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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 inspace 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.

Plain Language Summary In many groundwater-fed streams, tree water uptake can create daily fluctuations in streamflow. The lowest value in these fluctuations, occurring during the afternoon or early evening, typically correspond to the maximum tree water uptake, while the peaks correspond to minimum tree water uptake during the night. The presence of these fluctuations in streamflow suggests that trees and streams are closely connected; however, because of limited access to the subsurface it is difficult to determine how these fluctuations propagate through the hillslope and into the stream. We found that trees in upslope areas rely on soil water that is hydraulically disconnected from groundwater, and thus fluctuations from transpiration are not transferred to groundwater and the stream from upslope. The creation of daily fluctuations in groundwater was limited to near-stream areas. By identifying the physical processes that control the expression of these transpiration signals, we can improve our ability to determine the water reservoirs that trees rely on.