Steps towards the measurement and modelling of soil crusts

Soil crusting is a global issue which results in reduced infiltration, runoff, erosion, reduced seedling emergence, poor irrigation efficiency, and difficulty with crop-soil modelling. Soil crusts are thin transient soil-surface layers that are less porous than the material immediately beneath them. Their formation results in considerable modification to the physical properties of the soil surface including lower porosity, high bulk density, reduced size and continuity of macropores and lower degree of aggregation, compared to the underlying bulk soil (Hussein, et al., 2010; Miralles-Mellado, et al., 2011). Surface crusts control infiltration processes and thus play a critical role in partitioning rainfall into infiltration and runoff.

Despite the obvious importance of soil crusts on hydrological process few crop simulation models, catchment hydrological models or soil hydrological models explicitly include soil crust routines. Of the models that do account for crusting many simply use generic USDA curve numbers to arbitrarily partition rainfall into runoff and infiltration based on generic soil type and surface condition. Currently no model considers the soil crust to be a spatially explicit soil layer possessing unique soil water retention and hydraulic conductivity properties as described by van Genuchten-Mualem parameters. This lack of routines for simulating the occurrence of soil crusts is in large part due to limited understanding of the effects of crusting on soil pore size and function, and by a lack of in situ and laboratory approaches for measuring the physical properties of soil crusts.