Environmental flows planning to improve ecological realism and address operational reality

Abstract

Environmental flow studies typically define a single minimum flow regime required to support ecological processes based on the required frequency, timing and duration of particular flow events within each year, irrespective of prevailing climate conditions. However, this approach overlooks natural inter-annual hydrologic variability, and has resulted in the frequent inability to meet recommended flows during dry periods (environmental shortfalls), and led to seemingly excessive entitlements during wet periods. In this paper we present a case study for the Wimmera River (Victoria, Australia) in which an assessment of inter-annual variability in ecological water requirements was used to define four distinctive flow-regimes matched to the prevailing climatic conditions. The climatic conditions spanned drought years to wet years and were defined based on a simple quartile split of a long-term modelled predevelopment flow. We characterised the modelled predevelopment flow patterns associated with each of the defined climate conditions, and identified the specific ecological and physical processes (such as species recruitment and channel formation) for which such flow variability was considered important. The analyses presented here compare the total environmental water demand (defined in terms of managed releases from storage and quantified using eflow Predictor) for both the annually variable approach, and the more typical annually averaged approach. While the long-term water demand was similar for the two approaches, annual watering requirements under the climatically specific allocation rules differed by up to three-fold, and resulted in much reduced environmental flows during dry periods, and substantially longer high flow spells during wet years. From a water management perspective environmental demand is much reduced in dry years when water costs are typically high and shortfalls more likely. Inclusion of inter-annual variability thus leads to recommendations reflecting the 'boom and bust' nature of many river systems, while also being more parsimonious with inter-annual variability in operational water availability and water costs.