Water movement in porous materials III. Evaporation of water from soil

Abstract

The equation of vertical flow is adapted to describe the evaporation of water from a semi-infinite porous material during the second stage of drying when water loss is limited by the distribution of water in the material. Flow measurements on long columns of aggregates of six soil materials confirm the theory, and enable the transport parameters controlling flow to be found over the range of volumetric water content θ (cm³ water/cm³) from aggregate saturation θ_C to dryness (i.e. from near pF 2 to near pF 7). Evaporation from porous materials initially at a uniform water content θ_C produces profiles of θ invariant with zt ^−1/2 (where z is the depth in cm and t the time in seconds) which differ for each material. As θ decreases from θ_C diffusivities (D cm² s⁻¹) calculated from these profiles fall to a minimum, then rise to a subsidiary maximum close to dryness, before falling again as θ approaches zero. Such behaviour accords with prediction. Hydraulic conductivities (k cm s⁻¹) fall continuously as θ decreases from θ_C to zero.