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

82 Chapter 3
Evapotranspiration and drought stress
The model of the soil moisture balance according to Thornthwaite & Mather (1957) was
described in detail by Fischlin et al. (1994). It is used with two minor modifications in the
FORCLIM-E model:
First, soil moisture content is not reset to field capacity at the beginning of every simulation
year, but the soil water content of the previous December is used as a starting point
for the soil water balance in the next year. Thus, soil moisture content (SM, Fig. 3.4) is
a true state variable in FORCLIM-E. While this change may be insignificant in the European
Alps under current climate because the soil is recharged to field capacity in most
places during the winter, it still is important because it relaxes the implicit assumption that
climate does not change: In a changing climate with decreasing winter precipitation or
increasing winter temperatures, the new formulation tracks soil moisture content more
realistically and has the potential to produce more and earlier drought stress.
Second, it is well known that slope and aspect of the terrain have a considerable effect on
the amount of incident radiation. Evapotranspiration rates depend not only on air temperature
and precipitation (Thornthwaite & Mather 1957), but also on the incoming solar
radiation (Penman 1948, Mintz & Serafini 1992). In a study comparing north and south
slopes, Running et al. (1987) found that radiation was 8-34% higher on southern slopes
than on northern slopes, which affected potential evapotranspiration rates (cf. Schädler
1980). Thus, the calculation of the potential evapotranspiration (PET m,y,l ) described in
Fischlin et al. (1994) was modified according to Eq. 3.73.
PET' m,y,l = kPMod · PET m,y,l (3.73)
kPMod =
1 + kSlAsp · 0.125 kSlAsp > 0
1 + kSlAsp · 0.063 else
(3.74)
where kPMod is the fractional change in PET with increasing (or decreasing) incident
solar radiation, and kSlAsp is a parameter describing this change on a qualitative basis in
the range [-2…+2]. The parameter kPMod thus causes PET m,y,l to decrease by a maximum
of 12.5% on steep northern slopes (kSlAsp = -2), and to increase by a maximum
of 25% on steep southern slopes (kSlAsp = +2) as compared to flat terrain (where
kSlAsp = 0) (Running et al. 1987). It is acknowledged that this formulation is purely
empirical; however, it provides a tool to explore the sensitivity of the FORCLIM model to