Abstract
Temperature has been used to characterize groundwater and stream water exchanges for years. One of the many methods used analyzes propagation of the atmosphere-influenced diurnal signal in sediment to infer vertical velocities. However, despite having good accuracy, the method is usually limited by its small spatial coverage. The appearance of fiber optic distributed temperature sensing (FO-DTS) provided new possibilities due to its high spatial and temporal resolution. Methods based on the heat-balance equation, however, cannot quantify diffuse groundwater inflows that do not modify stream temperature. Our research approach consists of coupling groundwater inflow mapping from a previous article (Part I) and deconvolution of thermal profiles in the sediment to obtain vertical velocities along the entire reach. Vertical flows were calculated along a 400 m long reach, and a period of 9 months (October 2016 to June 2017), by coupling a fiber optic cable buried in thalweg sediment and a few thermal lances at the water–sediment interface. When compared to predictions of hyporheic discharge by traditional methods (differential discharge between upstream and downstream of the monitored reach and the mass-balance method), those of our method agreed only for the low-flow period and the end of the high-flow period. Our method underestimated hyporheic discharge during high flow. We hypothesized that the differential discharge and mass-balance methods included lateral inflows that were not detected by the fiber optic cable buried in thalweg sediment. Increasing spatial coverage of the cable as well as automatic and continuous calculation over the reach may improve predictions during the high-flow period. Coupling groundwater inflow mapping and vertical hyporheic flow allows flow to be quantified continuously, which is of great interest for characterizing and modeling fine hyporheic processes over long periods.
Highlights
Groundwater has great impact on stream ecology since it supports baseflow throughout the year [1,2,3] and stabilizes stream water quality [4,5], providing habitat for many species [6,7,8]
The processes involved were groundwater inflows from the nearby hillslope or water drained from agricultural fields
Despite some suspected to be either groundwater inflows from the nearby hillslope or water drained from uncertainty about certain inflows, we assumed herecertain for convenience that all thermal anomalies
Summary
Groundwater has great impact on stream ecology since it supports baseflow throughout the year [1,2,3] and stabilizes stream water quality [4,5], providing habitat for many species [6,7,8]. Tools from hydrological and ecological communities need to be combined to understand groundwater effects on stream flow and predict its future change. Many techniques such as seepage meters, mini-piezometric analysis [10,11], differential gauging [12,13], and chemical tracing [14,15] have been used to quantify groundwater inflow into streams. Differential gauging allows for measurements at the reach level (a few m to km) but does not allow for detailed description along the longitudinal profile It requires additional measurements when considering the effects of tributaries. Variable t is the time and z is the distance along the direction of flow
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