4. Conclusions
At temperate locations around the world, high wet-canopy evaporation
losses have been observed from forests using canopy water balance
methods during large and extreme rainfall events and are associated with
significant variability. Wet-canopy evaporation of up to approximately
40 mm d-1 have been recorded for large rainfall events
(>50 mm d-1) and across all events range
between approximately 2 and 38% of gross rainfall. Taken at “face
value” these evaporation losses are qualitatively significant in
the context of flood mitigation resulting from tree planting.
Theoretical wet-canopy evaporation estimates made using the
Penman-Monteith model for large and extreme events in mountainous
regions of the UK suggest consistency with these high observed losses
but uncertainties associated with the estimation of, in particular,
aerodynamic exchange are so large that this test of consistency remains
weak. During 4 major flood events in the Cumbrian Mountains, UK,
meteorological conditions were favourable for high rates of wet-canopy
evaporation: high windspeeds prevailed and surprisingly low relative
humidity was observed at some locations. Thus the disparity regarding
the significance of wet-canopy evaporation for flood mitigation between
conclusions based upon results from catchments studies of forest cover
effects and results from forest plot studies remains.
Our results suggest that it is possible for high rates of Ewcover forest to occur during large flood events in mountainous regions of
the UK but not in all locations and not for all events. To be able to
determine the potential of tree planting scenarios on flood hydrographs
using hydrological models, estimates of the spatial and temporal
patterns of wet-canopy evaporation through sequences of rainfall events
are needed. Appropriate estimates require simulation of the control
imposed by meteorological variables on wet-canopy evaporation to be made
necessitating interpolation and extrapolation from (normally) sparse
meteorological observation sites. This is difficult to implement with
any accuracy and the uncertainties associated with this step are
compounded with the large uncertainties associated with the estimation
of aerodynamic exchange with forest canopies such that wet-canopy
evaporation estimates may be represented best as scenarios. Simulations
must also be able to represent the limitation on evaporation imposed by
storage on the surfaces of different vegetation canopies (e.g. between
foliated and unfoliated deciduous trees) meaning that scenarios of
parameterisations must be incorporated into the simulations. Simulation
scenarios must be associated with a confidence weighting that can be
propagated to simulation results as expressions of modelling
uncertainty.
If the considerable uncertainties associated with estimating
meteorological conditions and Ewc across large areas such are to
be constrained, collection and analysis of a larger number of
well-placed and well-distributed meteorological observations is
required, combined with concurrent wet-canopy evaporation observations.