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UBC Theses and Dissertations

Evapotranspiration from a dry Douglas fir forest Curtis, John Reeves


Evapotranspiration measurements using the Bowen ratio/energy balance technique were, made over a dry Douglas fir forest. Soil water matric potential over the period of measurement varied from 0 to -10.5 bars. With the premise that the equilibrium evaporation rate associated with adequate water supply essentially expresses evapotranspiration as a fraction of daily net radiation, the ratio of latent heat flux to net radiation for 24-hour periods is examined as a function of soil water matric potential. This ratio, is reduced to approximately half its maximum value as the matric potential approaches -10 bars for the experimental site. The reduction is not as great-as that implied by changes in soil water storage, indicating that evapotranspiration of water not originating in the root zone is carried on during periods of considerable water stress. While some water may be flowing upward from below the root zone, it is felt that over half of the daily evapotranspired water is released from storage in tree stems during the most severe period.

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