Impact of Water Retention Curves on Evaporation Under Diurnal Atmospheric Forcing

Abstract:

Water retention and unsaturated hydraulic conductivity curves dictate soil moisture dynamics, whose accurate description in both the liquid and vapor phases is crucial to properly estimate soil water evaporation. When classical water retention curves that approach infinitely negative matric potentials at nonzero residual water content (e.g. Van Genuchten or Brooks Corey) are employed to model soil moisture dynamics, evaporation from arid soil is not satisfactorily described because no soil drying below residual water content is allowed. Ciocca et al., GRL, [2014] showed how, for the isothermal case, more physically sound dynamics are predicted by employing modified retention models allowing the drying below the residual water content by vapor diffusion. The impact of these modified water retention models on the description of the moisture dynamics is numerically investigated in a more complex and realistic framework, in which a diurnal atmospheric forcing is applied at the soil surface and the soil heat dynamics (coupled to the moisture dynamics) are considered. For different soils, results are compared both with predictions from the classical retention curves and with a steady (i.e. not diurnally oscillating) atmospheric forcing. The impact of the significantly larger vapor fluxes predicted by the modified retention models on the soil temperature and consequently on the latent, sensible and ground heat fluxes is presented. A detailed analysis of the hourly liquid, vapor and temperature dynamics with depth is provided in order to assess whether the modified retention curves may help to reconcile the theory with some still debated field experimental results (e.g. soil moisture content rises at midday) without invoking for any empirical liquid gain and/or vapor enhancement factor.