Dynamic evolution of the unsaturated hydraulic conductivity of a developing crust

Understanding the evolution of hydraulic parameters during the different stages of crust formation is necessary to predict infiltration in crusted soil. However, only a few studies have focused on the dynamics of parameters that control infiltration through a developing crust. This study reports the results of laboratory experiments carried out with a specific device, the drip infiltrometer, which enables a crust to form at the surface, and the evolution of the pressure head and the hydraulic conductivity of the soil horizon underneath the crust to be followed simultaneously. Two soils of different susceptibility to crusting were used in the experiments: a silt loam and a silty clay loam of the Bassin de Paris (France). The results show matric potential in the unsaturated range developing below the surface from the early stage of crust formation. The values of matric potential show an increasing trend from the early stage of crust formation and at the late stage respectively for the silt loam and the silty clay loam soils. The hydraulic conductivity of the sub‐crust shows significant changes from the early stage for the silt loam soil, but only at the final stages for the silty clay loam. The initial value of the hydraulic conductivity of the sub‐crust drops by more than 50 per cent for silt loam compared to about 35 per cent for silty clay loam. The decrease of hydraulic conductivity beneath the surface crust is related to the changes in the soil matrix, induced by the migration of particles detached from aggregate breakdown. The different stages of crust formation were characterized by the variation of the unsaturated conductance of the soil crusted layer, calculated from the relationship between the flux density through the crust and the sub‐crust pressure head. The results revealed rapid sealing at the surface of the silty loam soil, whereas sealing occurs gradually at the surface of the silty clay loam. The difference in the susceptibility to crusting and the difference in moisture characteristics between the two soils are probably the causes of the contrast in temporal variation of the sub‐crust matric potential. Copyright © 2004 John Wiley & Sons, Ltd.