooler, wetter summer of 1999.