| Abstract/Description or Keywords |
Tracer dilution methods using salt and Rhodamine WT (RWT) are commonly used to measure discharge in steep mountain streams. This research addressed knowledge gaps associated with dilution methods using original field data collected on nine streams in southwest British Columbia and discharge measurements conducted by Northwest Hydraulic Consultants. Laboratory experiments were conducted to evaluate the uncertainties associated with different procedures for calibrating the relation between salt concentration and electrical conductivity (EC) for dry salt injection, and to evaluate the effects of RWT decay due to sorption and photolysis. For salt dilution, calibration should be conducted at the in-situ stream temperature for greatest accuracy. The calibration factor varied linearly with background EC for water samples with EC less than 1000 µS/cm. For higher background EC, factors plotted below the fitted relation, likely due to differences in the relative ionic abundances. Minimum mixing lengths ranged between 6.5 and 24.5 stream wetted widths, but determining the mixing length can be confounded by surface-subsurface water fluxes. Probes need to be placed on opposite sides of the stream to verify adequate mixing, because probes located at different locations on the same of the stream sometimes suggested complete mixing had occurred when it in fact had not. For probes located downstream of complete mixing, breakthrough curves (BTCs) for probes located in the main current differed significantly from probes in zones with recirculating flow, even though they yielded discharge values within ± 10%. The peak of the BTC is a function of the mass of tracer injected, reach length, channel cross-sectional area, and the integral of a non-dimensional BTC, A*. The distribution of A* derived from analysis of 175 BTCs can be used, in conjunction with estimates of channel geometry and desired increases in EC, to estimate dosing requirements to avoid under- or over-dosing a stream reach. The calibration factor for RWT varied with turbidity, indicating that calibration is essential for each discharge measurement. Laboratory and field experiments focused on RWT decay were confounded by other factors, so no firm conclusions could be drawn. |
