Proceedings zur 6. Fachtagung BIOMET - Deutsche ...

only to a short increase of water in the upper soil layer) after the precipitation event on 20th
August (DOY 232) the high LE differences between the two simulations remain.
The course of these differences can be explained by the reaction of the vegetation over the
day. The increasing smaller water content of the soil leads to closing of the stomata and thus
to a decrease of the transpiration. In combination with high solar radiation (fig. 2) and high
values of vpd (not depicted here) it results in an increasing water stress in the period from 8th
to 14th August (DOY 220 – 226) which is described by the model comparison of the two soil
moisture regimes (constantly wet and increasingly dry).
modelled LE (PSN6)
400 Y=1.115*X
350
300
250
200
150
100
50
R²=0.24
0
0 50 100 150 200 250 300 350 400
measured LE
Fig. 4: Comparison between measured
and modelled LE (half-hour
means for irradiation > 0)
Abb. 4: Vergleich zwischen gemessenem
und modelliertem LE
(Halbstundenwerte für Globalstrahlung
> 0)
Water content [%]
16
14
12
10
2
210 213 216 219 222 225 228 231 234
Day of Year
4 Conclusions
The results of this study clearly show that the upgraded model HIRVAC is able to simulate
the coupling between soil moisture and evapotranspiration in a dry period. The modified
parameterisation of the stomatal resistance in the sub model PSN6 reflects the link between
soil moisture, atmospheric humidity and ET via stomatal control fairly well. This simple
modification leads to a noticeable improvement of the simulated canopy air moisture regime.
It allows to “reproduce” measurement of turbulent fluxes without the use of new “tuning”
parameters. The reduction of ET by dryer soil ranges between 30 to 150 W/m² in the forest
Tharandter Wald. This leads to a new partition of modelled water budget components of forested
catchments and has possible consequences for runoff simulations.
Several SVAT models (e.g. SIB2, SELLERS et al. 1996) consider the effect of soil water on ET
in a simple form. But they are usually not able to simulate feedback effects between vegetated
surface and the atmospheric boundary layer (ABL) because of the lack in vertical resolution
and physical description of ABL.
Results of this study identify several problems which must be examined in further investigations.
One problem is the nocturnal offset of LE of the forest stand using FGRS (flux gradient
relationships) for calculation. Here the values do not decline under 50 W/m² at night which
8
6
4
115
measured water content
modelled water content for a dry soil
modelled water content for a moist soil
ΔLE between moist and dry soil
300
250
200
150
100
Fig. 5: Difference of LE between model runs with
moist and dry soil (solid line), compared
with modelled (moist soil: dashed line, dry
soil: dots) and measured (triangle) soil water
content
Abb. 5: LE-Differenz zwischen Modellläufen mit
feuchtem und trockenem Boden (durchgezogene
Linie), verglichen mit simuliertem
(feuchter Boden: gestrichelte Linie, trockener
Boden: Punkte) und gemessenem
Bodenwassergehalt
50
0
Δ LE between moist and dry soil [W/m²]