Analysis of subsurface stormflow from 147 storms at the 20 m long trench in the Panola Mountain Research Watershed by Tromp‐van Meerveld and McDonnell (2006a) showed that there was a distinct 55 mm precipitation threshold for significant subsurface stormflow production. This second paper in the series investigates the processes responsible for this threshold response. We installed a dense spatial array of maximum rise crest stage gauges and recording wells on the hillslope and studied the temporal and spatial patterns of transient saturation at the soil‐bedrock interface and its relation to subsurface stormflow measured at the trench face. Results show that while transient groundwater developed on parts of the hillslope during events smaller than 55 mm, it was not until more than 55 mm of rain fell before bedrock depressions on the hillslope were filled, water spilled over microtopographic relief in the bedrock surface, and the subsurface saturated areas became connected to the trench. When connectivity was achieved, the instantaneous subsurface stormflow rate increased more than fivefold compared to before the subsurface saturated areas were connected to the trench face. Total subsurface stormflow was more than 75 times larger when connectivity was achieved compared to when connectivity was not achieved. The fill and spill hypothesis presented in this paper is a process explanation for the observed threshold behavior of Tromp‐van Meerveld and McDonnell (2006a), thereby linking patterns and processes.

This study investigates the role of soil moisture on the threshold runoff response in a small headwater catchment in the Italian Alps that is characterised by steep hillslopes and a distinct riparian zone. This study focuses on: (i) the threshold soil moisture-runoff relationship and the influence of catchment topography on this relation; (ii) the temporal dynamics of soil moisture, streamflow and groundwater that characterize the catchment's response to rainfall during dry and wet periods; and (iii) the combined effect of antecedent wetness conditions and rainfall amount on hillslope and riparian runoff. Our results highlight the strong control exerted by soil moisture on runoff in this catchment: a sharp threshold exists in the relationship between soil water content and runoff coefficient, streamflow, and hillslope-averaged depth to water table. Low runoff ratios were likely related to the response of the riparian zone, which was almost always close to saturation. High runoff ratios occurred during wet antecedent conditions, when the soil moisture threshold was exceeded. In these cases, subsurface flow was activated on hillslopes, which became a major contributor to runoff. Antecedent wetness conditions also controlled the catchment's response time: during dry periods, streamflow reacted and peaked prior to hillslope soil moisture whereas during wet conditions the opposite occurred. This difference resulted in a hysteretic behaviour in the soil moisture-streamflow relationship. Finally, the influence of antecedent moisture conditions on runoff was also evident in the relation between cumulative rainfall and total stormflow. Small storms during dry conditions produced low stormflow amounts, likely mainly from overland flow from the near saturated riparian zone. Conversely, for rainfall events during wet conditions, higher stormflow values were observed and hillslopes must have contributed to streamflow.

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.