Mechanisms of flash-flood generation in a gullied high-plains grassland: evidence for partial contributing area runoff

Abstract:

Flash floods commonly cause rapid gully erosion, creating headwalls that erode into previously stable surfaces, thereby reducing arable land and releasing sediment that can contaminate water supplies. In semi-arid landscapes, gully erosion tends to be driven by flash floods. Here, we study the mechanisms for flash-flood generation, seeking to answer two questions: (1) how spatially variable is runoff production, and (2) what combination of rainfall intensity and duration is required to produce runoff? To answer these questions, we combine field data from a study site on the Colorado High Plains, USA, with numerical modeling. The site is characterized by patchy, dryland shrub vegetation dispersed throughout the otherwise bare slopes and gullies. Analysis of six years of rainfall and runoff data indicate that flash flood generation requires a 15-minute intensity of approximately 38 mm/hr. Sprinkler experiments on isolated bare and vegetated plots revealed a large contrast between infiltration capacities: bare areas can produce runoff when the rainfall exceeds 10-15 mm/hr, whereas vegetated areas permit infiltration of at least 45 mm/hr during relatively brief, intense events. These findings imply that high-intensity rainstorms associated with summertime moist convective systems drive gully incision. They also suggest that a self-enhancing feedback may exist in which initial incision creates steep and relatively bare slopes that tend to generate more runoff, leading to more aggressive gully incision.