Micro- and Macro- Models in Geomorphology and Hydrology

Abstract
Even the most 'physically based' current models rely on gross simplifications of the detailed process mechanics, and substantial advances in understanding are achieved mainly through this paradigm of scientific simplification rather than through developing more complex model structures. The priorities for computationally intensive research are thought to lie in two other areas.

1. Reconciling models at different scales
   Relevant models exist at scales which range from the soil crumb to continental or global scales, a range of 10¹⁰x. Most models have time or space scopes of at most 10³x, so that fundamentally different structures are used across the full range. Reconciliation is by formal integration or, increasingly, by heavy computation. The greatest challenge is to understand structural changes associated with non-linearity.
2. Calibration and validation
   To date only some hydrological models have been simple enough to allow through investigation of the parameter space, often using parallel processing facilities. There is a need to extend this process to a wider class of models, and to explore computationally efficient ways of doing this where models have a substantial number of parameters and/or significant run times. There is also a need to consider the use of qualitative data for calibration, rather than rely exclusively on hydrological outputs, which suppress much of the richness of potential change in the landscape.