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Time Domain Reflectometry (TDR) for Remote,
Real-time Water Quantity/Quality Monitoring of Ephemeral Streams
Initiated: October 2001
Team: Vince Tidwell, Jim Brainard, Jesse Roberts, Doug Ruby, Larry Desonier
Accurate and timely measurement of stream flows and water quality are critical to efficient management of water resources. Accurate measurements of discharge are integral to demonstrating deliveries to meet interstate compacts and international treaties, equitable allocations among holders of water rights, and monitoring of in-stream flows to protect endangered species habitat. Current methods for estimating discharge are based on measurement of stream stage (i.e., depth of water), the accuracy of which depends on knowledge of stream channel morphology. Although channel morphology changes with each storm event, its frequency of measurement is generally monthly at best, as it must be surveyed by hand. TDR system
Schematic of TDR System for monitoring the
quantity and quality of ephemeral stream flows.
Total Maximum Daily Loads (TMDLs), which define a watercourse's water quality standards, are being more stringently enforced. Two of the leading causes of TMDL violation are sediment load and salinity; however, in-stream real-time methods for monitoring these contaminants are almost non-existent. As an alternative to these current approaches, Time Domain Reflectometry (TDR) is proposed for remote, real-time monitoring of the quantity and quality of ephemeral stream flows. In Time Domain Reflectometry, a fast rise step voltage pulse in the form of an electromagnetic wave is propagated along a transmission line. The transmission line is terminated with a waveguide embedded in the medium of interest, in our case the stream channel. The voltage pulse travels to the end of the waveguide where it is reflected back to TDR cable tester for analysis. Propagation of the voltage pulse is dependent on the dielectric properties of the medium along the waveguide. Specifically, the pulse is susceptible to reflection by dielectric discontinuities (air-water and water-sediment interfaces) and attenuation by clays and salts. Measured TDR traces are interpreted using a multisection model based on the S11 scatter function and Cole-Cole parameters for dielectric dispersion and loss. In this way, the number of media segments along the waveguide (sediment, water, air), their associated dielectric properties, from which the sediment concentration can be inferred, and their electrical conductivities (salinity and clay concentration) are determined. By partially burying a waveguide in a stream channel, the depth of water, channel depth, sediment concentration, and salinity can be measured simultaneously with the TDR. By placing several waveguides along a cross section of the stream, the evolving morphology of the stream channel can be monitored in time.

The goal of this project is to develop an improved means of measuring stream stage, channel morphology, sediment concentration and salinity of ephemeral streams using time domain reflectometry.

TDR graph
Graph of five TDR traces.

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