The ISBA-Ags model: INTRODUCING CO2 ASSIMILATION IN ISBA FOR INTERACTIVE VEGETATION
1. INTRODUCTION
The soil-vegetation-atmosphere transfer
(SVAT) schemes now employed in meteorology are
designed to describe the basic evaporation processes
at the surface together with the water partitionning
between vegetation transpiration, drainage, surface
runoff and soil moisture change. Recent advances in
SVAT modeling consisted in accounting for vegetation-
climate-CO2 feedbacks in order to: (1) describe, in a
more realistic way, the canopy stomatal conductance by
considering the functional relationship between stomatal
aperture and photosynthesis (Wong et al. 1979); (2)
make the SVATs more general in order to properly
simulate the surface processes in contrasting CO2
concentration, air temperature, and air humidity
conditions, with the same calibrated model; (3) use the
estimated assimilation rate to simulate the plant growth
and mortality, and diagnose the leaf area index (LAI)
consistently with the prescribed climate and CO2
concentration
(Calvet et al. 1998b).
The ISBA (Interactions between Soil,
Biosphere, and Atmosphere) scheme
(Noilhan and Planton 1989) was modified in order to account for the
atmospheric carbon dioxide concentration on the
stomatal aperture. The physiological stomatal
resistance scheme proposed by Jacobs (1994) was
employed to describe photosynthesis and its coupling
with stomatal resistance at leaf level. In addition, the
plant response to soil water stress was accounted for
by a normalized soil moisture factor applied to the
mesophyll conductance. The computed vegetation net
assimilation was used to feed a simple growth
submodel, and to predict the density of vegetation
cover. Only two parameters are needed to calibrate the
growth model: the leaf life expectancy and the effective
biomass per unit leaf area. The new scheme, called
ISBA-Ags, was tested against data from six
micrometeorological databases for vegetation ranging
from temperate grassland to tropical forest.
2. datasets
The datasets used to calibrate the model were
all used and described in previous papers: MUREX
(Calvet et al. 1998a) is an
agricultural fallow in south-
western France, PILPS/Cabauw
(Chen et al. 1997) is a
grassland in the Netherlands, HAPEX-MOBILHY/Caumont
(Mahfouf and Noilhan 1996)
is a soybean field of
south-western France, INRA/Avignon (Olioso et al.
1996) is a late soybean field in south-eastern France,
INRA/Castanet (Cabelguenne et al. 1990) is a maize
field in south-western France, and ARME (Shuttleworth
et al. 1984) is undisturbed Amazon forest in northern
Brazil. For all the sites, continuous meteorological data
(air temperature and humidity at screen level,
atmospheric pressure, precipitation, wind speed, and solar
and atmospheric radiation) are available over periods
ranging from 2 months to 3 years. Such data constitute
the atmospheric forcing employed to run the SVAT
models. However, the CO2 concentration was not
measured in any of these experiments. In this study, a
value of 340 ppm at screen level was assumed.
Since the purpose of this study is to validate
models of both hydrological and physiological
processes, most appropriate datasets should comprise
measurements or estimates of soil moisture in the root-
zone (w2), evapotranspiration (LE) and heat surface
fluxes, net assimilation of CO2
(An), and leaf area index
(LAI). Only one dataset provides observations of the
four model output variables: INRA/Avignon (Olioso et al.
1996). Evapotranspiration could be estimated in all the
datasets, except over the maize of Castanet. Root-zone
soil moisture was not fully characterized during PILPS-
Cabauw and ARME. But in both cases, severe soil
water stress are not likely to occur, because of the
rather large amount of precipitation and also, as far as
the Amazon forest is concerned, because of the deep
root-zone. The leaf area index was either directly
measured or estimated. The MUREX fallow was cut in
1995 at the end of the spring growth. The resulting
reduction of LAI was prescribed in ISBA-Ags. The
other agricultural canopies were grown in optimal
conditions by providing the required tillage, irrigation
water amount, nitrogen fertilization, and pesticide
application. The irrigation amount was included in the
precipitation of the SVAT forcing file.
3. THE COUPLED MODEL
3.1 At leaf level
The purpose of the Jacobs' Ags model is to
simulate the leaf net assimilation of CO2,
An, and the
associated leaf conductance to water vapour gs, as a
function of air density ra, concentration of CO2 at the
leaf surface Cs, leaf
temperature Ts, saturation deficit at
the leaf surface Ds (g kg-1), global radiation Rg (W m-2),
and plant type (C3 or
C4). The saturation deficit
Ds
represents the difference between the saturation
specific humidity at leaf temperature and the air specific
humidity close to the leaf surface. Along with An and
gs,
the Jacobs' model computes the leaf internal CO2
concentration (Ci). The stomatal conductance is
estimated using a flux-gradient relationship, modified to
account for the effect of Ds on stomatal aperture.
