The effect of soil macro-structure on bare soil evaporation

Evaporation of soil water is often characterised by water losses over time for a defined soil volume where soils are assumed to be homogeneous in texture and structure. In this study, we hypothesised that evaporation depends not only on climatic conditions, soil texture, and soil hydraulic properties but also on the soils’ macro-structure. Specifically, that the different distribution of air-filled macropores, stones, and the connectivity of soil matrix will affect bare soil evaporation and herewith the transition from stage 1 to stage 2 evaporation. In a climate constant room, we measured evaporation characteristics of undisturbed soil cores taken under various land uses and soil textures (clay and sandy loam) and compared the evaporation rates to columns with sieved soil repacked to the same bulk density. Tensiometers installed in two different depth provided information about the hydraulic gradient along the columns, while weight measurements continuously recorded the mass loss. Soil structure of undisturbed columns was determined by X-ray computed tomography (X-ray µCT) at a voxel size of 50 µm. In addition, we evaluated the effect of macro-structure on bare soil evaporation for unsaturated condition, i.e. visible porosity was air-filled, by 3D image-based simulations using HYDRUS 3D.  The lab study showed that the well-sorted repacked samples lost significantly more water as the undisturbed samples. The differences cannot be explained by the total porosity and thus the total water reservoir. When using the time, the hydraulic gradient along the undisturbed columns was exponentially increasing, it was shown that the well-connected macropore volume could explain most of the evaporation characteristics. In addition, the presence of denser soil clods significantly shortened the time to build up the gradient. Neither stone nor particulate organic matter content had a significant effect on evaporation characteristics. The 3D image-based simulation indicated that air-filled macropores act as barriers for upward water flow and that the loss of water was limited by the connectivity of the soil matrix. It can be concluded that not only soil texture effects bare soil evaporation but also the soil macro-structure.