The Influence of Climate and Micro-climate (aspect) on Soil Creep Efficiency

Abstract:

Although hillslope evolution has occupied geoscientists for over a century, the effect of climate on the morphology of soil-mantled, transport-limited hillslopes remained poorly-constrained. In this study we utilize numerical simulations of volcanic cinder cones in the Golan Heights (Eastern Mediterranean) to estimate soil creep efficiency across a strong north-to-south gradient in mean annual precipitation (1100-500 mm). Our model utilizes the cinder cone initial hillslope profile (constrained by the dip of the ash layers), the current hillslope profile (measured with cm scale accuracy) and the known eruption age (K-Ar and Ar-Ar constraints) to predict the best-fit final hillslope profile by changing the soil creep diffusion coefficient. Hillslope evolution is described using a diffusion equation.

Our results indicate that the best-fit soil creep diffusion coefficient varies from 0.5 to 7 m²/kyr. Soil diffusivity depends on both climate (precipitation) and aspect-related microclimate as the diffusion coefficient decreases with mean annual precipitation and is higher on south facing hillslopes. This climate dependency seems to reflect an increase in the apparent soil cohesion at higher precipitation rates due to vegetation coverage (the density of root networks) which co-varies with rainfall and aspect. We demonstrate this strong co-variance utilizing the spatial distribution of NDVI vegetation index calculated from ASTER images. In addition, 650-750 kyr cinder cones display lower diffusion coefficient relative to late Pleistocene cinder cones (120-150 kyr). This temporal variance may reflect the unique climatic conditions of the last glacial and inter-glacial period. Future analysis of additional cones is expected to contribute further to our understanding of hillslope form-process relation and the morphologic signature of climate.