Controls On The Runoff Response of The Ephemeral Arroyo de Los Pinos Watershed To High-Intensity Rain

Runoff from basins drained by ephemeral channels is generated in a particularly non-linear manner. Typically, rainfall intensity in excess of infiltration capacity is the dominant control, as Hortonian overland flow contributes rapid hillslope runoff that can trigger damaging flash floods. Secondary controls, such as watershed area and channel conveyance losses are also significant.

The relative influence of rainfall characteristics vs. watershed characteristics in controlling runoff production in ephemeral channel systems is difficult to interrogate with field data. 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, duration of high-intensity rainfall (or, equivalently, total depth of high-intensity rainfall), and spatial extent of high-intensity rainfall. As a result, there is rarely enough data to fully cover this variable space. Long-term monitoring is required, but long-term research stations are by their nature site-specific. The influence of watershed size can be investigated in most long-term stations by monitoring nested sub-basins. But influences such as lithology, vegetation, and soil and their effect on infiltration – both on hillslopes and in channels – are difficult to include. The establishment of additional dryland research watersheds in unstudied lithologies can help bridge this gap.

We are developing a new runoff and rainfall monitoring dataset for the Arroyo de los Pinos watershed in central New Mexico with the goal of using the diverse bedrock lithology of the basin to advance understanding of runoff generation and channel conveyance loss. The 32 km2 watershed has three important lithologic sub-units: limestone bedrock, sandstone-shale bedrock, and alluvial basin fill. Here, we present five 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. Runoff is produced most readily in limestone sub-basins, followed by sandstone, and finally alluvial fill. Within sub-basins, rainfall intensity and rainfall volume are co-dominant controls on the runoff ratio. The relative strength of these two influences changes with watershed size and lithology. Channel bed infiltration rates appear less important, particularly in small sub-basins, but this conclusion is complicated by questions that remain about subsurface storage capacity. In braided alluvial reaches, infiltration affects flood hydrograph shape by slowing the rate of wetting front advance under initially-dry bed conditions.