CONTROLS ON FLOW GENERATION IN EPHEMERAL WATERSHEDS

Ephemeral channels produce runoff 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 in many cases.

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 axes of variability in rainfall character that affect runoff: rainfall intensity, duration 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 therefore 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 vegetation or lithology and their effect on infiltration on hillslopes and in channels are difficult to include. A growing list of dryland research watersheds is helping to 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 incorporating additional dimensions of variability into rainfall-runoff investigations. The 32 km² watershed has three important lithologic sub-units: limestone bedrock, sandstone-shale bedrock, and semi-lithified rift fill. Here, we present five years of monitoring data from this watershed. Runoff only occurs during the monsoon season, when high-intensity thunderstorms move slowly over the watershed. Runoff is produced most readily in limestone sub-basins, followed by sandstone, and finally alluvial fill. Within sub-basins, rainfall intensity is the primary control on the runoff ratio. But the strength of this influence changes with watershed size and lithology. Channel bed infiltration rates appear less important, but are complicated by questions that remain about subsurface storage capacity.