Towards setting environmental water temperature guidelines: A South African example

Abstract

Water temperature is a primary factor affecting the number and kinds of species in a stream. A key step towards including water temperatures in environmental flow assessments is to develop metrics which describe natural variability in a river's thermal regime. This is best achieved using time series analyses, where metrics are defined based either on time series disaggregation, or shapes of regimes defined using agglomerative techniques. The aim of this paper was to refine approaches in setting environmental water temperature guidelines for inclusion in defining environmental flows assessments. Annual water temperature series from 82 sites sampled across 48 rivers (mainstems and tributaries) in ten catchments in the southern Cape region of South Africa were described using 39 metrics based on the magnitude, frequency, duration and timing of thermal events. Sites were classified into thermal groups using their similarity in multivariate temperature regime and variation amongst groups along important temperature gradients examined. Deviation from a natural range of variability using a thermal confidence envelope is a suitable approach for broad evaluation of thermal guidelines. The approach presented can be applied at multiple levels of complexity to assess which elements of a thermal time series fall outside of reference conditions. Further steps in this approach are to link thermal patterns to biotic metrics, and gain a clearer understanding of interactions between flows, temperatures and biota, particularly below impoundments. Research on improving approaches in defining thermal regions is recommended.