Chappell, N.A., Bidin, K., Sherlock, M.D., Lancaster, J.W. and Vertessy, R. 2000. Parsimonious, spatial representation of tropical soils within dynamic, catchment hydrological models. In Forests-Water-People in the humid tropics, UNESCO-IHP / IUFRO2000 workshop 30 July - 8 August 2000, Kuala Lumpur, Malaysia. (for publication by UNESCO / Cambridge University Press 2002)



The functional relationship between catchment rainfall and the river-discharge generated, requires very few (3 to 5) parameters to be described. The use of greater numbers of parameters within more complicated model-structures, therefore, should necessitate that these parameter- sets / model-structures are tested and hence 'justified' by distributed (thus inevitably, semi-quantitative) field observations. As model complexity increases, the riverflow time- series becomes predictable from a very wide range of parameter sets, so that each sensitive parameter becomes more and more uncertain. There is, therefore, value in limiting the complexity of models (i.e., parsimony) to allow evaluation of the component structures or parameters. Topographically-based hydrological models, such as TOPMODEL, WETZONE and TOPOG can have more parsimonious model structures. While the value of their topographic indices in predicting saturated areas has been tested by numerous studies over the last decade (mostly within temperate climates), their spatial distributions (in 1 to 3 dimensions) of the other model component, the 'soil transmissivity' and/or 'soil permeability', has received much less attention, often because of the limitations of the field data. Yet, soil permeability is often seen as the most important soil (porous media) property specified within physics-based models.

As a result, this study will focus on the parameterisation of soil permeability within topographically-based hydrological models applied to tropical catchments, its affect on riverflow time-series, and the relationship between point-measures of permeability and effective values capable of simulating the riverflow time-series. The results of this analysis, will potentially demonstrate that the spatial distribution of soil permeability derived from inversion of (parsimonious) catchment-scale models is very different to that derived from tests on small-samples (even allowing for the severe limitations of the inversion process). The differences are consistent with the presence of zones of preferential flow (i.e., natural soil pipes, percolines and fractures) that are inadequately characterised by tests on small-samples. Hillslope-scale tracer tests yield results that support these conclusions, but as yet, do not provide sufficient data to parameterise the models.

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