Abstract: Simple relationships between mean climate and erosion rates have failed to emerge from either historic sediment yield studies or millennial-scale erosion rate studies using cosmogenic radionuclides. One confounding factor is the role of vegetation, whose type and density are also strongly linked to climate. In this presentation, I will demonstrate how plant water use generates an important vegetation feedback with river incision by altering flood frequencies. Soil water modeling of the plant root zone shows that the relationship, or the lack thereof, between the right tails of daily rainfall and daily streamflow distributions are consistent with an empirical analysis of large hydroclimatic datasets in the U.S. Proper attribution of which hydroclimatic metrics best correlate with flood frequencies is important to relating climate to erosional processes. For example, the shape of the right tail of streamflow distributions directly influences the relationship between steady state relief and long-term erosion rates predicted by stream power models that include erosion thresholds. When driven by modern hydroclimatology, such stochastic-threshold models successfully explain observed relationships between topography and erosion rates in steep, mountain landscapes that span large gradients in climate (desert to tropical rainforest). These field examples highlight that climatic controls on erosion rates may be so difficult to quantify because seemingly large differences in mean climate actually corresponds to much smaller differences in the geomorphically effective climate.
Biosketch: Matthew Rossi is a geomorphologist who recently joined the Earth Lab initiative at the University of Colorado - Boulder as a Post-Doctoral Research Scholar in Project Erosion. He received his B.S. in Geology from the College of William and Mary (2003) and his Ph.D. in Geological Sciences from Arizona State University (2014). Prior to coming to the University of Colorado, he served as a Visiting Assistant Professor in the Program in Environmental Sciences at Northwestern University. His research focuses on developing quantitative understanding of the processes and process interactions that control the shape of Earth’s surface and is motivated by a desire to explain how landscapes record and respond to climatic and tectonic forcing.