ACCOUNTING FOR TEMPORAL COMPLEXITY WHEN MODELING EROSION AND SEDIMENT TRANSPORT IN EVOLVING LANDSCAPES

Surface processes express complex interactions between the lithosphere and atmosphere. As geoscientists we know this, we teach this, but modeling this is truly one of the greatest grand challenges, and the question of how to explore the feedbacks among climate, erosion, and tectonics has emerged as a grand challenge in Geomorphology. One major problem is that while various processes are defined by certain temporal and spatial scales under which they occur, they cumulatively contribute to the rates of change of landform surfaces and ultimately to landscape topography through time. Long term averages ignore short term processes that may play significant roles in long term surficial changes. This presentation outlines the results of a recent case study that applied numerical modeling to erosion and sediment transport of regolith on a glacial-interglacial timescale. A standard hillslope diffusion equation that was used to model hillslope degradation is based on a linear sediment transport law which states that sediment flux is proportional to hillslope gradient and a proportionality constant called topographic diffusivity. While expressing the cumulative effects of climate and biota on a substrate, the constant is unspecific about lithologic material, vegetation, or local climate variability. By convention, topographic diffusivity is applied in the equation as a constant coefficient invariable through time. In the present case study time-varying coefficients were employed to model hillslope degradation in eastern Sierra Nevada – a mid-latitude region with a glacial chronology providing a significant record of climate fluctuations. In the model, a hillslope was degraded for 85 ka and the crest elevation results were compared between model runs using constant and time-varying parameters. The results indicated some variation and the question arose of how patterns of past climate variability might have contributed more significantly as driving forces for erosion in other regions around the world. Future directions for the approach presented include identifying such locations with climate histories that might have generated past erosion rates varying significantly from a long term average. These locations could provide an opportunity to assess the contributions of climate versus those of tectonics to erosion.