Assessing the significance of wet-canopy evaporation from forests during
extreme rainfall events for flood mitigation in mountainous regions of
the UK
Abstract
There is increased interest in the potential of tree planting to help
mitigate flooding using nature-based solutions or natural flood
management. However, many publications based upon catchment studies
conclude that, as flood magnitude increases, benefit from forest cover
declines and is insignificant for extreme flood events. These
conclusions conflict with estimates of evaporation loss from forest plot
observations of gross rainfall, throughfall and stem flow. This study
explores data from existing studies to assess the magnitudes of
evaporation and attempts to identify the meteorological conditions under
which they would be supported. This is achieved using rainfall event
data collated from publications and data archives from studies
undertaken in temperate environments around the world. The
meteorological conditions required to drive the observed evaporation
losses are explored theoretically using the Penman-Monteith equation.
The results of this theoretical analysis are compared with the
prevailing meteorological conditions during large and extreme rainfall
events in mountainous regions of the UK to assess the likely
significance of wet canopy evaporation loss. The collated dataset showed
that Ewc losses between approximately 2 and 38% of gross rainfall (1.5
to 39.4 mm d-1) have been observed during large rainfall events (up to
118 mm d-1) and limited data for extreme events (> 150 mm
d-1). Event data greater than 150 mm, where duration was not reported,
showed similarly high percentage evaporation losses. Theoretical
estimates of wet-canopy evaporation indicated that, to reproduce these
high losses, relative humidity and the aerodynamic resistance for vapour
transport needed to be within an envelope of approximately 90 to 97.5%
and 0.5 to 2 s m-1 respectively. Surface meteorological data during
large and extreme rainfall events in the UK suggest that conditions
favourable for high wet-canopy evaporation are not uncommon and indicate
that significant evaporation losses during large and extreme events are
possible but not for all events and not at all locations. Thus the
disparity with the results from catchment studies remains.