Litter evaporation
Water stored in the forest-floor litter layer will be evaporated back to
the atmosphere or drained by gravity. Height profiles of absolute
humidity and VPD suggest that the litter layer contributes significantly
to evaporation in our forest stand (Figure 8). Water vapor evaporated
from the forest-floor litter layer significantly reduces VPD below the
forest canopy, by exchanging latent for sensible heat and thus
increasing absolute humidity and lowering temperature. If we assume that
all litter layer storage evaporates (or, equivalently, we assume that
drainage occurs only during precipitation events when the maximum
storage capacity is reached), Table 2 yields an estimate of 191 mm
y‑1 for annual litter evaporation. Together with
evaporation from the forest canopy (assumed to be 20% of
precipitation), this sums to a total of 38 % of annual precipitation or
a total of 400 mm y-1, with some seasonal variation.
This is consistent with previous studies. For example, Gerrits et al.
(2010) reported that around 22% of annual evaporation at a beech forest
site originated from water retained in the litter layer; combined with
canopy interception this sums to a total of 26% and 36% of
precipitation in winter and summer, respectively.