SCHRIFTENREIHE Institut für Pflanzenernährung und Bodenkunde ...

Chapter 6 Modeling of Coupled Water and Heat Transfer in Freezing and Thawing Soil
Simulated Soil Temperatures and Water Contents
Soil water and heat fluxes are numerically simulated for the whole
hydrological year in 2006. This is exemplified by two sites of UG 79 (Fig. 4) and
WG (Fig. 5). In general, the simulated and measured soil water contents (SWC)
are comparable during the studied periods (Figs. 4-5; RMSE of 0.02-0.07 cm 3
cm -3 for UG 79 and 0.02-0.08 cm 3 cm -3 for WG). Particularly, the freezing model
simulates well the diurnal water dynamics caused by phase changes between
water and ice, i.e. soil moisture increases with increasing soil temperature and
vice versa (Figs. 4b and 5b). This suggests the freezing and thawing processes
can be accurately simulated by the applied freezing code. However, the snow
routine is only available for the simulation of soil water contents under unfrozen
condition. Under frozen condition, there is a clear disparity between the liquid
water content line simulated by the freezing model and the total water content
line simulated by the snow routine (Fig. 4a and 5a). This discrepancy is
apparently caused by the function of the two models, and may be used to
approximate the ice content in the soil. Thus, the comparison between the two
model approaches just provides a way to calculate the ice content. For instance,
the SWC simulated by the freezing model drops shortly after 6 th November
associated with soil freezing in UG 79 (Fig. 4a). However, the SWC simulated by
snow routine keeps constant. From which, an ice content of 0.07 cm 3 cm -3 can
be estimated. Similarly, the ice content in WG is 0.05 cm 3 cm -3 (Fig. 5a). This
also gives an evidence of grazing effects on soil freezing point and ice content.
In late March, coinciding with soil temperature increasing above 0°C, both
simulated (freezing model) and measured SWC raise, while simulated SWC by
snow routine keeps constant. This again proofs that the freezing model can
predict well the increase in SWC by soil thawing. An increase in SWC in the
deeper soil layers (20 and 40 cm) accompanied by soil thawing is also clearly
predicted (Figs. 4-5). However, there is no indication of vertical water movement
since soil water content is low and it can be held by soil.
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