SUPERPOSITION OF DIEL SIGNALS IN STREAM DISCHARGE – INSIGHTS INTO WATERSHED BEHAVIOR

Evapotranspiration-driven daily fluctuation in groundwater levels represents a linkage between the biosphere and hydrosphere. Characteristics of these fluctuations when transmitted to springs and streams may be useful for assessing ecosystem vulnerability as well as the vulnerability of spring or stream waters to contamination. The spring and stream examined for this study are located in the Glaciated Pocono Plateau section of the Appalachian Plateau physiographic province in Pennsylvania, U.S.A. The stream is bounded by a bedrock valley wall, which hosts the spring, on one side and alluvium or till on the other. The amplitude of the spring diel signal varied with seasonal vegetation changes and discharge. Lag, the timing of minimum daily flow measured relative to daily maximum temperature, was generally zero. The spring diel signal shape (i.e. maximum to minimum to maximum daily discharge) was approximately symmetrical throughout the growing season (March to October). The amplitude of the stream diel signal varied with seasonal vegetation changes, discharge, and lag. The timing of daily lag in the stream signal varied from zero in March and April, increased to approximately +6 hours by July and then decreased to zero again by October. The shape of the stream signal resembles the spring signal in March and October, and in the intervening months shows the superposition of a second signal with changing lag as described above. Simulation of the transmission of a diel signal in a groundwater system using MODFLOW illustrated the sensitivity of amplitude, lag, and shape to aquifer characteristics, hydraulic gradient, and distance from the perturbation. I hypothesize that the relatively constant “spring” signal in the stream originates from the bedrock hillsides while the superposed variable signal originates from small seasonal changes in groundwater discharge from floodplain alluvium and till. Detection of diel signals and their characteristics may be useful when evaluating water sources for contamination vulnerabilities such as the potential for surface water influence of springs.