It was necessary to modify the Jacobs' model
in order to introduce a response to soil water stress.
The soil water stress effect was applied to the
mesophyll conductance (the only tunable parameter of
the Ags model). In this version of the Ags model, gm is
multiplied by the normalized soil moisture theta2, also
employed in the standard version of ISBA to represent
the soil water stress:
|
(1) |
where w2 is the soil volumetric moisture in the root-
zone, and wfc and
wwilt are the root-zone moisture
content at field capacity and wilting point, respectively.
In seasonal simulations, w2 is a value predicted by
ISBA.
3.2 From the leaf to the canopy
The leaf conductance and CO2 assimilation are
not homogeneous within the canopy because of the
factors acting on photosynthesis: the photosynthetically
active radiation (PAR) is attenuated by the vegetation;
also, the leaf temperature and the air CO2 concentration
and saturation deficit may vary within the canopy. It is
assumed that the input variables of the physiological
model Ts, Cs, and Ds do not vary in the vegetation
canopy layer together with the parameters of the
Jacobs' model. In the coupled model, Ts is the surface
temperature calculated from the resolution of the
surface energy balance, similar to the standard version
of ISBA. The Cs and Ds variables at canopy level are
obtained from their values at reference level from a
simple flux-gradient relationship by using the
aerodynamic resistance ra computed by ISBA and the CO2 and
water vapour fluxes of the previous time step. The PAR
extinction by the canopy is described in the model.
Assuming an homogeneous leaf vertical distribution, the
integrated canopy net assimilation AnI and conductance
gsI can be written:
|
(2) |
|
(3) |
where h is canopy height and z is the distance to the
ground. Variability in Eqs. (2) and (3) is assumed to be
due to the light levels only. Possible air temperature and
humidity differences within the canopy are not
accounted for. Finally, the canopy conductance gsI (the
driving factor of transpiration), is employed in the
standard algorithm of ISBA to compute the heat and
water vapour surface fluxes.
3.3 Growth and mortality
Growth may be described as the accumulation
of carbon obtained from the atmospheric CO2
assimilation, and senescence as the result of a deficit of
photosynthesis (due to external factors). In this study,
the bulk biomass B (kg m-2) is obtained from a
differential equation:
|
(4) |
In the growth increment term of Eq. (4), PC is the
proportion of carbon in the dry plant biomass (a
constant value of 40% is assumed) and MC and
MCO2
are the molecular weights of carbon and CO2 (12 and
44 g mol-1), respectively. The mortality increment term
of Eq. (4) represents an exponential extinction of B
characterized by a time-dependent effective life
expectancy:
|
(5) |
where tauM is the maximum effective life expectancy,
Anfm(t) the maximum leaf net assimilation reached on
the day before time t and An,max the optimum leaf net
assimilation. The optimum leaf net assimilation An,max is
a constant. It corresponds to the leaf net assimilation
given by the Jacobs' model with optimal input values: Ds
= 0 g kg-1, Ia(h) = 500 W m-2, Ts = 25 C for C3 plants
and Ts = 35 C for C4 plants.
The value of LAI is obtained from B by
assuming that for a given vegetation canopy, the ratio
between B and LAI is a constant: alpha. Therefore, LAI is
given by:
|
(6) |
Only two parameters, the alpha ratio and tauM have to be
determined for each canopy type from LAI
measurements or estimations. These vegetation
parameters have to be retrieved during the model
calibration, together with the mesophyll conductance gm.
4. CALIBRATION
The estimated values of the plant parameters
are presented in Table 1. The obtained values are
rather disparate, and correspond to different plant-
development strategies. In the studied datasets, the
highest obtained values of a correspond to evergreen,
strongly rooted canopies (fallow, grassland, Amazon
forest). Also, the Amazon forest exhibits the lowest
value of gm, associated with the highest value of
tauM: the
high leaf life span compensates for the low biomass
production per leaf area unit, and contributes to
maintain a stable, nearly constant value of LAI. Unlike
the plants considered before, both soybean crops (late
and precocious cultivars of INRA/Avignon and HAPEX-
Mobilhy/Caumont, respectively) show low alpha values.
