Geoscience Reference
In-Depth Information
The last response function describes the dependence of the canopy resistance on
soil moisture availability. This is used as a proxy for the leaf water potential (see
Chapter 6
). The empirical function
f
4
is given by:
0
if
θθ
<
pwp
θθ
θθ
−
−
−
1
pwp
=
if
θθθ
≤<
f
4
(
θ
(9.28)
pwp
fc
fc
pwp
1
θθ
≥
if
fc
where
θ
is the root zone mean water content,
θ
pwp
is the volumetric soil moisture con-
tent at permanent wilting point and
θ
fc
is the water content at ield capacity (note: the
reciprocal of
f
4
is given to show the similarity to the Warrilow model used to model
the reduction of evapotranspir
a
tion; see
Section 4.2
). The method of determining the
root zone mean water content
θ
differs between models. In some models the volumet-
ric water content is weighted with the soil layer thickness, whereas in others the frac-
tion of the total root length in each layer is taken into account as an extra weighting.
The distribution of roots over the different soil layers may differ between vegetation
types (e.g., trees have deeper rooting systems than grass). It appears that the results
in terms of luxes can be sensitive to the choice of root density distribution (e.g.,
Desborough,
1997
and Ek and Holtslag,
2005
).
The time scales at which each of the response functions discussed in the preced-
ing text is active differs (see, e.g., Schüttemeyer et al.,
2006
). The radiation response
function mainly follows the diurnal cycle as do the temperature and VPD responses.
But temperature and VPD exhibit variations with longer time scales as well, including
the annual cycle. Generally, the soil moisture response shows a variation on the time
scale of weeks and more because soil moisture does not vary rapidly (except when a
dry period is followed by signiicant rain).
Question 9.10:
Consider the expression for the canopy resistance in Eq. (
9.24
) (and the
expressions for the response functions).
a) Explain why LAI is the nominator of the expression in Eq. (
9.24
).
b) Verify that the expressions in Eqs. (
9.25
)-(
9.28
) are similar to the responses of the
stomatal
resistance as given in
Figure 6.15
.
A-g
s
Approach
The
A-g
s
approach makes use of the fact that the stomata are the gateway not only
for water vapour, but for CO
2
as well. The stomata have to strike a balance between
optimal CO
2
uptake and minimal water loss. Thus some of the stomatal behaviour is
related to photosynthesis, and other parts to water loss. Many variants exist for mod-
els of the stomatal behaviour based on plant physiology. These concepts have been
transferred to the meteorological applications by, for example, Collatz et al. (
1991
),
Search WWH ::
Custom Search