Gopher bioturbation: field evidence for non-linear hillslope diffusion

Abstract

It has generally been assumed that diffusive sediment transport on soil-mantled hillslopes is linearly dependent on hillslope gradient. Fieldwork was done near Santa Barbara, California, to develop a sediment transport equation for bioturbation by the pocket gopher (Thomomys bottae) and to determine whether it supports linear diffusion. The route taken by the sediment is divided into two parts, a subsurface path followed by a surface path. The first is the transport of soil through the burrow to the burrow opening. The second is the discharge of sediment from the burrow opening onto the hillslope surface. The total volumetric sediment flux, as a function of hillslope gradient, is found to be: qs (cm3 cm−1 a−1) = 176(dz/dx)3 − 189(dz/dx)2 + 68(dz/dx) + 34(dz/dx)0·4. This result does not support the use of linear diffusion for hillslopes where gopher bioturbation is the dominant mode of sediment transport. A one-dimensional hillslope evolution program was used to evolve hillslope profiles according to non-linear and linear diffusion and to compare them to a typical hillslope. The non-linear case more closely resembles the actual profile with a convex cap at the divide leading into a straight midslope section. Copyright © 2000 John Wiley & Sons, Ltd.