Flow variables for ecological studies in temperate streams: groupings based on covariance

In this paper we investigate similarities in streamflow variables, identify groups of variables that represent different aspects of the flow regime, a nd we discuss the ecological relevance of these groups. For the study we us ed 35 flow variables for 62 New Zealand perennial rivers, which cover a wid e range of flow regimes. All flow variables were calculated using daily mea n flows from a common 7-year period. They included general flow variables ( which express general features without focusing on extreme flows, such as m ean flow and coefficient of variation), high flow variables and low flow va riables. Four groups of variables were identified based on covariance among the sites through a principal components analysis. These groups were: (1) the size of the river (central tendency of magnitude); (2) the overall vari ability of flow (including magnitude of high and low flows); (3) the volume of high flows; and (4) the frequency of high flow events. The ecological r elevance of each of the four groups is discussed. We also examined statisti cal properties of the flow variables such as the inter-annual variability, which was low for the general flow variables and high for variables that ch aracterize the highest floods. We recommend that a suite of different varia bles representing each of the identified groups be used in riverine ecologi cal studies in order that the different facets of flow regimes be adequatel y represented. (C) 2000 Elsevier Science B.V. All rights reserved.