SCHRIFTENREIHE Institut für Pflanzenernährung und Bodenkunde ...

was estimated from the soil heat flux G [J m -2 h -1 ] as follows (Chung and Horton,
1987):
cm).
G Ts = + T
λ ( θ ) z *
[12]
where Tz* is the temperature measured at the uppermost soil depth, z* (2
Root water uptake was simulated using the model of Feddes et al. (1978).
The maximum rooting depth was considered to be 100 cm, with the highest root
density in the upper 30 cm. The critical pressure heads in the
water-stress-response function of Feddes et al. (1978) were design to be similar
to the ones for grass based on Wesseling (1991), and adjusted for the local
conditions with a value of -1500 kPa for the permanent wilting point.
Water and Heat Flow Simulations
In Zhao et al. (2008), grazing effects on soil hydraulic and thermal
properties have been parameterized by calibrating the model HYDURS-1D,
based on data for evaporation ratio (weighing lysimeter experiments), water
interception (obtained by the model SHAW), and hydraulic parameters
(laboratory-derived hydraulic properties). However, this work was limited to show
the effects of grazing on coupled water and heat fluxes during the growing period,
i.e. unfrozen condition. Here we particularly attend to the snow melt and soil
freezing and thawing processes. For this aim, the current HYDRUS-1D version
including soil frozen module (denoted as freezing model) was modified to
consider the phase changes between water and ice, as well as the surface
energy and water balances. The algorithms in HYDRUS-1D for the description of
soil temperature and water movement during the freezing and thawing process
will be tested and adjusted to the local conditions. To verify the function of
freezing model, the “normal” version without soil frozen module (snow routine)
was also run as a reference. In the algorithm of snow hydrology it was assumed
that all precipitation is in the form of snow when the air temperature (soil surface)
is below -2°C, and in the form of liquid when the air temperature is above 2°C.
There is a linear interpolation between -2 and 2°C, i.e. 50% of precipitation is
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