Towards a new generation of hydrological process models for the meso-scale: an introduction

The main motivation for this special issue of Hydrology and Earth System Sciences is the IAHS decadal research initiative “Predictions in Ungauged Basins” (Sivapalan et al., 2003), and more specifically the need to develop hydrological models that work for the “right reasons” (Kirchner, 2006). This means that a model shall “work” because its structure is based on appropriate representation of the hydrological functioning of a landscape/catchment and the associated dominant patterns and landscape structures, and not because several structural deficits are mutually compensated within the usual model calibration process. The central credo underlying our quest in this direction is that typical patterns of vegetation, soils and subsurface structures within a catchment cause a typical hydrological process spectrum or hydrological functioning represented as a generic feature of a landscape. This is known as the pattern-process-function paradigm in theoretical ecology (Watt, 1947; Turner, 1989; Turner and Gardner; 2001; Schroeder, 2006, this special issue). We, therefore, expect that model structures that offer a higher compatibility with landscape structures and hydrological functioning of catchments will allow, in the long term, much better predictions than may be achieved with currently available conceptual models. The latter type models have the tendency to compensate the model structural error by unrealistic parameter combinations, which are then reflected in large parameter uncertainty (Wagener et al., 2003). In this light, this special issue presents a series of articles that address the following themes: