Determining Rainfall-Induced Runoff Threshold with Distributed Acoustic Sensing

In the southwestern United States, flash flooding frequently occurs when localized thunderstorms generate rainfall with intensities that exceed the infiltration capacity of dry soils. During these events, overland flow occurs, coalescing in rills and eventually larger channels where ephemeral flows can create downstream hazards and drive significant geomorphic change. Here, we explore the opportunistic use of distributed acoustic sensing (DAS) to monitor high-frequency strain oscillations along the length of a kilometer-long fiber-optic cable for the purpose of identifying flow in small rills with widths less than 1 m immediately following rainfall events. We demonstrate that for sufficiently high rainfall intensities, rill-flow in hectare-scale basins can be identified at multiple locations simultaneously with DAS. Our observations suggest that DAS can be used, alongside rainfall measurements, to determine the rainfall intensity threshold at which runoff and the resulting rill-flow begins for a particular site. By analyzing a season of DAS data alongside rainfall measurements, we determined a preliminary rainfall intensity threshold of 0.5 mm/min with 5-min duration to generate runoff at our study site. Although we recorded only four runoff events throughout the season, this method highlights the potential future value of DAS for determining hillslope hydrologic properties such as the threshold of runoff generation at relevant spatial scales for modeling and management. We also explore spectral properties of rill-generated DAS signals, finding that rill-flow produces primarily low frequencies (<50 Hz) with some signals observed as high as 200 Hz. This suggests that the rill-driven signals we observed are primarily a result of turbulence, with secondary influence from small (<2 mm) bedload grains impacting the fiber-optic cable and adjacent bed. These findings demonstrate that DAS can be a useful tool for determining high-resolution hydrologic and geomorphic properties across multiple hillslopes with a single instrument.