This study examined the characteristics of daily water level fluctuations produced by evapotranspiration measured in riparian areas and within a stream (Buck Hill Creek). The goal was to understand how diel signal characteristics Buck Hill Creek relate to watershed characteristics and predict how the diel signal characteristics might be affected by changing climate. Water levels were recorded at 5-minute intervals at eight riparian stations and five stream locations from spring to fall of 2016 and 2017 and a vegetation assessment was completed. A U.S. Geological Survey model was utilized to simulate the effects of future climate change on stream discharge. An analytical model was used to simulate modulation of dial signals in the stream.
Diel fluctuations in water levels in Buck Hill Creek are produced by the superposition of diel fluctuations in baseflow from adjacent riparian areas and hillslopes. Diel signals discharged from bedrock areas exhibited variable but predominantly long lag times (difference between daily temperature maximum and daily water level minimum). During high to moderate flow conditions, those areas that exhibited long lag times were out of phase with and destructively interfered with the diel fluctuation in the stream. Diel signals from till or alluvial deposits exhibited lag times that were shortest during wet conditions and longest during low-flow conditions. Diel signals in baseflow from all locations during low-flow conditions were approximately in phase and constructively interfered to produce high amplitude fluctuations in the stream. Simulated future climatic changes indicated decreased stream discharge from April through October. The predicted changes may be accompanied by diel fluctuations in the stream with increased amplitudes and long lag times. Concomitant shifts in vegetation are likely to occur and may also affect evapotranspiration-driven diel signals. Observations and modeling results indicated that variability in the timing and amplitude of diel signals from baseflow were more important than modulation of dial signals within the stream.
The results showed that monitoring diel signals is useful for assessing changes in watersheds and may be a valuable component for watershed monitoring programs.