Criteria for a hydrologically sound structuring of large scale land surface process models

The assessment of land surface heterogeneity in large scale modelling of moisture and energy fluxes at the soil–vegetation–atmosphere interface is one of the primary tasks in recent world climate and global change modelling. After a brief characterization of scale ranges applied in hydrology and of their relation to different categories of hydrological models, the application of lumped models and the extrapolation of the validity of microscale process descriptions to the macroscale are criticized with reference to selected examples of heterogeneous land surfaces. Appropriate alternative modelling principles which take into account the areal pattern of atmospheric forcing as well as landscape patchiness and intra-patch heterogeneity are then briefly described and a hierarchy for areal discretization in land surface modelling as well as criteria to delineate zones of “uniform” atmospheric forcing are introduced.