Canal Discharge Measurements with Radioisotopes

The Bureau of Reclamation program to improve water-measurement practices includes the study of the use of radioisotopes to measure discharges in irrigation canals. Measurements made in a concrete-lined canal, designed for a discharge of 620 cfs, are described. One-half-capacity tests, made in February, 1962, were followed by tests at full capacity in June, 1963. The pulse or total count radioisotope method was used to obtain a total of 65 discharge measurements. The consistency of measuring discharge by the radioisotope method was evaluated by using up to four portable Geiger counting systems. The tests indicated that with adequate mixing of the radioisotope and canal water, discharge measurements may be made with approximately 97% accuracy or greater when compared to a limited number of current meter discharge measurements. A sufficient number of measurements were made to permit the computation of diffusion coefficients that can be used to help determine the minimum required mixing length for straight canals of similar configuration. Further investigation is necessary, however, to satisfactorily refine the limits of the variables that control mixing. Mixing is the most important factor in obtaining accurate discharge measurements in low-velocity open channels.