Geoscience Reference
In-Depth Information
Box 3.1 The Penman-Monteith Combination Equation for Estimating Evapotranspiration Rates
The Penman-Monteith equation is based on a combination of a simplified energy balance
equation for the surface and equations for the transport of sensible heat and latent heat away
from the surface. It is what has been called a
big leaf
model, in that there is an assumption
that a complex vegetation canopy can be represented as if it were acting as a single transpiring
surface at some effective height above the ground. The energy balance equation, illustrated in
Figure B3.1.1, can be written as
H
=
R
n
−
A
−
G
−
S
(B3.1.1)
where
H
is the total energy available for evapotranspiration,
R
n
is net radiation (ranging from
−
50Wm
−2
on a clear night to more than 500Wm
−2
at midday in summer),
A
is heat loss due
to
advection
(
1Wm
−2
for a downwind temperature gradient of 1
◦
Ckm
−1
),
G
is heat loss
into the ground (usually positive during the day and negative at night) and
S
is the energy flux
into physical and biochemical storage in the vegetation (up to 15Wm
−2
during the day, and
perhaps 3Wm
−2
at night).
It is assumed that total available energy can be partitioned into two components: the transport
of sensible heat to or from the surface (that is energy directly involved in heat or cooling of the
air above the surface by conduction and convection) and the transport of latent heat (that is
energy used in vaporising water lost from the surface by evaporation or transpiration). Thus:
≈
H
=
C
+
E
(B3.1.2)
where
C
is the sensible heat flux and
E
is the latent heat flux as a product of the latent heat
of vaporisation,
(
=
2
.
4710
6
, Jkg
−1
) and the evapotranspiration rate (kgm
−2
s
−1
≈
mms
−1
)
Figure B3.1.1
Schematic diagram of the components of the surface energy balance. R
n
is net radiation,
E is latent heat flux, C is sensible heat flux, A is heat flux due to advection, G is heat flux to ground storage,
S is heat flux to storage in the vegetation canopy. The dotted line indicates the effective height of a “big
leaf” representation of the surface.