This characteristic corresponds to their rapid life cycle
(2 and 5 months, respectively): the production of leaves
is favoured, and the growing cycle can be completed
rapidly in spite of low values of gm. The late cultivar
presents a lower value of alpha and (especially) of tauM than
the precocious one, consistent with its shorter growing
cycle.
5. MODEL VALIDATION
The ISBA-Ags model computes the surface
fluxes, including net assimilation, and the leaf area
index LAI. Once calibrated over a given canopy, it may
be assumed that the plant growth parameters (alpha and
tauM) and mesophyll conductance gm at 25 C are intrinsic
plant characteristics, which remain unaffected by
changes in the climatic forcing. This hypothesis was
tested by running ISBA-Ags over the 1996 growing
cycle of the MUREX fallow (for the calibration, 1995
MUREX data were employed). In 1996, the MUREX
vegetation was not cut, and the precipitation regime
was more favourable to plant growth (a precipitation
total of 860 mm was registered in 1996, against 770
mm in 1995). The simulated LAI and w2 are presented
in Fig. 1 for the two years, together with the available
observations. Both variables are properly simulated.
The obtained results confirm that once calibrated, the
ISBA-Ags model is able to simulate the main
vegetation responses to changes in the atmospheric
conditions, and that it represents a rather robust
approach.
TABLE 1
Estimation of the mesophyll conductance gm and of the
plant growth parameters for the 6 studied sites.
Dataset | plants | gm (mm s-1) | (g m-2) | (d) |
MUREX-95 | Fallow | 20.0 | 365 | 90 |
PILPS Cabauw-87 | Grassland | 7.2 | 365 | 90 |
HAPEX- Mobilhy-86 | Soybean | 1.0 | 63 | 250 |
INRA Avingon-90 | Soybean (late) | 2.6 | 25 | 15 |
INRA Castanet-86 | Maize | 5.7 | 100 | 30 |
ARME-83 | Amazon forest | 0.8 | 250 | 365 |
|
6. SENSITIVITY TO CO2CONCENTRATION
From the water balance viewpoint, an increase
in [CO2] has two immediate conflicting effects: (1) LAI
may increase because of the photosynthesis
enhancement (at least for C3 plants); (2) the leaf
conductance decreases. The first effect tends to
increase transpiration, and the second effect tends to
decrease transpiration. Therefore, the resulting change
in evapotranspiration and soil moisture may be totally
different from one plant type to another, for given
climate conditions. Also, there is a third effect related to
the feedback beetween transpiration and w2: changes in
transpiration affect soil moisture availability. This effect
occurs over longer periods of time and may trigger
seasonal differences in the resulting water balance
change. Such a feedback is accounted for in
ISBA-Ags. Fig. 2 presents a
comparison between LAI and w2
computed by ISBA-Ags in the case of MUREX-95
under different prescribed values of [CO2]. Despite a
rather strong LAI response to CO2 enrichment the
simulated soil moisture seldom departs from its
reference value, because stomatal closure limits the LAI
increase effect on transpiration.
7. CONCLUSION
The ISBA-Ags model presented in this study
seems to be a good tool to analyse the plant response
to climate change. Although the model uses a limited
number of parameters, it is able to depict very distinct
situations.
The main advances of ISBA-Ags in relation to
the standard version of ISBA are: (1) a more realistic
representation of the leaf conductance; (2) a better
integration to the canopy scale by using a (simple)
radiative transfer parameterization; (3) a climate-
derived leaf area index may now contribute to better
account for the biological control of evapotranspiration;
(4) the new scheme may be validated through the
computed net assimilation by using in situ
measurements of the CO2 flux; (5) the complex
phenomena associated with an increase of the
atmospheric concentration of CO2 may possibly be
accounted for.
8. REFERENCES
Cabelguenne, M., Jones, C.A., Marty, J.R., Dyke, P.T.
and Williams, J.R., 1990. Calibration and validation of
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Retrieving the root-zone soil moisture from surface
soil moisture or temperature estimates: A feasibility
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37(4), 371-386.
Calvet, J.-C., Noilhan, J., Roujean, J.-L., Bessemoulin,
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vegetation SVAT model tested against data from six
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FIGURES
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FIGURE 1:
Leaf area index and bulk soil moisture over MUREX in 1995 and 1996: observed (boxes and +), and computed by
ISBA-Ags (line).
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FIGURE 2:
Leaf area index (left) and root-zone soil moisture (right) simulated by ISBA-Ags over MUREX-95 for different
prescribed values of the CO2 atmospheric concentration at reference level: 340 ppm (fine line), 680 ppm (dashed
fine line) and 1020 ppm (dashed thick line).
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