High-Intensity Rain Storm Connects Hillslopes to Channels in a Steep Semi-Arid Catchment

Abstract:

Hydrologic transport in ephemeral streams is inherently limited to times of stream flow. Hence, the conditions that shape their channels and the effects of overland flow on water, sediment, and organic matter delivery to the channel are rarely observed. An intense storm centered on the 0.45-km² Betasso catchment in the Boulder Creek Critical Zone Observatory provided an opportunity to study a 200-year precipitation event during which hillslope-channel connectivity was high. A convective storm on August 30, 2016 delivered about 95 mm of rain, nearly 17% of the catchment’s mean annual precipitation, in just over two hours. The Betasso catchment drains to Boulder Creek, where a stream gage, located 0.2 km downstream of the confluence, recorded a 7-fold increase in discharge from 0.8 to 5.8 m³s^-1, with most of the increase occurring during the 15-minute interval of highest rain intensity. Radar maps and analysis of nearby rain gages suggest that the Betasso catchment was the primary source of the rise in discharge, despite representing only 0.2% of the drainage area to the Boulder Creek gage.

The Betasso catchment, at ~1930 m in the Colorado Front Range, has an open Ponderosa pine forest, with grass growing in canopy gaps, and small meadows. Evidence of overland flow during the storm is widespread, particularly in areas with canopy gaps. Some rills, delineated by bent grasses, scour of mineral soil, or levees of pine needles and small woody debris, connected to the main gully system. While some rills were deep enough to expose mineral soil, others removed only the top layer of pine needles, revealing underlying matted, partially degraded organic matter. In other areas, discontinuous rills redistributed organic matter but terminated above the channel. Debris dams of pine needles and small woody debris were interspersed along these rills. Few rills were found in grassy meadows or dense stands of trees. This preliminary study aims to 1) reconstruct peak discharge in the Betasso gully system from mapped flow markers, 2) map rills on hillslopes, and 3) characterize sediment and organic matter on hillslopes, rills, and in gullies. These analyses will offer insight into the hydrology of extreme climatic events and their role in the redistribution and export of sediment and organic matter in small, mountain catchments.