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Abstract Here we use Richards Equation models of variably saturated soil and bedrock groundwater flow to investigate first‐order patterns of the coupling between soil and bedrock flow systems. We utilize a Monte Carlo sensitivity analysis to identify important hillslope parameters controlling bedrock recharge and then model the transient response of bedrock and soil flow to seasonal precipitation. Our results suggest that hillslopes can be divided into three conceptual zones of groundwater interaction, (a) the zone of lateral unsaturated soil moisture accumulation (upper portion of hillslope), (b) the zone of soil saturation and bedrock recharge (middle of hillslope) and (c) the zone of saturated‐soil lateral flow and bedrock groundwater exfiltration (bottom of hillslope). Zones of groundwater interaction expand upslope during periods of precipitation and drain downslope during dry periods. The amount of water partitioned to the bedrock groundwater system a can be predicted by the ratio of bedrock to soil saturated hydraulic conductivity across a variety of hillslope configurations. Our modelled processes are qualitatively consistent with observations of shallow subsurface saturation and groundwater fluctuation on hillslopes studied in our two experimental watersheds and support a conceptual model of tightly coupled shallow and deep subsurface circulation where groundwater recharge and discharge continuously stores and releases water from longer residence time storage.
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Water Table Depth and Bedrock Permeability Control Magnitude and Timing of Transpiration‐Induced Diel Fluctuations in Groundwater
Abstract The subsurface processes that mediate the connection between evapotranspiration and groundwater within forested hillslopes are poorly defined. Here, we investigate the origin of diel signals in unsaturated soil water, groundwater, and stream stage on three forested hillslopes in the H.J. Andrews Experimental Forest in western Oregon, USA, during the summer of 2017, and assess how the diurnal signal in evapotranspiration (ET) is transferred through the hillslope and into these stores. There was no evidence of diel fluctuations in upslope groundwater wells, suggesting that tree water uptake in upslope areas does not directly contribute to the diel signal observed in near‐stream groundwater and streamflow. The water table in upslope areas resided within largely consolidated bedrock, which was overlain by highly fractured unsaturated bedrock. These subsurface characteristics inhibited formation of diel signals in groundwater and impeded the transfer of diel signals in soil moisture to groundwater because (1) the bedrock where the water table resides limited root penetration and (2) the low unsaturated hydraulic conductivity of the highly fractured rock weakened the hydraulic connection between groundwater and soil/rock moisture. Transpiration‐driven diel fluctuations in groundwater were limited to near‐stream areas but were not ubiquitous in space and time. The depth to the groundwater table and the geologic structure at that depth likely dictated rooting depth and thus controlled where and when the transpiration‐driven diel fluctuations were apparent in riparian groundwater. This study outlines the role of hillslope hydrogeology and its influence on the translation of evapotranspiration and soil moisture fluctuations to groundwater and stream fluctuations.
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- Award ID(s):
- 2025755
- PAR ID:
- 10452702
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Water Resources Research
- Volume:
- 56
- Issue:
- 5
- ISSN:
- 0043-1397
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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