Using Measurements of Heat and Pressure to Characterize Hyporheic Exchange through a Riffle-Pool Sequence in the Truckee River, NV
Abstract
Flow in and out of the hyporheic zone is largely influenced by streambed topography, hydraulic conductivity, and stream discharge. The timing and magnitude of stream discharge, and the character and distribution of streambed material can significantly affect the chemical, physical, and biological gradients within the hyporheic zone, and ultimately, the structure and function of aquatic ecosystems. In this study, the spatial and temporal distribution of nutrients was observed along a riffle-pool sequence on the Truckee River, NV. The study area was selected due to the presence of significant algal blooms, which are attributed to increased nutrient loading to the stream in this area. Piezometers were installed into the streambed and the surrounding riparian floodplain to monitor nitrate and dissolved oxygen concentrations, temperature, and pressure at multiple depths beneath the streambed. Measurements of temperature and pressure were used to calibrate a 2-dimensional water- and heat-flow model. The model domain consists of a longitudinal profile that traverses a riffle-pool sequence. The purpose of the model was to determine the relationship between vertical and horizontal seepage velocity, and the nitrate and dissolved oxygen concentrations in the stream sediments. The flow model was calibrated using a uniform random sampling approach to explore the feasible parameter space and to estimate the uncertainty in the velocity estimates. A uniform distribution was selected from ranges in hydraulic and thermal parameters reported in the literature based on sediment texture. Using this approach, streambed heterogeneity, parameter identifiability and parameter sensitivity were determined. Results indicate a trade off exists between matching observed temperatures and observed pressures beneath the streambed. However, the estimated parameters were more unique when both temperature and pressure were used as observations, relative to using solely temperature or pressure observations. Episodic changes in streamflow changed the direction and magnitude of streambed seepage in the upwelling zone. Additionally, increases in stream stage associated with flow events suppressed seepage in the upwelling zones and affectively increased the length of the hyporheic flow paths in this ripple-pool sequence. These changes suggest upwelling zones are not static and vary spatially and temporally. At higher discharge, the upwelling zones transition towards neutral or down-welling conditions. Nitrate concentrations were greater in the riffle section of the streambed relative to surface water and appear to be correlated to larger vertical seepage velocities, which suggest nitrate production. The nitrate and dissolved-oxygen concentrations dramatically decrease as a function of depth along the riffle-pool transition, suggesting denitrification.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.H32C..07N
- Keywords:
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- 1830 HYDROLOGY / Groundwater/surface water interaction;
- 1831 HYDROLOGY / Groundwater quality;
- 1847 HYDROLOGY / Modeling;
- 1873 HYDROLOGY / Uncertainty assessment