ABSTRACT This paper gives a general overview of how parameter optimization using inverse modelling can be used to infer scale-dependent effective soil hydraulic functions. In this approach, it is assumed that macroscopic flow behaviour as represented by Richards' equation is valid at all spatial scales. Thus, solution of the continuity equation with appropriate boundary conditions is independent of the size of the physical system. In this manner, for example, watershed effective soil hydraulic properties can be estimated. Since the optimization requires areal-averaged boundary conditions and measured areal-averaged flow attributes, such as water content, it is shown how remote sensing can be applied to infer these area-averaged hydrological variables.

In this way effective soil hydraulic properties are obtained from inverse modelling using the dynamical one-dimensional soil–water–vegetation model SWATRE. Areal evaporation can be obtained from reflection and thermal infrared remote sensing, while areal soil water content is estimated from microwave remote sensing techniques. The combined remote sensing and inverse modelling approach is illustrated with data taken from the HAPEXEFEDA experiment in Spain and from the catchment Hupselse Beek in the Netherlands. It is concluded that small-scale soil physics may adequately describe mesoscale behaviour.