FOREST FLOOR EVAPORATION IN A DENSE DOUGLAS-FIR STAND

Forest floor evaporation was measured with an accurate weighing lysime ter during 44 days in early spring and summer. The Penman-Monteith app roach was used to model the evaporation rates as well as the temperatu re difference between forest floor surface and air at Im height. Value s of resistance parameters were slightly different when the Penman-Mon teith model was optimized for measured evaporation rates or for measur ed temperature differences. These discrepancies were partly due to fie ld variability in forest floor water contents but also because our app roach considered the forest floor to be isothermal. With the appropria te parameter sets, the model was able to predict measured hourly fores t floor evaporation rates and surface temperature dynamics satisfactor ily. We show that in the forest discussed in this paper the Penman-Mon teith ventilation term dominates over the available energy term. As a result the evaporation flux is matched by an almost equal, sensible he at flux but in opposite direction. Forest floor water content dynamics have a strong control over the evaporation flux. Spatial variability in forest floor water contents cause the 44-day average forest floor e vaporation to range from 0.19 mm d(-1) in a dry part of the forest to 0.3 mm d(-1) in a wet part with 0.23 mm d(-1) as a site representative value. (C) 1997 Elsevier Science B.V.