NUMERICAL MODELING OF LANDSCAPE EVOLUTION ON GEOLOGICAL TIME-SCALES -A PARAMETER ANALYSIS AND COMPARISON WITH THE SOUTH-EASTERN HIGHLANDS OF AUSTRALIA

Surface-process models (SPMs) have the potential to become an importan t tool in predicting sediment flux to basins, but currently suffer fro m a lack of quantitative understanding of their controlling parameters , as well as difficulties in identifying landscape properties that can be used to test model predictions. We attempt to constrain the parame ter values that enter a SPM by comparing predictions of landscape form las expressed by hypsometric and fractal measures) and process rates obtained for different parameter sets with observations from the south -eastern Australian highlands, a rifted margin mountain belt that has remained tectonically stable during Cenozoic times. We map the hypsome try and fractal characteristics of south-eastern Australia and find th at the roughness amplitude (G) correlates well with local relief, wher eas the hypsometric integral (H) correlates slightly better with eleva tion than with relief. The fractal dimension (D) does not correlate wi th any other morphometric measure and varies randomly throughout the r egion. Variograms generally show three kinds of scaling behaviour of t opography with increasing wavelength, with topography only being truly self-affine at wavelengths between similar to 1 and 10 km. From a rev iew of the available data on long-term denudation rates in south-easte rn Australia, we infer that these have been 1-10 m Myr(-1), and averag e escarpment retreat rates 0.2-1.0 km Myr(-1), throughout the Cenozoic . Model predictions, using a SPM that includes hillslope diffusion and long-range fluvial transport, suggest that landscape form evolves wit h time; after an initial phase where D, G and relief increase, all mor phometric measures decrease with increasing denudation. The behaviour of G and Hin the models is qualitatively compatible with the observati ons; D, however, varies predictably in the models, in contrast with it s random behaviour in the real world. The observed present-day morphol ogy of SE Australia does not impose quantitative constraints on parame ter values. The fractal analyses do impose general conditions of relat ive parameter values that have to be met in order to create 'realistic ' topographies. They also suggest that there is no theoretical basis f or including hillslope diffusion in SPMs with a spatial resolution coa rser than 1 km. A comparison of the observed denudation and retreat ra tes with model predictions places order-of-magnitude constraints on pa rameter values. Thus, data pertaining to landscape evolution are much more valuable than static present-day topography data for calibrating SPMs.