Parsimonious modelling of water and suspended sediment flux from nested catchments affected by selective tropical forestry

The ability to model the suspended sediment flux (SSflux) and associated wa ter flow from terrain affected by selective logging is important to the est ablishment of credible measures to improve the ecological sustainability of forestry practices. Recent appreciation of the impact of parameter uncerta inty on the statistical credibility of complex models with little internal state validation supports the use of more parsimonious approaches such as d ata-based mechanistic (DBM) modelling. The DBM approach combines physically based understanding with model structure identification based on transfer functions and objective statistical inference. Within this study, these app roaches have been newly applied to rainfall-SSflux response. The dynamics o f the sediment system, together with the rainfall-river flow system, were m onitored at five nested contributory areas within a 44 ha headwater region in Malaysian Borneo. The data series analysed covered a whole year at a 5 m in resolution, and were collected during a period some five to six years af ter selective timber harvesting had ceased. Physically based and statistica l interpretation of these data was possible given the wealth of contemporar y and past hydrogeomorphic data collected within the same region. The results indicated that parsimonious, three-parameter models of rainfall -river flow and rainfall-SSflux for the whole catchment describe 80 and 90% of the variance, respectively, and that parameter changes between scales c ould be explained in physically meaningful terms. Indeed, the modelling ind icated some new conceptual descriptions of the river flow and sediment-gene ration systems. An extreme rainstorm having a 10-20 year return period was present within the data series and was shown to generate new mass movements along the forestry roads that had a differential impact on the monitored c ontributory areas. Critically, this spatially discrete behaviour was captur ed by the modelling and may indicate the potential use of DBM approaches fo r (i) predicting the differential effect of alternative forestry practices, (ii) estimating uncertainty in the behaviour of ungauged areas and (iii) f orecasting river flow and SSflux in terrain with temporal changes in rainfa ll regime and forestry impacts.