Modelling the Environmental Water Reserve: A Case Study Exploring the Effects of the Environment’s Water Entitlement in a Complex Water Supply System

Abstract

An 8800 km pipeline has been constructed in western Victoria to replace 17,500 km of open channel system. The Wimmera Mallee Pipeline Project (WMPP) will provide reticulated water to 36 towns and about 6000farms across an area of approximately 2 million hectares. With this pipeline will come vastly improved efficiencies in the supply of water, with water savings being returned to the environment, existing consumptive use and new development. One of the challenges for managers of water recovery projects is to determine the most effective form of entitlement for the beneficiaries of the water savings. In regulated systems, consumptive entitlements are generally supplied from water held in storage, where as the environment has a share of the run-of-river flow specified in the form of a minimum passing flow rule. However, in some cases, such as in the Wimmera-Mallee system, the environment may have the option of both regulated and unregulated entitlements. The objective of the present study was to demonstrate the effect the environment’s entitlement configuration has on the volume of water supplied to the environment, and the overall efficiency of the water supply system under the historic climatic sequence and climate change. The modelling work is undertaken using the REALM simulation package, with the results presented in case study form based on the Wimmera-Mallee system, outlining the changes in the total system water balance post-Wimmera Mallee Pipeline, changes in the environment’s reliability of supply, and exceedance plots for environmental flows and headworks loss. The outcomes of this study demonstrate the need to consider the trade-offs between large entitlements of low reliability and small entitlements of high reliability as part of the system reconfiguration process, given the effect it has on total system efficiency, particularly in an uncertain climate future.