312 R.J. Harding et al. Agricultural and Forest Meteorology 100 2000 309–322
The Hydra system also recorded hourly average hu- midity, air temperature, wind speed and net radiation.
The humidity was recorded with a Vaisala RH sen- sor, the air temperature with the thermocouple sensor
also used for the measurement of temperature fluc- tuations and the net radiation, with a REBS Q6 ra-
diometer. To provide the continuous record of forcing data required for the model runs, these data were sup-
plemented by data from an automatic weather station AWS operated at a site 200 m north-east of the flux
tower Fig. 1. Comparisons showed that the agree- ment between the data taken at the flux mast and the
AWS was excellent. There was a small underestimate of the net radiation by the AWS at high radiation lev-
els which was almost certainly due to inadequacies in the design of the AWS Shenk radiometer. However the
hours of high radiation were sufficiently few that the totals from the two radiometers agreed well.
3. Model description
The complete MOSES scheme is a comprehensive land surface model describing the exchanges of en-
ergy, water and carbon at the land surface Cox et al., 1999. It contains the parameterisation of tran-
spiration, interception, direct soil evaporation, soil water movement, soil thermal regime including soil
freezing and the melting of snow. The full model is described by Cox et al. 1999, only the important
features of the soil and photosynthesis model are described further.
Soil water movement is described using a four layer model with vertical transfers described by the Darcy
equation. Plant roots extend throughout the top three layers but water can move upwards from the bottom
layer into the root zone through diffusion. The neces- sary soil parameters are calculated from soil texture
classes following Cosby et al. 1984. Transpiration is reduced from a non-limited rate once the average soil
moisture in the root zone drops below a critical value, θ
crit
which itself is a function of soil texture, using a ‘soil water availability factor’, β, which decreases lin-
early between the critical soil moisture and the wilting point, θ
wilt
. Transpiration is based upon a surface energy parti-
tioning equation, which splits available radiation into evaporative flux, sensible heat flux and soil heat flux.
A fractional coverage of vegetation is prescribed, and for the grass field, this is set as 0.95. With a value
so near to unity, bare soil evaporation is only a small component of the total evaporative flux. The surface
energy equation used within MOSES is essentially an extended version of the Penman–Monteith equation
Monteith, 1965, and is given in Eq. 11 of Cox et al. 1999.
A key parameter within the surface equations is the stomatal conductance to water vapour. At the top
leaf level, the model of the stomatal conductance, g
s
m s
− 1
is based upon the work of Collatz et al. 1992 and Leuning 1995. This may be written as
g
s
= 1.6
1 − f ×
a
n
c
s
− c
∗
[1 + D
s
D
∗
f 1 − f
] .
1 Here a
n
is the net photosynthesis which contains an explicit dependence upon soil moisture; see Cox et al.,
1998, c
s
mol CO
2
m
− 3
is the CO
2
concentration at the leaf surface, c
∗
mol CO
2
m
− 3
is the CO
2
con- centration at the photorespiration compensation point
and D
s
the humidity deficit at the leaf surface. Net photosynthesis contains an explicit dependence upon
surface temperature T
1
K, photosynthetically active radiation and intercellular CO
2
concentration, c
i
mol CO
2
m
− 3
. The latter is found by combining Eq. 1, and that for diffusion through the stomata i.e. a
n
= g
s
1.6c
s
− c
i
to give a closure first found by Jacobs 1994 thus:
c
i
− c
∗
c
s
− c
∗
= f
1 − D
s
D
∗
. 2
Both f and D
∗
are empirical parameters derived from laboratory experiments e.g. Collatz et al., 1991 or
optimised on field data. Initial values of f =
0.95 and D
∗
= 0.075 kg kg
− 1
were selected to fit with published values of maximum stomatal conductance Schulze
et al., 1994 and the vapour pressure deficit response e.g. Dolman et al., 1991. Soil moisture through
parameter β was also modelled as directly affecting net leaf photosynthesis Cox et al., 1998, and there-
fore stomatal conductance as described by Eq. 1.
A number of parameters contained within MOSES were considered for calibration. The first is V
max
mol CO
2
m
− 2
s
− 1
, a parameter described by Schulze
R.J. Harding et al. Agricultural and Forest Meteorology 100 2000 309–322 313
et al. 1994 and set to a value of 0.4 × 10
− 5
mol CO
2
m
− 2
s
− 1
. This parameter appears within the description of the temperature dependent rubisco
capacity of the top leaf level of C
3
plants thus: V
T
= V
max
2
0.1T
s
− 25
[1 + e
0.3T
s
− T
1
] 3
where T
s
is the surface temperature. Parameter T
1 ◦
C represents stomatal closure at higher temperatures and
an initial value of T
1
= 36
◦
C e.g. Sellers et al., 1996 is prescribed. Except in strongly light-limited condi-
tions, net leaf photosynthesis a
n
mol CO
2
m
− 2
s
− 1
is broadly proportional to V
T
Cox et al., 1999 with all other factors invariant. Parameters f
and D
∗
in Eqs. 1 and 2 were also used in the optimisation.
The equations for g
s
were derived from observa- tions on individual leaves. A scaling up procedure is
adopted in MOSES to calculate the whole canopy con- ductance, g
c
. It is assumed that all limiting factors for photosynthesis decay within the canopy, follow Beer’s
Law, and so by integration of the canopy and noting that g
s
is linear in a
n
, then g
c
= 1 − e
− kL
c
k g
s
4 where k is an extinction coefficient set to 0.5 and L
c
is the local leaf area index.
Fig. 2. Values of measured total soil moisture within the top 1.4 m of soil for 1995–1997.
4. Data analysis
