Geoscience Reference
In-Depth Information
Case study
FORESTS AND RAINFALL VS EVAPORATION
If you stand watching a forest during a warm
summer shower it is common enough to see what
appear to be clouds forming above the trees (see
Plate 4). For many years it was believed that
somehow trees attract rainfall and that cloud-
forming was evidence of this phenomenon. As
described by Pereira (1989) 'The worldwide
evidence that hills and mountains usually have
more rainfall and more natural forests than do
adjacent lowlands has historically led to confusion
of cause and effect'. This idea was taken further so
that it became common practice to have forestry
as a major land use in catchments that were being
used to collect water for potable supply. In fact
the cloud formation that is visible above a forest
is a result of evaporation occurring from water
sitting on the vegetation (intercepted rainfall).
This 'wet leaf evaporation' can be perceived as a
loss to the hydrologist as it does not reach the soil
surface and contribute to possible streamflow.
Throughout the latter half of the twentieth
century there was considerable debate on how
important wet leaf evaporation is.
One of the first pieces of field research to
promote the idea of canopy interception being
important was undertaken at Stocks Reservoir,
Lancashire, UK. Law (1956) studied the water
balance of an area covered with conifers (Sitka
spruce) and compared this to a similar area covered
with grassland. The water balance was evaluated
for areas isolated by impermeable barriers with
evaporation left as the residual (i.e. rainfall and
runoff were measured and soil moisture assumed
constant by looking at yearly values). Law found
that the evaporation from the forested area was far
greater than that for the pasture and he speculated
that this was caused by wet leaf evaporation - in
particular that the wet leaf evaporation was far
greater from the forested area as there was a greater
storage capacity for the intercepted water.
Furthermore, Law went on to calculate the amount
of water 'lost' to reservoirs through wet leaf vege-
tation and suggested a compensation payment
from the forestry owners to water suppliers.
Conventional hydrological theory at the time
suggested that wet leaf evaporation was not an
important part of the hydrological cycle because
it compensated for the reduction in transpiration
that occurred at the same time (e.g. Leyton and
Carlisle, 1959; Penman, 1963). In essence it was
believed that the evapotranspiration rate stayed
constant whether the canopy was wet or dry.
Following the work of Law, considerable
research effort was directed towards discovering
whether the wet leaf/dry leaf explanation was
responsible for discrepancies in the water balance
between grassland and forest catchments. Rutter
(1967) and Stewart (1977) found that wet leaf
evaporation in forests may be up to three or
four times that from dry leaf. In contrast to this,
other work has shown that on grassland, wet leaf
evaporation is approximately equal to dry leaf
(McMillan and Burgy, 1960; McIlroy and Angus,
1964). In addition, transpiration rates for pasture
have been found to be similar to that of forested
areas. When all this evidence is added up it
confirms Law's work that forested areas 'lose' more
rainfall through evaporation of intercepted water
than grassland areas.
However there is still a question over whether
the increased wet leaf evaporation may lead to
a higher regional rainfall; a form of water re-
cycling. Bands
et al
. (1987) write that: 'Forests
are associated with high rainfall, cool slopes or
moist areas. There is some evidence that, on
a continental scale, forests may form part of a
hydrological feedback loop with evaporation
contributing to further rainfall'. Most researchers
