Geoscience Reference
In-Depth Information
A1 The energy available for evapotranspiration,
H
, can be estimated from knowledge of net
radiation, ground heat flux, advective heat flux and storage of heat in the vegetation
canopy. In applications, this is often reduced to an estimate based on the net radiation
alone, with other terms neglected as small.
A2 The latent heat and sensible heat fluxes can be estimated from first-order gradient flux
relationships with the gradients of vapour pressure and temperature, respectively, being
determined between some measurement height and some conceptual height within the
canopy (the “big leaf” assumption).
A3 The aerodynamic resistance for the movement of vapour and heat away from the canopy
are similar and can be estimated from the aerodynamic resistance for the downwards
flux of momentum as determined under the assumptions of a logarithmic velocity
profile.
A4 The controls of stomata on evapotranspiration rates can be represented by some effective
canopy resistance for the movement of vapour from stomata to the air within the canopy.
A5 The saturated vapour pressure of the air within the stomata is assumed to be consistent
with the effective canopy temperature and may be predicted by extrapolating from the
measurement height using the slope of the known theoretical saturated vapour pressure-
temperature relationship.
Box 3.2 Estimating Interception Losses
In Section 3.3, we have made the point that, in many environments, evapotranspiration is a
greater proportion of the catchment water balance than stream discharge. In turn, an important
component of the total actual evapotranspiration from a catchment can be due to evaporation
from water intercepted on the vegetation canopy, particularly from rough canopies that are
frequently wetted. In forest canopies subject to frequent wetting in the windy environment of
upland UK,
interception
may amount to more than 20% of the rainfall inputs (Calder, 1990). In
such circumstances, the vegetation canopy may have an important effect on the amount and
pattern of intensity of rainfall reaching the ground.
B3.2.1 Regression Models of Throughfall and Stemflow
There have been many experimental studies of interception, many of which have reported
results in the form of regression equations for storm throughfall and stemflow as
V
TF
=
B
TF
P
storm
−
C
TF
V
SF
=
B
SF
P
storm
−
C
SF
where
P
storm
is the storm rainfall total, the subscripts
TF
and
SF
refer to throughfall and stemflow
respectively,
V
TF
and
V
SF
are the volumes of throughfall and stemflow in the storm,
C
TF
and
C
SF
are minimum storage capacities for throughfall and stemflow, and
B
TF
and
B
SF
are coefficients.
This type of regression relationship can obscure considerable scatter in the measurements
for individual storms so that, in making predictions, the uncertainty in estimated throughfall
and stemflow predictions should be assessed. The storage capacities and coefficients vary
with vegetation type and seasonal vegetation growth. Not all such studies have differentiated
between throughfall and stemflow.
Most of these experiments were based on volumetric collection of throughfall and stemflow
with storm by storm measurements. Increasingly, however, hydrologists have made measure-
ments on a more continuous basis and this has allowed the development of more dynamic
interception models.
