Proceedings zur 6. Fachtagung BIOMET - Deutsche ...

Plants, especially trees usually experience a fluctuating water supply during their life cycle
due to continuously changing climatic factors. The water stress or dryness leads to closing the
stomata, whereby the transpiration is strongly reduced (Gottschalck et al., 2001). That is an
important strategy to protected trees against loss of water in dryness.
Results of the German network project VERTIKO (BERNHOFER and KÖSTNER, 2006) and
Eva_grips showed that commonly used meso-models like Lokalmodell (German Weather
Service, HERET et al., 2006) often overestimate evapotranspiration (ET) of vegetated surfaces
during dry periods. This is, among others, probably due to the lack of a plant-specific coupling
between the soil water content and the physiological reactions of leaf stomata in the
implemented simple SVAT modules. This leads to a continuous increase of ET caused by an
increase of the atmospheric saturation deficit. Consequently this leads to an overestimation of
water transport from the surface to the atmosphere. Therefore, meteorological models should
include a mechanism to couple the soil water with a physiologically based plant control of ET
for a more realistic simulation of water transport from the surface to the atmosphere.
This study presents an approach to describe the above mentioned coupling mechanism by
upgrading the coupled vegetation boundary layer model HIRVAC using a dependence of
stomatal reaction on soil moisture change in the included gas exchange model PSN6.
2 Methods
2.1 Model description
Fig. 1: Scheme of the HIgh Resolution Vegetation Atmosphere Coupler (HIRVAC)
Abb. 1: Schema des HIgh Resolution Vegetation Atmosphere Coupler (HIRVAC)
HIRVAC (HIgh Resolution Vegetation Atmosphere Coupler) is a 1.5 dimensional atmospheric
boundary layer model (MIX et al., 1994; GOLDBERG and BERNHOFER, 2001; BAUMS et
al., 2005) which is coupled with the mechanistic photosynthesis module for C3 plants PSN6
(FALGE et al., 1996). The model has a resolution of 120 layers between the lower (surface)
and the upper model boundary (typical 2 km above the ground) whereas the layer distance
increases with a geometric progression. Vegetation is considered by extension of the basic
model equations for momentum, temperature and humidity with additional source terms
which are solved numerically for each canopy model level. These terms are included in the
first calculation levels above the surface and are parameterized with vegetation parameters
dependent on type and vertical structure of vegetation. The high vertical resolution of
HIRVAC in the first decametres above the ground (60 layers between 0 and 30 m) permits a
very detailed differentiation of structured vegetation like forests (Fig.1).
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