MULTISCALE PERMEABILITY ESTIMATION FOR A TROPICAL CATCHMENT

Physically based and spatially distributed modelling of catchment hydr ology involves the estimation of block or whole-hillslope permeabiliti es. Invariably these estimates are derived by calibration against rain fall-runoff response. Rarely are these estimates rigorously compared w ith parameter measurements made at the small scale. This study uses a parametrically simple model, TOPMODEL, and an uncertainty framework to derive permeability at the catchment scale. The utility of expert kno wledge of the internal catchment dynamics (i.e. extent of saturated ar ea) in constraining parameter uncertainty is demonstrated. Model-deriv ed estimates are then compared with core-based measurements of permeab ility appropriately up-scaled. The observed differences between the pe rmeability estimates derived by the two methods might be attributed to the role of intermediate scale features (natural soil pipes). An alte rnative method of determining block permeabilities at the intermediate or hillslope scale is described. This method uses pulse-wave tests an d explicitly incorporates the resultant effects of phenomena such as s oil piping and kinematic wave migration. The study aims to highlight i ssues associated with parameterizing or validating distributed models, rather than to provide a definitive solution. The fact that the perme ability distribution within the Borneo study catchment is comparativel y simple, assists the comparisons. The field data were collected in te rrain covered by equatorial rainforest. Combined field measurement and modelling programmes are rare within such environments. (C) 1998 John Wiley gr Sons, Ltd.