Meteorological techniques were used to monitor evapotranspiration (ET) at two sites in a boreal forest drainage basin located in southeastern Manitoba, Canada. An energy balance method was used in which net radiation (RN) and ground heat flux (C) were measured directly. Sensible heat flux (H) was measured by the eddy correlation technique using a propeller anemometer and a fine-wire thermocouple. The energy components were calculated hourly on-line, and data were collected reliably over a five-month period.
The RN and H instruments were mounted above the forest canopy and simultaneous measurements of H at heights of 12 and 6 m were in good agreement. Measurements at an open bare rock site indicated that G could be a substantial fraction of the daily RN at some locations, but over longer time periods, it was a small fraction and, therefore, was ignored.
The two measurement locations represented upland (open bedrock/jack pine forest) and lowland (aspen/willow forest) sites in the drainage basin. The mean daily value of R, -H at the upland site was 0.57 times the value at the lowland location owing to differences in RN, H, and G. The mean ratio of daily H/R, was 0.6 for the upland site and 0.4 for the lowland site. A basin-wide ET was calculated by weighting the values for the two sites in proportion to their areas. The measured ET agreed well with precipitation minus runoff for the basin. Differences between these two quantities in summer and fall were attributed to water release and storage by the ground, respectively. * Issued as AECL-9153.