Factors Affecting the Runoff Response of an Ephemeral Watershed to High-Intensity Rain: Arroyo De Los Pinos, NM

The relative influence of rainfall and watershed characteristics in controlling runoff in ephemeral channel systems is difficult to interrogate with current field datasets. First, runoff-producing rainstorms are rare in the environments that host ephemeral channels. Compounding this, there are at least three dimensions of variability in rainfall that affect runoff: rainfall intensity, total depth of high-intensity rainfall (or, equivalently, duration of high-intensity rainfall), and spatial extent of high-intensity rainfall. As a result, there is rarely enough field data to fully cover this variable space. Beyond this, influences such as lithology, vegetation, and soil and their effect on infiltration – both on hillslopes and in channels – are difficult to incorporate. We are developing a new runoff and rainfall monitoring dataset for the Arroyo de los Pinos watershed in central New Mexico to help bridge this gap. Our goal is to use the diverse geology of the basin to advance understanding of runoff generation and channel conveyance loss. The 32 km 2 watershed has three important lithologic classes: limestone bedrock, sandstone-shale bedrock, and weakly-lithified alluvial basin fill. Here, we present two years of monitoring data from this watershed. Runoff only occurs during the summer monsoon season, in instances when high-intensity thunderstorms linger long enough over the watershed. An approximate 15-minute intensity threshold for runoff production is 0.2 mm/min. Runoff is produced most readily in limestone sub-basins, followed by sandstone, and least readily in alluvial fill, a pattern that is consistent with the increasing hydraulic conductivity of the three lithologies. Rainfall intensity is a stronger predictor of the runoff ratio than rainfall depth, particularly in smaller subbasins and in limestone-dominated subbasins. This is consistent with observations of infiltration-excess overland flow throughout the watershed during high-intensity storms. In general, larger subbasins have lower runoff ratios, due to high transmission losses as bed infiltration in the ephemeral channel network. However, the mix of lithologies in the larger subbasins complicates the interpretation