Diel discharge variations in dormant and growing seasons in a headwater catchment suggest potential sources of an evapotranspiration signal

Abstract

AbstractDiel fluctuations in discharge not associated with precipitation have been identified in streams around the world and attributed to evapotranspiration of both hillslope and near‐stream vegetation. Several mechanisms have been hypothesized to explain the generation of these signals, but a consensus has not been reached. In order to investigate the origin of these daily discharge fluctuations, we measured discharge and groundwater levels in a 43‐ha headwater catchment near Cullowhee, NC, USA. The catchment was logged at least twice in the past 120 years, allowing the evergreen invasive Chinese privet (Ligustrum sinense) to become established in near‐stream areas. Stream discharge was measured at the outlet, and stream stage and near‐stream groundwater level were measured at a second location higher in the catchment. Daily fluctuations in discharge were quantified and found to increase in magnitude with daily mean discharge. Interestingly, daily discharge fluctuations were observed in the dormant season, and our analysis suggests that their magnitude may increase more with discharge than during the growing season. Two previously hypothesized processes may explain our observations. A decrease in signal dampening due to faster transmission of an evapotranspiration signal at elevated flows may explain the increased diel fluctuation magnitude at higher flows. Additionally, because the primary active vegetation in our catchment in the dormant season is in near‐stream areas, we suggest that this indicates most of the evapotranspiration signal in stream discharge is derived from near‐stream vegetation, not hillslopes.