Simulating bare soil evaporation for undisturbed soil cores—Using HYDRUS 3D simulation on X‐ray µCT determined soil macrostructures

Evaporation of soil water depends not only on climatic conditions, soil surface roughness, soil texture, and soil hydraulic properties but also on the soils’ macrostructure. Evaporation is characterized by water losses over time for a defined soil volume, where soils are assumed to be homogeneous in texture and structure. In this technical note, we investigated the potential and limitations of 3D modeling of evaporation processes on 250 cm3 soil cores with structural features ≥480 µm determined by X‐ray computed tomography. For this, we used isothermal Richards equation as the main governing equation, accounting also for isothermal vapor flow. We simulated two evaporation experiments with same soil texture but contrasting macrostructures, that is, the spatial arrangement of voxels classified as soil matrix and air‐filled voids, of a ploughed and non‐ploughed grassland soil with HYDRUS 3D. In both simulations, we fixed the potential evaporation rates to the experimental rates and evaluated simulation results with measured matric potential data at two depths (1.25 cm and 3.75 cm) continuously recorded at 10 min intervals. We could show that the simulations of bare soil evaporation were able to predict the tensiometer dynamics and water losses for the full experimental time of 7 days. The simulation provided unique spatial information of water content and flow velocities as a function of time, which are important when studying the effect of air‐filled macropores, macro‐connectivity of soil matrix, and water dynamics on soil evaporation.