Agricultural and Forest Meteorology 103 2000 229–247
Investigating soil and atmospheric plant water stress using physiological and micrometeorological data
Jean-Christophe Calvet
∗ Météo-FranceCNRM, 42 Av. Coriolis, F-31057 Toulouse Cedex 1, France
Received 28 May 1999; received in revised form 24 February 2000; accepted 29 February 2000
Abstract
The effect of drought on the parameters of commonly used models of plant stomatal conductance is investigated, based on a large number of data obtained at the leaf level by plant physiologists, and at the canopy level by micrometeorologists.
Sixty-three case studies are analysed in order to understand the intra- and inter-specific variations of the conductance and photosynthesis parameters in both stressed and unstressed conditions. It appears that the intraspecific variability is as large as
the interspecific variability, and that the stomatal sensitivity to air humidity may depend on soil water content. Soil resistance to rooting seems to be an important factor of the variability. Field studies are consistent with leaf-scale ones. A simple
representation of these effects is implemented into an interactive vegetation model and then applied to simulate three annual vegetation cycles on a fallow site. © 2000 Elsevier Science B.V. All rights reserved.
Keywords: Stress; Drought; Stomatal conductance; Rooting; Soil moisture; Saturation deficit
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 partitioning between
vegetation transpiration, drainage, surface runoff and soil moisture change. In current operational SVAT
models, a Jarvis-type parameterisation is often used to compute the leaf stomatal conductance g
s
Noil- han and Planton, 1989. It is assumed that various
environment factors act independently on g
s
. In re- ality, measurements suggest that strong interactions
may occur Collatz et al., 1991; Jacobs et al., 1996. Additionally, the value of the prescribed leaf area
index LAI is often a crude estimate, which does not
∗
Tel.: +33-561079308; fax: +33-561079626. E-mail address: calvetmeteo.fr J.-C. Calvet
account for rapid changes in the vegetation cover as- sociated with climatic events droughts in particular.
Another limitation of such parameterisations is the lack of feedback with the atmospheric concentration
of CO
2
, especially in climate studies. The current trend in SVAT modelling is the integration of bio-
logical processes such as photosynthesis and plant growth, and hydrological transfers, in the same sur-
face model. The ‘classical’ part of the SVAT performs the atmospheric interface calculations, while new
modules provided by research in physiology and hy- drology simulate interactive vegetation and river flow.
This is why improving SVAT modelling would ben- efit meteorology, climatology, hydrology, and agron-
omy. In semi-empirical physiological models of the leaf net assimilation of CO
2
A
n
, both physiological responses to external parameters and non-linear inter-
actions between the various factors are handled, based on rather general relationships. For example, g
s
may
0168-192300 – see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 1 9 2 3 0 0 0 0 1 3 0 - 1
230 J.-C. Calvet Agricultural and Forest Meteorology 103 2000 229–247
be calculated according to A
n
, consistent with obser- vations showing the strong correlation between water
use and CO
2
assimilation Cowan, 1982. The net as- similation computed by a coupled physiology-SVAT
model such as ISBA–A–g
s
Calvet et al., 1998 or other models Ji, 1995; Dickinson et al., 1998 can
be used to estimate LAI according to the prescribed climate and CO
2
concentration and, hence, to ex- plore biosphere feedback mechanisms in response
to changes in rainfall patterns, temperature, and soil water storage. Since, in most cases, the hydrology of
the root-zone and the surface fluxes are controlled by vegetation, modelling the rate of soil water extraction
by the plant roots and the stomatal feedback is impor- tant for atmospheric, hydrological, and environmental
studies De Rosnay, 1999. In SVAT models, the effect of soil water stress on plant transpiration is generally
represented by applying a function depending on soil moisture or soil water potential to stomatal conduc-
tance or to the parameters of photosynthesis. The shape of this ‘stress function’ varies a lot from one
SVAT model to another Mahfouf et al., 1996. The rather straightforward assumptions defining the stress
function in present SVAT models may be adequate to represent large scale phenomena. However, more thor-
ough parameterisations may be useful for mesoscale meteorological or hydrological applications, in which
coherent landscape units may be identified.
In this paper, a large number of results obtained at the leaf level, and micrometeorological data at the
canopy level, are analysed in order to better under- stand the intra- and inter-specific variations of the pho-
tosynthesis parameters in both stressed and unstressed conditions. Three models of the stomatal conductance
are employed to discriminate the soil water stress from the atmospheric water stress. In the last section a sim-
ple representation of these effects is implemented into ISBA–A–g
s
and the model is applied to three annual vegetation cycles on the MUREX modelling the us-
able soil reservoir experimentally fallow site Calvet et al., 1999.
2. Modelling g g
