The ultimate aim of many computational techniques in engineering hydrology is the derivation of river discharges, and it might appear that, once these are obtained, the hydrologist’s work is done. However, whether they are gained indirectly from considerations of other hydrological variables (to be described in following chapters) or directly from river discharge measurements, the discharge data are only samples in time of the behaviour of the river. The hydrologist then must assess the utility of the data and their representativeness over the period for which the information is required, usually the expected life of a water engineering project.