The dilution method is generally used for purposes of calibration or for spot gaugings mainly because of the costs of performing a gauging and a chemical analysis of the tracer samples. Nevertheless the method can often provide very accurate results given a suitable reach of river. The outstanding advantage of the dilution technique is that it is an absolute method, because discharge is computed from volume and time only. Tracer concentrations need be determined only in dimensionless relative readings. In rock-strewn shallow streams, the dilution method may provide the only effective means of estimating flow. The main disadvantages of the method are the difficulties in obtaining complete mixing of the tracer without loss of tracer and the problem of obtaining permission in some countries to inject tracers into rivers. There are two basic injection techniques, several sampling techniques and a large number of possible tracers of three main types – chemical, fluorescent and radioactive. The technique is normally carried out by specially trained personnel and although the method is mostly used for smaller rivers, discharges of up to 2000 m3 s−1

have been measured with confidence.