Effects of gravel mulch on water vapor transfer above and below the soil surface

Laboratory experiments were conducted to investigate the effects of gravel mulch on evaporation with emphasis on resistance to water vapor transfer in a soil-mulch–atmosphere continuum. Based on measurements of evaporation rate using weighing soil columns with and without the surface mulch, the resistance to vapor flow by each of atmosphere, mulch and soil sections was determined. Comparison of experimental results of bare soil with mulched surface indicated that gravel mulch increases the resistance not only above the soil surface but also below the surface. The resistance of the mulch layer to water vapor flow above the soil surface increased exponentially with the thickness of the mulch layer, although so-called “aerodynamic resistance” (i.e. resistance above the mulch–atmosphere interface) slightly decreased compared to the bare soil because of large roughness of gravel mulch. The increase in the resistance below the soil surface, which is conventionally called “soil surface resistance”, is attributed to the development of a dry surface layer. Partial covering of the soil surface with gravel induced concentrated evaporation flux and increased depth of the drying front, even if the soil is relatively wet on spatial average. The nonhomogeneity in soil moisture movement may be reinforced when gravels are partially embedded into the soil.