SCHRIFTENREIHE Institut für Pflanzenernährung und Bodenkunde ...

Chapter 5 Modeling Grazing Effects on Coupled Water and Heat Fluxes in Inner Mongolia Grassland
where θm, θo, and θw represent the volume fractions of the mineral, organic
matter, and water in soil, respectively. The terms on the right-hand side of Eq. [4]
represent soil heat flow by conduction, convection of sensible heat with flowing
water, and uptake of energy associated with root water uptake, respectively.
Transfer of latent heat by vapor movement is ignored. λ(θw) denotes the soil
thermal conductivity and q the water flux density while T is the soil temperature.
The λ(θw) is described with a simple equation given by Chung and Horton
(1987):
1
2
3
0.
5
λ ( θ w ) = b + b θ w + b θ w
[6]
where b1, b2, and b3 are empirical regression parameters.
Initial and Boundary Conditions
Initial conditions were set in the model in terms of measured water
contents at the beginning of the simulation period. Value-specified boundary
conditions were used for the top and bottom boundary of the flow domain. At the
soil surface, an atmospheric boundary condition was imposed using daily data of
precipitation, soil surface temperature, potential evaporation and transpiration.
Water fluxes at the soil-atmosphere interface were corrected accounting for daily
water interception by the canopy and litter using the SHAW model (Flerchinger
and Saxton, 1989). Actual evaporation equaled the sum of intercepted
evaporation and soil evaporation. Furthermore, the ratio between potential
transpiration and soil evaporation was based on radiation partitioning according
to Beer’s law as a function of LAI (Ritchie, 1972). The partitioning results were
subsequently calibrated using in situ measurements from the weighing
mini-lysimeter experiments. A free drainage condition and the measured soil
temperature at 100 cm depth were used as bottom boundary conditions
assuming that the water table is located far below the domain of interest and that
heat transfer across the lower boundary occurs only by convection of liquid water.
Soil surface temperature Ts ( 0 C) was estimated from the soil heat flux G (J m -2
h -1 ) as follows (Chung and Horton, 1987):
T λ +
[7]
G
s = − ( θ ) Tz*
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