Forest floor evaporation in a dense Douglas fir stand

Forest floor evaporation was measured with an accurate weighing lysimeter during 44 clays in early spring and summer. The Penman-Monteith approach was used to model the evaporation rates as well as the temperature difference between forest floor surface and air at 1 m height. Values of resistance parameters were slightly different when the Penman-Monteith model was optimized for measured evaporation rates or for measured temperature differences. These discrepancies were partly due to field variability in forest floor water contents but also because our approach considered the forest floor to be isothermal. With the appropriate parameter sets, the model was able to predict measured hourly forest floor evaporation rates and surface temperature dynamics satisfactorily. We show that in the forest discussed in this paper the Penman-Monteith ventilation term dominates over the available energy term. As a result the evaporation flux is matched by an almost equal, sensible heat 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 evaporation to range from 0.19 mm d^-1 in a dry part of the forest to 0.3 mmd^-1 in a wet pan with 0.23 mmd^-1 as a site representative value.