Threshold relations in subsurface stormflow: 1. A 147‐storm analysis of the Panola hillslope

Subsurface stormflow is a dominant runoff‐producing mechanism in many upland environments. While there have been many trench‐based experimental studies, most of these investigations have examined only a handful of storms. We analyzed subsurface stormflow in response to 147 rainstorms at a trenched hillslope in the Panola Mountain Research Watershed between February 1996 and May 1998. We used this unique long‐term data set to examine how often the hillslope delivers water, the contribution of pipe flow to total flow, and the persistence of spatial patterns of flow at the trench face. The long‐term data set showed a clear threshold response of subsurface stormflow to storm total precipitation. For storms smaller than the precipitation threshold of 55 mm, little subsurface stormflow was observed. For events exceeding the threshold, there was an almost 2 orders of magnitude increase in subsurface flow compared to subsurface flow from storms smaller than the threshold. Pipe flow was an important component of total subsurface flow and showed a similar threshold behavior. We observed a linear relation between total pipe flow and total subsurface stormflow. Contributions of different trench segments to total trench flow changed seasonally and with changes in precipitation and antecedent conditions. Our results suggest that the threshold relation at the hillslope scale may be an emergent behavior of combined processes internal to the hillslope and perhaps point the way toward how to characterize hillslope processes. A companion paper (Tromp‐van Meerveld and McDonnell, 2006) explores the physical mechanisms responsible for the threshold behavior.