Runoff and erosion thresholds dictated by the balance between stochastic rainfall statistics and Critical Zone architecture

Geomorphic processes like landslides, rill erosion, and fluvial incision each rely on erosion thresholds (e.g., in pore water pressure, shear stress) that are intimately related to hydrologic thresholds (e.g., soil saturation, infiltration capacity). The frequency of exceeding these thresholds depends both on the stochastic climate forcing (e.g., rainfall intensity, duration, inter-storm period) and Critical Zone architecture of a site (e.g., land-cover, soil texture, soil depth). Because we often only observe a forcing variable like total rainfall depth and a response variable like flood magnitude, and not the complex suite of intervening variables impacted by spatially heterogeneous rainfall, soil moisture state, soil thickness, direct attribution of flood climatology to rainfall climatology is challenging and limited to the few settings where detailed hydrologic observations enable flood forecasting and attribution. In this work, we attempt to identify the key hydro-climatological parameters controlling flood frequency statistics in the Colorado Front Range, USA. This site is well-suited to analysis for three reasons: (1) the largest rainfall-runoff magnitudes occur at low to intermediate elevations and generate the largest river floods; (2) a dense network of sub-daily rain gauges exists at low to intermediate elevations; and (3) LiDAR coverage along Boulder Creek facilitates the development of topographic proxies for exposed rock (i.e., high runoff generation efficiency). We find that while an orographic gradient in rainfall extremes is concordant with spatial patterns in runoff generation, it is likely not sufficient to explain flood climatology by itself. We hypothesize that an orographic gradient in land-cover and soil properties acts to amplify the efficiency of runoff generation at low to intermediate elevations during extreme rainfall events. This gradient in Critical Zone properties is attributable to a transient wave of incision migrating up river systems in the Colorado Front Range. Such feedbacks have important implications for the response timescales of steep landscapes to changes in base level lowering rates.