THE SOIL PRODUCTION FUNCTION AND LANDSCAPE EQUILIBRIUM

Hilly and mountainous landscapes are partially to completely covered w ith soil under a wide range of erosion and uplift rates, bedrock type and climate. For soil to persist it must be replenished at a rate equa l to or greater than that of erosion. Although it has been assumed for over 100 years that bedrock disintegration into erodable soil decline s with increasing soil mantle thickness(1-9), no field data have shown this relationship. Here we apply two independent field methods for de termining soil production rates to hillslopes in northern California. First, we show that hillslope curvature (a surrogate for soil producti on(7)) varies inversely with soil depth. Second, we calculate an expon ential decline of soil production rates with increasing soil depth fro m measurements of the in situ produced cosmogenic Be-10 and Al-26 conc entrations in bedrock sampled under soils of different depths. Results from both methods agree well and yield the first empirical soil produ ction function. We also illustrate how our methods can determine wheth er a landscape is in morphological equilibrium or not.