SCHRIFTENREIHE Institut für Pflanzenernährung und Bodenkunde ...

Chapter 6 Modeling of Coupled Water and Heat Transfer in Freezing and Thawing Soil
capacity decreases with increasing grazing intensity due to high water content at
the ungrazed sites and similar soil textural classes for each site (Eqs. 7-10).
Thermal conductivity, which shows a non-linear dependency on, e.g., water
content and geometrical arrangement of particles, has also been included in the
current freezing model (e.g., Eq. 9).
Comparing the two treatments of UG 79 and WG, soil water and heat fluxes
are clearly affected by grazing management. The relatively weak prediction in
WG might ascribe to the weak parameterization of hydraulic parameters, which
is associated with a platy structure created by the growth of ice segregation
blades due to winter grazing (Krümmelbein et al., 2006). Furthermore,
desiccation also results in the formation of vertical shrinkage fissures and a
prismatic structure in deeper soil depths in WG. This can be observed by that a
decrease in soil moisture at 20 cm depth is slower in WG than in UG 79 because
of relatively low evaporation by the less continued pore system in WG (Figs. 4-5).
The decrease in soil moisture at 5 cm depth after rainfall is slower in WG than in
UG 79 since water supplied to the topsoil layer lasts longer in WG due to a lower
hydraulic conductivity (Zhao et al., 2008). However, the constant water contents
at 40 cm depth in both sites show that residual water content is reached and
maintained in the deeper layers due to slight influences either by water
infiltration or by soil evaporation. Moreover, soil water dynamics in WG is less
responsive to freezing and thawing processes than in UG 79. Due to the lower
unsaturated hydraulic conductivity in WG, there is much less moisture migration
to the freezing front than in UG 79. Consequently, water content increases much
smaller in WG than in UG 79 (Figs. 4-5).
CONCLUSION
We used an extended frozen soil module of HYDRUS-1D to govern coupled
flow equation which solves water and heat transport under both frozen and
unfrozen conditions simultaneously. The model was evaluated using field data of
soil water and temperature at a long-term experimental site in Inner Mongolia
grassland (North China). The results showed that both freezing model and snow
routine reflected well the measured soil water and temperature under unfrozen
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