THE SOIL PRODUCTION FUNCTION AND LANDSCAPE EVOLUTION: A TWENTY-YEAR PERSPECTIVE (Invited Presentation)

The thickness of mobile regolith or “colluvial soil” is a fundamentally important characteristic of the critical zone that plays an essential role in hydrology, ecology, the action of bio-geochemical cycles, and erosion and transport processes. We use the term “colluvial soil”, often shortened to simply “soil”, rather than the equivalent “mobile regolith” because it is deeply entrenched in the geomorphic literature. In our usage soil production is the process of mechanical breakdown and entrainment of rock or regolith into the mobile, active colluvial soil – soil production thus is distinct from pedogenesis and governed by different processes. The thickness of such colluvial soil on hillslopes reflects a balance between the rate or erosion and the rate of soil production at the soil/weathered rock interface. The soil production function refers to the relationship between soil thickness and the rate of soil production. It has a long history of theoretical development and a more recent, twenty-two-year history of empirical testing. For the past two decades, many studies have quantified this function across landscapes, climate zones and tectonic regimes, and used the quantified relationship to predict how landscapes evolve. Well-established theory also predicts that (1) mean soil thickness on erosional landscape elements (convex and planar hillsides) decreases with increasing catchment-mean erosion rate and (2) soil-mantled landscapes give way to rocky ones when catchment-mean erosion rate exceeds the climate- and lithology-controlled maximum soil production rate that occurs on bare-rock surfaces or under a thin soil cover. According to theory, this critical erosion rate is thus associated with a sudden and profound change in hydrology, ecology, biogeochemical processes, and erosion/sediment transport processes. This presentation offers a twenty-year perspective on quantifying the soil production function and using it around the world to help understand how landscapes evolve.