A network of shallow groundwater wells in a headwater catchment at the Hubbard Brook Experimental Forest in New Hampshire, U.S. was used to investigate the hydrologic behavior of five distinct soil morphological units. The soil morphological units were hypothesized to be indicative of distinct water table regimes. Water table fluctuations in the wells were characterized by their median and interquartile range of depth, proportion of time water table was present in the solum, and storage-discharge behavior of subsurface flow. Statistically significant differences in median, interquartile range, and presence of water table were detected among soil units. Threshold responses were identified in storage-discharge relationships of subsurface flow, with thresholds varying among soil units. These results suggest that soil horizonation is indicative of distinct groundwater flow regimes. The spatial distribution of water table across the catchment showed variably connected/disconnected active areas of runoff generation in the solum. The spatial distribution of water table and therefore areas contributing to stormflow is complex and changes depending on catchment storage.

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The critical zone features that control run-off generation, specifically at the regional watershed scale, are not well understood. Here, we addressed this knowledge gap by quantitatively and conceptually linking regional watershed-scale run-off regimes with critical zone structure and climate gradients across two physiographic provinces in the Southeastern United States. We characterized long-term (~20 years) discharge and precipitation regimes for 73 watersheds with United States Geological Survey instream gaging stations across the Appalachian Mountain and Piedmont physiographic provinces of North Carolina. Watersheds included in this analysis had <10% developed land and ranged in size from 14.1-4,390 km 2 . Thirty-four watersheds were located in the Piedmont physiographic province, which is typically classified as a low relief landscape with deep, highly weathered soils and regolith. Thirty-nine watersheds were located in the Appalachian Mountain physiographic province, which is typically classified as a steeper landscape with highly weathered, but shallower soils and regolith. From the United States Geological Survey daily mean run-off time series, we calculated annual and seasonal baseflow indices (BFI), minimum, mean, and maximum daily run-off, and Pearson's correlation coefficients between precipitation and baseflow. Our results showed that Appalachian Mountain watersheds systematically had higher minimum daily flows and BFI values. Piedmont watersheds displayed much larger deviations from mean annual BFI in response to year-to-year variability in precipitation. A series of linear regression models between 21 landscape metrics and annual BFIs showed non-linear and complex terrestrial-hydrological relationships across the two provinces. From these results, we discuss how distinct features of critical zone architecture, with specific focus on soil depth and stratigraphy, may be dominating the regulation of hydrological processes and run-off regimes across these provinces.

We investigated potential source areas of dissolved organic carbon (DOC) in headwater streams by examining DOC concentrations in lysimeter, shallow well, and stream water samples from a reference catchment at the Hubbard Brook Experimental Forest. These observations were then compared to high-frequency temporal variations in fluorescent dissolved organic matter (FDOM) at the catchment outlet and the predicted spatial extent of shallow groundwater in soils throughout the catchment. While near-stream soils are generally considered a DOC source in forested catchments, DOC concentrations in near-stream groundwater were low (mean 5 2.4 mg/L, standard error 5 0.6 mg/L), less than hillslope groundwater farther from the channel (mean 5 5.7 mg/L, standard error 5 0.4 mg/L). Furthermore, water tables in near-stream soils did not rise into the carbon-rich upper B or O horizons even during events. In contrast, soils below bedrock outcrops near channel heads where lateral soil formation processes dominate had much higher DOC concentrations. Soils immediately downslope of bedrock areas had thick eluvial horizons indicative of leaching of organic materials, Fe, and Al and had similarly high DOC concentrations in groundwater (mean 5 14.5 mg/L, standard error 5 0.8 mg/L). Flow from bedrock outcrops partially covered by organic soil horizons produced the highest groundwater DOC concentrations (mean 5 20.0 mg/L, standard error 5 4.6 mg/L) measured in the catchment. Correspondingly, stream water in channel heads sourced in part by shallow soils and bedrock outcrops had the highest stream DOC concentrations measured in the catchment. Variation in FDOM concentrations at the catchment outlet followed water table fluctuations in shallow to bedrock soils near channel heads. We show that shallow hillslope soils receiving runoff from organic matter-covered bedrock outcrops may be a major source of DOC in headwater catchments in forested mountainous regions where catchments have exposed or shallow bedrock near channel heads.