Surface Runoff Threshold Responses to Rainfall Intensity, Scale, and Land Use Type, Change and Disturbance

The dominance of infiltration-excess (Hortonian) overland flow can be determined by how well a rainfall intensity threshold predicts streamflow response. Areas in which we would expect infiltration-excess overland flow to dominate include urban, bedrock, desert pavement, and lands disturbed by vegetation removal (e.g., after a fire burn or fallow agricultural lands). Using a transferable method of identifying the existence of thresholds, we compare the following sites to investigate their hydrologic responses to 60-minute rainfall intensities: desert pavement sites in Arizona (Walnut Gulch and Yuma Proving Ground), post-fire sites in a forested, mountainous burn area in north-central Colorado (High Park Fire), an area of northeastern Colorado Plains that has transitioned from dryland agriculture to conservation reserve (Drake Farm), and watersheds in suburban Baltimore, Maryland which range from less than 5% to over 50% impervious surface cover. We observed that at desert sites, the necessary threshold of rainfall intensity to produce flow increased with watershed size. In burned watersheds, watershed size did not have a clear effect on rainfall thresholds, but thresholds increased with time after burning, with streamflow no longer exhibiting clear threshold responses after the third year post-fire. At the agricultural site, the frequency of runoff events decreased during the transition from cultivated crops to mixed perennial native grasses. In an area where the natural land cover (forested) would be not dominated by infiltration-excess overland flow, urbanization greatly lowered the rainfall thresholds needed for hydrologic response. This work contributes to building a predictive framework for identifying what naturally-occurring landscapes are dominated by infiltration-excess overland flow, and how land use change could shift the dominance of infiltration-excess overland flow. Characterizing the driving mechanism for streamflow generation will allow better prediction of hydrologic response to rainfall events.