Threshold changes in storm runoff generation at a till-mantled headwater catchment

A small research watershed in the Hubbard Brook Experimental Forest in New Hampshire was equipped with a spatially distributed instrument network designed to continuously monitor hydrometric responses in the shallow subsurface. We analyzed rainfall events during seasonal wet up from late summer through autumn to investigate the mechanisms of runoff generation and the patterns of rainfall-runoff response at the catchment outlet. Our results show that storm quick flow depths displayed a threshold relationship with two independently measured soil moisture indices: a maximum water table height index and the sum of gross precipitation and antecedent soil moisture. Quick flow depths during events with below-threshold criteria were not significantly correlated with either index, while quick flow depths during events with above-threshold criteria were strongly correlated with both indices (r >= 0.98). The effective runoff contributing area (estimated by event runoff ratios) also changed significantly between above-and below-threshold conditions, as did the synchronicity between groundwater fluctuations and streamflow. Below the threshold, we inferred that catchment runoff was generated primarily in the near-stream zones, while above the threshold the contributing area likely expanded laterally onto neighboring hillslopes. Our results show that the effective saturated hydraulic conductivity appeared to increase significantly during runoff events with above-threshold conditions, possibly owing to water tables rising into highly transmissive near-surface soils. We believe the observed threshold pattern may partially be explained as a transmissivity feedback mechanism and/or preferential flows through macropore networks which allowed for a rapid expansion of the runoff contributing area onto hillslopes, resulting in increased runoff yields.