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important. Their formula for potential evaporation
is shown in equation 3.13.
sometimes referred to as stomatal resistance. Various
researchers have established canopy resistance values
for different vegetation types (e.g. Szeicz
et al
.,
1969), although they are known to vary seasonally
and in some cases diurnally. Rowntree (1991)
suggests that for grassland under non-limiting
moisture conditions the range of
r
c
should fall some-
where between 60 and 200 s/m. The large range is
a reflection of canopy resistance being influenced by
a plant's physiological response to variations in
climatological conditions (see earlier discussion of
stomatal control p. 40). Some values of canopy
resistance for different vegetation types are given in
Table 3.3.
( )
( )
*
Q
G
−
∆
(3.13)
=
α
λγ
PE
∆
where
Q
G
is the soil heat flux term (often ignored by
Penman but easily included if the measurements are
available) and
is the Priestly-Taylor parameter, all
other parameters being as defined earlier. The
term is an approximation of the sensible heat
transfer function and was estimated by Priestly and
Taylor (1972) to have a value of 1.26 for saturated
land surfaces, oceans and lakes - that is to say, the
sensible heat transfer accounts for 26 per cent of the
evaporation over and above that from net radiation.
This value of
has been shown to vary away from
1.26 (e.g.
= 1.21 in Clothier
et al
., 1982) but to
generally hold true for large-scale areas without a
water deficit.
Reference evaporation
The Penman-Monteith equation is probably the
best evapotranspiration estimation method avail-
able. However for widespread use there is a need to
have the stomatal resistance and aerodynamic
resistance terms measured for a range of canopy
covers at different stages of growth. To overcome
this, the idea of reference evaporation has been intro-
duced. This is the evaporation from a particular
vegetation surface and the evaporation rate for
another surface is related to this by means of crop
coefficients. The Food and Agriculture Organisation
(FAO) convened a group of experts who decided that
the best surface for reference evaporation is close-
cropped, well-watered grass. This is described in
Allen
et al
. (1998) as a hypothetical reference crop
with an assumed crop height of 0.12 m, a fixed
canopy resistance of 70 s/m and an albedo of 0.23.
Using these fixed values within the Penman-
Monteith equation the reference evaporation (ET
o
in mm/day) can be calculated from equation 3.15.
Penman-Monteith
Monteith (1965) derived a further term for the
Penman equation so that actual evaporation from a
vegetated surface could be estimated. His work
involved adding a canopy resistance term (
r
c
) into
the Penman equation so that it takes the form of
equation 3.14.
*
Q
∆
+
ρδ
c
/
r
ρ
e
a
=
E
t
(3.14)
r
r
c
a
λγ
∆
++
1
Looking at the Penman-Monteith equation you
can see that if
r
c
equals zero then it reverts to the
Penman equation (i.e. actual evaporation equals
potential evaporation). If the canopy resistance is
high the actual evaporation rate drops to less than
potential. Canopy resistance represents the ability
of a vegetation canopy to control the rate of
transpiration. This is achieved through the opening
and closing of stomata within a leaf, hence
r
c
900
273
−
( )
+⋅
+
0 408
.
∆
QQ
*
γ
u
⋅
δ
(3.15)
G
e
T
ET
=
+
( )
o
∆
γ
1034
.
u
where
Q
* is net radiation at the crop surface (MJ/m
2
/
day)
is
