Use of fluorometric techniques for streamflow measurements in turbulent mountain stteams
Though the use of tracers for water discharge measurements is not a new concept. Measurements with current-meters are easier and commonly used. Current-meters are however difficult to apply, especially in torrential regimes. To measure water discharge in mountain watersheds, we have developped an integrated device using rhodamine-wt as a tracer. The rhodamine-wt is nontoxic at the levels employed, has low sorption tendencies, and ifs determination of low concentrations (0.01 ppb) can be clone with fluoroemtric techniques. Fluorescence intensity decreases with increasing temperature and can be formulated, according to Feuerstein and Selleck (1963) as : FS = Ft. en(ts-t), where n = 10((logFs-log Ft)/(ts-t) In the laboratory, in order to determine the constant n for rhodamine-wt we worked with three solutions of rhodamine-wt (0.115, 0.23 and 0.46 μg · l-1) with increasing temperature from 5 to 17.5 °C. Thus we found : n = -0.024. °C-1. In the same way, we established the relationship between concentration and fluorescence. For rhodamine concentrations between 0 and 1 μg · l-1 and concentration of suspended matter between 0 and 2 μg · l-1 this relation is : C μg · l-1 = 0.0047 x F1.0049 Rhodamine-wt has low sorption tendencies, nevertheless we measured the attenuation effect due to the presence of suspended matter in the pumped water. In the range of 0.2 to 2 g · l-1 of suspended matter (diameter < 200 μm), with rhodamine-wt concentrations of less than 0.3 μg · l-1, we found the water discharge was overestimated of 6.4 % at most. In the field, we have organized a continuous survey of tracer concentrations in the river using the fluorometer with a continuous-flow attachment. The fluorescence signal was recorded using a plotter and a computer. When all the dye has passed through the sampling site, the operator starts the automatic calculation process of the mean concentration and the transit time of the dye to obtain finally the water discharge. In this way, the total recovery method enables us to measure water discharge several times during a flood and to obtain easily and quickly the stage-discharge relation even for flood crest. This device has been applied on various rivera in the French part of the lake Geneva watershed, for a large range of waster discharges from 0.1 m3 · s-1 to 300 m3 · s-1.