On the Ecology of Mountainous Forests in a Changing Climate: A ...

Behaviour of FORCLIM along a transect in the European Alps 97
600
correlated T and P
500
Frequency
400
300
200
100
0
-.1 0 .1 .2 .3 .4 .5 .6 .7
uDrStr
Fig. 4.3: Frequency distribution of simulated drought stress (uDrStr) at the site Sion.
The time series of soil water content as simulated by FORCLIM-E at these sites
(Tab. 4.2) are shown in Fig. 4.4. The Thornthwaite & Mather (1957) model tracks
measured soil moisture fairly well. The deviations from the measurements should be
interpreted considering both the accuracy of the measurements and their representativeness:
The measured soil moisture content is influenced considerably by the water demand
of the trees surrounding the measuring devices, whereas the simulated water content corresponds
to the average over a larger area. Moreover, it should be kept in mind that the
FORCLIM-E model has a monthly time step only, whereas the measurements have a much
finer resolution that can not be produced by the model. Generally speaking, FORCLIM-E
appears to be capable of simulating realistic time series of soil moisture under conditions
of moderate drought. It can be expected that results of a similar quality would be obtained
for other sites on the Swiss Plateau because they are characterized by similar drought
stress (Tab. 4.1). However, the precision of the calculation for sites with much more
drought (e.g. Sion) can not be inferred from the above experiments and would have to be
assessed separately.
Tab. 4.2: Derivation of the field capacity parameter (kFC) at the sites used for testing the behaviour of
the soil water balance model.
Site
Rooting
depth [cm]
Field capacity
[%]
kFC
[cm]
Observation
years
Reference
Wallbach-Möhlin 350 38 132.3 1971, 1972 Germann (1976)
Möhlin 300 38 114.0 1975, 1976 Borer (1982)
Kaisten 150 25 37.5 1978, 1979 Vogelsanger (1986)
Eiken-Laufenburg 150 25 37.5 1982–1984 Hurst (1988)