GLOBAL CONTROLS ON DAILY RUNOFF VARIABILITY AND ITS IMPRINT IN TOPOGRAPHY

The possibility for bidirectional coupling between climatically-mediated surface processes and tectonically driven rock uplift has driven decades of geoscience research. The degree to which tectonics can be coupled to climate depends on the sensitivity of topography to changes in rock uplift. In un-glaciated landscapes, this sensitivity will largely reflect how river channels steepen in response to increasing rock uplift rates. However, there are an increasing number of studies showing sublinear relationships between basin averaged millennial scale erosion rates (E) and normalized channel steepness (k_sn), leading some authors to argue for a global threshold in channel steepness. We recently argued using data from the Greater Caucasus that pseudo-threshold behavior may arise due to feedbacks among increasing topographic relief, enhanced snowmelt runoff lowering daily runoff variability, and subsequent decreased sensitivity to climate at higher rock uplift rates. To examine whether our results from the Greater Caucasus are indicative of a more general phenomenon, we present new results from an analysis of mean runoff and daily runoff variability using the WaterGAP3 reanalysis dataset that estimates daily runoff globally. We first validate model data against empirical records of precipitation and runoff. Using multidimensional analysis, we are then able to disentangle the complex hydrological runoff response to precipitation inputs. We find that isolating simple and general relationships between mean precipitation, daily precipitation variability, and precipitation phase is challenging due to the importance of other climatic variables (e.g., temperature, vapor pressure deficit), topography (e.g., elevation, relief, aspect), ecology (e.g., plant community structure, canopy cover), and scaling properties (e.g., drainage density, drainage area) in setting the hydrological response. Nevertheless, our results highlight that some of the most robust predictors for daily runoff variability under modern climate are climatic and topographic proxies for snowmelt. Incorporating such hydro-climatic relationships into landscape evolution models promises to refine expectations for where and when we should expect low potential for climate-tectonic coupling.