Numerical analysis of air‐water‐heat flow in unsaturated soil: Is it necessary to consider airflow in land surface models?

Numerical analysis of air‐water‐heat flow in unsaturated soil: Is it necessary to consider airflow in land surface models?

From a subsurface physical point of view, this paper discusses the necessity of considering the two‐phase heat and mass transfer process in land surface models (LSMs). The potential‐based equations of coupled mass and heat transport under constant air pressure form the basis of the proposed model. The model is developed considering dry air as a single phase, and including mechanical dispersion in the water vapor and dry air transfer. The adsorbed liquid flux due to thermal gradient is also taken into account. The set of equations for the two‐phase heat and mass transfer is formulated fully considering diffusion, advection, and dispersion. The advantage of the proposed model over the traditional equation system is discussed. The accuracy of the proposed model is assessed through comparison with analytical work for coupled mass and heat transfer and experimental work for isothermal two‐phase flow (moisture/air transfer). The influence adding airflow has on the coupled moisture and heat transfer is further investigated, clearly identifying the importance of including airflow in the coupled mass and heat transfer. How the isothermal two‐phase flow is affected by considering heat flow is also evaluated, showing the influence of heat flow only to be significant if the air phase plays a significant role in solving the equations of the water phase. On the basis of a field experiment, the proposed model is compared with the measured soil moisture, temperature, and evaporation rate, the results showing clearly that it is necessary to consider the airflow mechanism in soil‐atmosphere interaction studies.

Figure 1. Diurnal changes in meteorological variables: (a) air temperature, (b) relative humidity, (c) wind speed, (d) precipitation, (e) atmospheric pressure, and (f) surface temperature. They are recorded every 30 min from 13 to 19 June 2008. The solid lines are the approximation and the dots are the measurement.

(a)                                                                                                    (b)

Figure 2. (a) Comparison between simulated (i.e., by model with and without considering airflow) and measured soil temperature during 13–19 June 2008, at selected depths: (top to bottom) 2 cm, 5 cm, 10 cm, 20 cm and 50 cm. The solid black line is the simulation with airflow, the solid gray line is the simulation without airflow, and the red open circle is the measurement; (b) Same as Figure 2. (a) but for soil moisture content at selected depths: (top to bottom) 10 cm, 20 cm, 30 cm, 40 cm and 50 cm. The solid black line is the simulation with airflow, the solid gray line is the simulation without airflow, and the red open circle is the measurement.

Figure 3. Comparison between simulated evaporation rates and actual measurements from 13 to 19 June 2008.

Citations: Zeng Y., Z. Su, L. Wan and J. Wen, (2011): Numerical Analysis of Air-Water-Heat Flow in the Unsaturated Soil Is it Necessary to Consider Air Flow in Land Surface Models. Journal of Geophysical Research – Atmosphere, 116(20), D20107, doi: 10.1029/2011JD015835.