Orographic Controls on Subdaily Rainfall Statistics and Flood Frequency in the Colorado Front Range, USA

Generalizable relationships for how subdaily rainfall statistics imprint into runoff statistics are lacking. We use the Colorado Front Range, known for destructive rainfall-triggered floods and landslides, to assess whether orographic patterns in runoff generation are a direct consequence of rainstorm climatology. Climatological analysis relies on a dense network of tipping-bucket rain gauges and gridded precipitation frequency estimates from the National Oceanic and Atmospheric Administration to evaluate relationships among subdaily rainfall statistics, topography, and flood frequency throughout the South Platte River basin. We find that event-scale rainfall statistics only weakly depend on elevation, suggesting that orographic gradients in runoff extremes are not simply a consequence of rainfall patterns. In contrast, bedrock exposure strongly varies with elevation in a way that plausibly explains enhanced runoff generation at lower elevations via reduced water storage capacity. These findings are suggestive of feedbacks between bedrock river evolution and hillslope hydrology not typically included in models of landscape evolution. Plain Language Summary Large floods generated along the eastern slopes of the Colorado Front Range, USA, threaten communities near the transition between the Great Plains and Colorado Rockies. While seasonal snowpack and snowmelt control mean water supply and streamflow, runoff generated during rare, intense rainstorms at lower mountain elevations generates the largest floods in this region. In this work, we assess how well spatial patterns in flood frequency mirror rainstorm properties. We find that patterns in subdaily properties of rainstorms (i.e., intensities, durations) fail to explain the differences in the sizes of the largest floods. Instead, we show how rockiness strongly decreases with increasing elevation in the unglaciated part of the landscape and hypothesize that systematic differences in land surface properties (such as infiltration capacity and soil depth) may amplify the runoff response to rare rainfall events at low to intermediate mountain elevations. These results highlight the need for better constraints on elevation-dependent properties of the land surface. Such properties may be as important to explaining spatial patterns in the frequency of floods as is understanding the climatology of extreme rainfall itself.