Quantifying Variability in Streamflow Distributions from Climate Models to Understand the Relationships Between Climate, Topography and Erosion

Abstract:

The role of climate in the tectonic evolution of a mountain range is a compelling topic as there are numerous characteristics of climate that may influence the erosional processes that shape mountainous environments. Quantifying these characteristics proves challenging because obtaining high resolution measurements in both time and space is either too computationally expensive to capture in modeling studies or often not possible in the field. Studies typically overcome these challenges by using averages, such as an effective discharge or mean annual precipitation that reflect long temporal averaging of events of different magnitudes and frequencies. The use of these averaging approaches leaves out the complexities of climate variability and is insufficient when erosion thresholds are included in a landscape evolution model. In this study, we provide a quantitative method for measuring river discharge variability and analyze the importance of this variability in fluvial incision. We use the Weather Research and Forecasting (WRF) atmospheric model coupled with the Landlab modeling framework to produce high resolution discharge distributions at 5 latitudes between 0º and 40ºS, both with and without a topographic barrier. We fit these distributions using a stretched exponential model and use this fit to extrapolate the discharge data to higher flood values, that are then used to drive incision in a 1D river model. Fluvial relief, channel concavity and bed shear stress are measured in this model and compared to discharge characteristics to determine relationships between mean discharge, discharge variability, and channel erosion in the context of an erosion threshold. We note a strong latitudinal dependence on the magnitude of change in mean discharge and variability when a topographic barrier is added. Surprisingly, we find that in most simulations, mean discharge dictates the erosional power of a channel, regardless of the inclusion of an erosion threshold. In cases where mean discharge is similar between two distributions, the more variable distribution will be more erosive. These findings provide a first order analysis of the relative importance of mean discharge and discharge variability in driving incision in bedrock rivers and shaping mountainous landscapes.