An experiment has been carried out in order to both measure the unsaturated hydraulic conductivity of a soil crust when it is developing and to characterize its macro- and microstructural evolution. Five stages of a structural crust developing in a loamy clay soil have been identified. Mercury intrusion experiments and micromorphological descriptions have provided information on the structural evolution of the crust at the microscale. Quantitative image analysis from images of the crust microstructure has led to the definition of a particle erosion index at the microscopic scale, which is the mean chord length calculated in the solid phase. This index decreases from 60 to 11 mu m during the whole experiment, the lowest value being recorded at the fourth stage. Specific measurements of the percolation threshold in the soil crust at different stages during its formation have given quantitative data to interpret the modifications of the unsaturated hydraulic conductivity of this crust when it develops. The percolation threshold decreases from 130 mu m at the second stage down to 40 mu m at the fifth stage. Between the same stages, the unsaturated hydraulic conductivity at -5 kPa decreases from 12.2 to 4.7 cm/h. Both the structural information given by the chord size distribution and the functional information obtained from the percolation threshold can be related to propose a new model of soil crusting. (c) 2005 Elsevier B.V. All rights reserved