Chappell, N.A., Douglas, I., Walsh, R., and Bidin, K. 1994. Scaling water and sediment flux across rainforest mosaics. In Proceedings of the Royal Society / UKM conference Development of Research in Danum Valley, Kota Kinabalu, Malaysia, 7-8 December 1994.



Selective timber logging leaves a mosaic disrupted terrain within largely non-impacted rainforest. Assessment of the role of the disrupted terrain within the dynamics of a region requires that whole elements of terrain (e.g., a skid trail, a logging road, a disrupted headwater, or a forest block with or without canopy disturbance) are characterised with a small/unique set of parameters. And that the true distribution of these is understood over the region. Understanding the recovery of the terrain, would be then, a simple extension of such parameter estimation methods.

Current attempts to validate distributed hydrological or contaminant models show that the derivation of mosaic or slope scale parameters by either 'down- or up-scaling' is subject to severe constraints: 1. Application of physics-based models to catchment problems is based on limited measurement, thus rendering true parameter distributions un-identifiable. 2. Examination of work where mosaic scale parameters are derived by response upscaling, again highlights the dearth of data. Often the data allows the derivation of 'effective parameters' for only one mosaic element 3. Where large uncertainties in the process definitions exist, both techniques (up- and down- scaling) will propagate and magnify the error. In conclusion, there is a need for direct derivation of parameters at the scale they are required - the scale of the terrain disturbance.

A new research project was established in July 1994 with the aim of deriving distributions of mosaic-scale parameters of water and sediment flow. The field data programme is centred on the Barn catchment, Danum Valley, Sabah. The catchment lies within a stand of dipterocarp rainforest, now recovering from selective logging in 1988. Manchester University have derived water and sediment budgets from catchment monitoring since 1987. The new instrument network will comprise of 18 mosaic-scale elements, each with continuos logging of discharge and turbidity. The responses observed for a distribution of the elements will be parameterised using a combination of transfer-function (microCAPTAIN) and conceptual (TOPMODEL) models. Uncertainty in the response at the regional (or catchment) scale will be predicted. Small-scale measurements of hydro- geomorphological parameters (e.g., saturated hydraulic conductivity and erodibility) will be incorporated within small-scale physics-based modelling, to (1) predict uncertainties at the mosaic scale, and (2) allow the large-scale response functions to be related to 'physics-based parameters'. The work aims both to quantify uncertainties in large-scale predictions of water/sediment flux and develop a better understanding of the patterns of these flows within a recovering rainforest.

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