Evaporation, and Moisture and Heat Fields in the Soil

Recent developments in the study of moisture and heat fields in the soil are applied to the problem of evaporation from bare soil surfaces. Most of the analysis is confined to steady conditions, but it definitely suggests that there are three phases to the desiccation of a given soil profile: 1. So long as the soil is sufficiently moist, the evaporation rate, E, is indistinguishable from that from a saturated surface, E3. 2. At intermediate moisture contents, E is independent of E3 and depends only on the soil-moisture distribution. 3. When the surface layers of the soil are sufficiently dry, E is sensitive to the heat flux in the soil, and a negative correlation between E and E3 may follow. The first and second of these correspond to the well-known constant and falling rate phases of the “isothermal” drying of initially saturated soils and other porous media, which thus receive quantitative physical explanation. There is experimental evidence of the existence of the third phase also. Other results of the article include : 1. Development of a (steady state) mode1 of the energy balance at imperfectly evaporating surfaces (E < E3). This leads to a simplified but suggestive approach to the microclimate of bare soils. 2. Analysis of the spatial distribution of evaporation sites within the soil. 3. Analysis of the modification of the energy balance which results from the location of evaporation sites within the soil rather than at the surface.