The analysis of streamflow records is extremely important in order to characterise the flow regime for a particular river. Hydrograph analysis involves dissecting a hydrograph to distinguish between stormflow and baseflow. This is often a precursor to using the unit hydrograph, a technique using past stormflow records to make predictions on the likely form of future storm events. Flow duration curves are used to look at the overall hydrology of a river - the percentage of time a river has an average flow above or below a certain threshold. Frequency analysis is used to look at the average return period of a rare event (or the probability of a certain rare event occurring), whether that be extremes of flooding or low flow. Each of the methods described in this chapter has a distinct use in hydrology and it is important that practising hydrologists are aware of their role.
Computer modelling offers a methodology to investigate hydrological processes and make predictions on what the flow might be in a river given a certain amount of rainfall. There are different types of models, with differing amounts of complexity, but all are a simplification of reality and aim to either make a prediction or improve our understanding of biophysical processes.
A key use of flow and hydraulic data is to assess the needs of aquatic fauna within a river system. This information is used by managers of regulated rivers to set flow regimes that are beneficial, or at least non-detrimental, to particular flow species. Models such as PHABSIM have been used successfully in many places around the world to achieve this. They combine hydrological and ecological knowledge to provide vital information to resource managers.