Determination of discharge rates in turbulent streams by salt tracer dilution applying a microcomputer system. Comparison with current meter measurements

R. BENISCHKE & T. HARUM Institute for Geothermics & Hydrogeology, Joanneum Research, Elisabethstr. 16/2, 8010 Graz, Austria ABSTRACT The attempt to determine exactly discharge rates of turbulent streams in mountainous regions based on current meter measurements can lead to serious problems rising from a badly defined runoff channel geometry and irregular flow patterns. Therefore systematic errors influence the results and make it sometimes impossible to get reliable stage-discharge relations at gauging stations. The development of commercially available computerized systems made it possible to apply the salt tracer dilution method with instantaneous or continuous injection and to monitor the tracer passage in the time-concentration or time-conductivity mode directly at the field site. The result of the measurement is available immediately at the end of the measuring procedure. The experience showed that the salt tracer dilution method is the appropriate method in almost all cases of turbulent flow conditions and irregular channel geometry. The article describes the theoretical basis of the method, the used measuring system and compares the results with current meter data from streams, where the latter method can be applied with sufficient precision and accuracy. In many cases both methods can be applied with good results within a 2% average deviation, but both methods have limitations, which will be discussed too. Salt tracer dilution and current meter do not exclude each other, they are complementary methods. INTRODUCTION Hydrological investigations sometimes suffer the lack on quantitative data. Beside meteorological data also discharge rates of a catchment runoff are needed. An often applied method to determine this is the current meter method, but in runoff channels with turbulent flow or irregular flow pattern some errors will influence the reliability of the results. As the current meter method depends strongly on a defined channel geometry for the calculation of flow velocities this fact is frequently the source of erroneous data particularly when using such data to evaluate stage-discharge relations for gauging stations. A possible way to overcome these difficulties offers the long known tracer method (Rimmar, 1960, Andre, 1964, Behrens, 1971). This method can be applied in various forms, which will be described below. Up to now the method was hardly used comparing it with the well known current meter method. The reason was the necessary equipment to monitor the tracer passage or the uncertain results when collecting water samples for the difficult and time consuming subsequent analytical procedure. Now these problems could be reduced because of a great variety of computerized, handy, components with high precision and accuracy and the possibility to evaluate and calculate immediately the results after the measurement procedure (Benischke & Harum, 1984), This gives also the opportunity for the immediate decision in the field to repeat a doubtful measurement. In this article the focus is on the tracer method with instantaneous injection using salt tracer and conductivity measurement to monitor the tracer passage. 215