Comment on “A simulation analysis of the advective effect on evaporation using a two‐phase heat and mass flow model” by Yijian Zeng, Zhongbo Su, Li Wan, and Jun Wen

[1] Zeng et al. [2011a] (hereafter referred to as Z11) recently proposed a Representative Elementary Volume (REV) scale two-phase heat and mass flow model and then used the developed model to investigate the usually ignored advective airflow effect on soil evaporation. The proposed model, consisting of balance equations of water (liquid and vapor), dry air, and heat, together with corresponding constitutive equations, is developed mainly on the basis of Thomas and Sansom’s [1995] model by additionally accounting for thermal liquid film flow [Kay and Groenevelt, 1974; Milly, 1982], water vapor dispersion [Z11], dry air diffusion and dispersion [Z11], and heat of wetting [de Vries, 1958; Milly, 1982]. Subsequently, the model was calibrated using the field measured soil temperature and soil moisture content collected from the Badain Jaran Desert, representing an extremely dry climate condition. The simulated soil temperature agreed reasonably well with the measured soil temperature at five depths (Z11, Figure 2), however, this is not the case for soil water content, particularly at depths of 20, 30, and 40 cm (Z11, Figure 3). Z11 ascribed the mismatch between simulated and measured soil water content to two possible reasons: one is the likely incorrect assumption of homogeneous soil hydraulic parameters along the 5 m simulation soil profile, the other is the possible low sensitivity of soil moisture sensor in measuring water content in extraordinarily dry environment. While we agree with the above two possible explanations, however, to our knowledge, other mechanisms such as adsorption component of the soil water retention which was ignored in Z11 could also be responsible for the mismatch between simulated and measured soil water content in desert soils (discussed later in section 4.1). Z11 then used the calibrated model to study advective airflow effect on evaporation in both low and high permeability soils and found that neglecting soil airflow could result in an underestimation of evaporation by 8.85% and 6.4% in low and high permeability soils, respectively, during the 6 day period. This underestimation error was more significant on the day right after a precipitation event (Z11, Figure 4) because the fine sand was moderately dry (not very dry) after this small rainfall event which occurred at the end of the first day. After systematically analyzing driving forces (soil air pressure gradient, soil matric potential gradient, and soil temperature gradient) and conductivity fields (mainly thermal and isothermal hydraulic conductivity), Z11 concluded that the underestimation error of evaporation was mainly caused by underestimation of isothermal hydraulic conductivity (which we agree with) by neglecting airflow (which we doubt). We commend Z11 for creatively investigating this often neglected advective airflow effect on soil evaporation and finding such insightful phenomena, however, it seems to us that the advective vapor flux on increasing isothermal hydraulic conductivity (hereafter denoted as KLh) is overestimated in Z11 analysis. In addition, the negligence of rainfall influence on discussing advective airflow effect and the neglect of extending the water retention function and KLh to oven-dry condition is unwarranted.