RATE OF SOIL TRANSPORT AT THE HILLSLOPE TO THE REGIONAL SCALE

A numerical model of soil production and transport on soil-mantled landscapes has been developed that combines processes such as linear soil creep, depth-dependent (or viscous) soil creep and transport by overland flow. Cosmogenic measurements at a site in the Bega Valley, southeastern Australia, were used to determine a production function that varies exponentially with soil thickness. Field observations also led to a well-constrained relationship between soil thickness and local surface curvature that is used to constrain the value of the various transport parameters.

The numerical model has been used to determine how soil production and transport is affected by rapid climatic, tectonic or environmental changes, which we incorporated in the model by artificially changing the value of the transport parameters or the imposed bedrock geometry. The results indicates that: (a) it takes between a few thousands to a few hundred thousand years for soil thickness to reach steady-state; (b) this "response time" of the system to rapid environmental changes is a strong function of the height and extent of the hill (i.e. the local relief) and, to a lesser degree, of the assumed transport parameters; (c) the response time is independent of the assumed production parameters, which confirms that in this region of southeastern Australia, landscape evolution is transport-limited (i.e. bedrock is rarely exposed); (d) each of the transport mechanisms is active on a specific part of the landscape; (e) the dominant transport mechanism varies with time during the recovery phase.

It is also shown under which conditions landscape evolution can be accelerated by rapidly changing environmental factors, such as a periodically varying climate, or by changing bedrock geometry to simulate movement on a discrete fault.

Finally, we show how the results of this local study are incorporated into a large-scale surface processes model, demonstrating how discrete geochronological and morphometric measurements can be used to constrain landscape evolution on a regional scale and on geological time scales.