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

Abstract

In this paper we investigate similarities in streamflow variables, identify groups of variables that represent different aspects of the flow regime, and we discuss the ecological relevance of these groups. For the study we used 35 flow variables for 62 New Zealand perennial rivers, which cover a wide range of flow regimes. All flow variables were calculated using daily mean flows from a common 7-year period. They included general flow variables (which express general features without focusing on extreme flows, such as mean flow and coefficient of variation), high flow variables and low flow variables. 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 variability 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 relevance of each of the four groups is discussed. We also examined statistical properties of the flow variables such as the inter-annual variability, which was low for the general flow variables and high for variables that characterize the highest floods. We recommend that a suite of different variables representing each of the identified groups be used in riverine ecological studies in order that the different facets of flow regimes be adequately represented.