Quantifying evaporation drying front dynamics using neutron radiography

Soil evaporation plays an important role in the hydrologic cycle and affects near surface energy balance. Changes in evaporative drying rates reflect complex interplay between soil pore space and transport properties and atmospheric conditions. Typically, initial drying rate from the wet soil surface is high and is limited by atmospheric conditions. Subsequently, when hydraulic connection between the receding drying front and atmosphere can no longer be maintained, the drying rate becomes dominated by vapor diffusion through the air-filled pores. We show that the depth of drying front where a transition from atmospheric (high) to diffusion controlled (slow) evaporation takes place, is related to air entry value of the medium (or to a critical pore size). Fine textured soils with small pores sustain higher drying rates than coarser ones. We delineated drying front position and morphology of sand-filled Hele-Shaw cells using neutron transmission technique. Insights on water content distribution and dynamics of drying front in sands of different textures will be presented and compared with theoretical predictions.