A simulation analysis of the advective effect on evaporation using a two-phase heat and mass flow model

A simulation analysis of the advective effect on evaporation using a two-phase heat and mass flow model

The concept of enhanced vapor transfer in unsaturated soils has been questioned for its reliance on soil temperature gradient, which leads to consideration of other mechanisms of vapor transfer, e.g., advective vapor transfer due to soil air pressure gradient. Although the advective flux is an important portion of evaporation, there is a lack of knowledge of its effect on evaporation. In order to assess the dependence of evaporation on the soil air pressure gradient, a vertical one-dimensional two-phase heat and mass flow model is developed that fully considers diffusion, advection, and dispersion. The proposed model is calibrated with field measurements of soil moisture content and temperature in the Badain Jaran Desert. The proposed model is then used to investigate the advective effect in both low- and high-permeability soils. The advective effect is reflected by underestimating evaporation when the airflow is neglected and is more evident in the low-permeability soil. Neglecting airflow causes an underestimation error of 53.3% on the day right after a rainfall event in the low-permeability soil (7.9 104 cm s1) and 33.3% in the high-permeability soil (2 103 cm s1). The comparisons of driving forces and conductivities show that the isothermal liquid flux, driven by the soil matric potential gradient, is the main reason for the underestimation error.

Figure 1 Advective effects on the diurnal evaporation in both high- and low-permeability soils

Figure 2 Spatial and temporal distributions of (a, b) soil air pressure head gradient, (c, d) soil matric potential head gradient, and (e, f) soil temperature gradient in both the high- and low-permeability soils when the airflow is considered.

Figure 3 Left Panel: Spatial and temporal distributions of (a) normalized scale index for soil temperature gradient difference, (b) soil matric potential gradient difference, (c) thermal hydraulic conductivity difference, and (d) isothermal hydraulic conductivity difference induced by neglecting soil airflow in the high permeability soil; Right Panel: The comparison of gradients and  conductivities in the top surface layer on the day right after rainfall for the high-permeability soil.

Figure 4 Left Panel: Spatial and temporal distributions of (a) normalized scale index for soil temperature gradient difference, (b) soil matric potential gradient difference, (c) thermal hydraulic conductivity difference, and (d) isothermal hydraulic conductivity difference induced by neglecting soil airflow in the low permeability soil; Right Panel: The comparison of gradients and  conductivities in the top surface layer on the day right after rainfall for the high-permeability soil.

Figure 5 The advective liquid and vapor fluxes in the top surface layer on the day right after rainfall for the high- and low-permeability soils.

Citations:

Zeng Y., Z. Su, L. Wan and J. Wen, (2011): A simulation analysis of the advective effect on evaporation using a two-phase heat and mass flow model. Water Resources Research, 47(10), W10529, doi: 10.1029/2011WR010701.

Zeng, Y. and Su, Z. (2013) Reply to comment by Binayak P. Mohanty and Zhenlei Yang on “A simulation analysis of the advective effect on evaporation using a two-phase heat and mass flow model”. Water resources research, 49: 7836-7840.