1.1 Forest plot studies of wet-canopy evaporation: losses during
large and extreme rainfall events
Forest plot studies estimate Ewc , using a canopy water balance
(CWB), as the difference between the gross rainfall (ππ) incident upon a
vegetation canopy and the fraction of ππ that reaches the ground asnet rainfall (Pn). Net rainfall comprises rainfall
that bypasses or drips from the canopy (throughfall : TF )
and that which flows via stems and trunks (stem flow : SF ).
As noted above, very few studies have focused on CWB estimatedEwc during large (> 50 mm d-1 of
ππ) or extreme (> 150 mm d-1 of ππ:
Collier, Fox & Hand, 2002) rainfall events. A notable exception is the
work of Keim, Skaugset, Link and IroumΓ© (2004) who report Ewc losses
above 30% of ππ at temperate sites in Chile and Northwest USA. Equally
high Ewc losses during large magnitude rainfall events at other
locations with a temperate climate have been reported (e.g. see Deguchiet al ., 2006 and Hashino et al ., 2002). Taken at
βface valueβ, these Ewc losses appear to be potentially
significant in the context of flooding: removal of such large fractions
of event rainfall from a catchment system are likely to have a
significant effect on a flood hydrograph where tree planting covers a
large proportion of a catchment (Hankin et al ., 2017).
Consequently, there is an apparent disparity between the publications
which conclude that forest effects on flood peaks are likely to be small
or insignificant for large and extreme events and the CWB observations
from forest plot studies.
The significance of forest Ewc for flood mitigation depends upon
the difference in Ewc between a given forest canopy and another
land cover. For example, moorland vegetation species such as Heather
(Calluna vulgaris ) tend to have long-termevapotranspiration losses approximately 30% to 60% of that for
tall forests and short semi-natural grasses lose around 10 to 40% of
that for forests (e.g. Calder, 1976, Calder & Newson, 1979; Calderet al ., 1981). However, as most comparative studies derive
estimates from catchment or lysimeter water balances over relatively
long periods, and do not separate Ewc and transpiration losses,
these estimates are of limited use when considering individual events.
Additionally, evaporation from a forest understorey or soil litter layer
can be significant (Bulcock & Jewitt 2012; Carlisle et al .,
1967; Gerrits et al ., 2007, 2010) and effectively increase the
difference in Ewc loss (e.g. between forest and grass). We
therefore assume that Ewc losses from tall canopies are likely to
be significantly higher than for short vegetation under meteorological
conditions favourable for wet-canopy evaporation and, consequently, that
the absolute magnitude of Ewc from tall forest canopies is of
primary relevance here. Thus it is important to determine the full
extent of evidence for significant Ewc from forest canopies
during large and extreme rainfall events as well as an understanding of
the meteorological conditions under which significant losses might be
supported.