Hydrology and Channel Head Erosion in a Semiarid Discontinuous Ephemeral Stream Network near Oracle, Arizona

We present results from monitoring of hydrology and erosion at channel heads in a discontinuous ephemeral stream system in southeastern Arizona rangelands. At this field site, alluvial headwalls ~1 m high mark the transition from unchannelized valleys to actively incising arroyos. The headwalls are vertical to overhanging in cross-section and amphitheater-shaped in planform. The local landscape is made up of well- consolidated clay-rich soil and sediment. Possible processes involved in headwall retreat include groundwater sapping, plunge-pool erosion, and soil tension-fracturing. To evaluate the relative efficacy of such processes and to test and refine models of incision and headcut migration rate, we deployed 40+ sensors at and immediately upstream and downstream of channel headwalls. Wired sensors and a datalogger recorded precipitation, overland flow, channel discharge, and soil moisture at a one minute interval. Additionally, a custom wireless sensor network with cellular telemetry was developed and deployed to measure shallow overland flow. At our field site, overland flow can occur from less than one centimeter of precipitation. We tracked landscape erosion through a combination of time-lapse photography, repeat RTK GPS and terrestrial LiDAR surveys. Over the monitoring period, headwalls retreated 10s of cm during several flow events. When the sediment becomes saturated (e.g., from flow down headwall faces and standing water in plunge pools), it has an order-of-magnitude lower shear strength than when dry. Erosion and retreat of channel heads appear to occur by a combination of erosive flow down saturated channel headwalls, plunge-pool erosion at high flows, and mass wasting along vertical tension cracks between runoff events. The amphitheater form is largely due to plunge-pool undercutting and vertical differences in soil material properties, particularly clay content. Though shallow groundwater sapping is widely thought to lead to amphitheater-shaped channel heads, we found no evidence of moisture flux below 40 cm depth directly upstream of the ~1 m channel headwalls. Our results have implications for understanding runoff- erosion dynamics, arroyo cutting, rangeland soil erosion, and may also provide insight into the formation of larger amphitheater-headed channels and channel initiation processes in general.