Over the past 30 years, ecologists have demonstrated the importance of flow and temperature as primary variables in driving running water, riparian and floodplain ecosystems. As it is important to assess the size and timing of discharge variations in relation to those in temperature, a method is proposed that uses multivariate techniques to separately classify annual discharge and temperature regimes according to their shape and magnitude, and which then combines the classifications. This paper: (i) describes a generally applicable method; (ii) tests the method by applying it to riparian systems on four British rivers using a 20-year record (1977-97) of flow and air temperature; (iii) proposes a hydroecological interpretation of the classification; (iv) considers the degree to which the methodology might provide information to support the design of ecologically acceptable flow regimes.
Regimes are defined for discharge and air temperature using monthly mean data. The results of applying the classification procedure to four British rivers indicates that the typical regimes for each of the four catchments are composite features produced by a small number of clearly defined annual types that reflect interannual variability in hydroclimatological conditions. Annual discharge patterns are dominated by three shape classes (accounting for 94% of the station years: class A, early (November) peak; class B, intermediate (December-January) peak; and class C, late (March) peak) and one magnitude class (70% of the station years fall into class 3, intermediate), with two subordinate magnitude classes: low-flow years (18%) and high flow years (12%). For air temperature, annual patterns are classified evenly into three shape and four magnitude classes. It is argued that this variety of flow-temperature patterns is important for sustaining ecosystem integrity and for establishing benchmark flow regimes and associated frequencies to aid river management.