A proposed method and case study for detection of change in waterway health due to flow

Abstract

Victoria has been actively implementing measures to protect and provide water for the environment. In 2005 the environmental water reserve was legally recognised, and the subsequent regional Sustainable Water Strategies (SWSs) formalised sharing of water between the environment and other uses. Identifying whether the balances determined in the SWSs need to be adjusted will involve multiple considerations. This paper examines one of these considerations: has waterway health continued to deteriorate since the introduction of environmental flows, and how do we detect the influence of changes in the flow regime? The logic in deciding on an appropriate methodological approach is described. Existing datasets were assessed for suitability under several criteria. A case study is presented using measures of invertebrate diversity against a number of flow measures to illustrate how a proposed step-wise regression method may be used to determine change over time, change with introduction of the SWS, and change relative to flow components. Co-variates were controlled for where data were available. Numerous factors complicate this assessment, and their influence on the approach used are also described. These factors include: lack of a monitoring program designed and implemented specifically for this purpose; existing datasets being highly variable in time, space, quality and relevance; that defining waterway health is complex; that each waterway functions differently; that there are numerous confounding factors involved; that application of water for the environment has been incremental; and that the period since the introduction of environmental water management to many systems is relatively short, such that there may not yet be an impact to detect or that any change may still be small. These complexities mandate use of additional layers of evidence to contextualise and interpret the quantitative analysis.