Apportioning allocations to users of multi-storage water supply systems: A case study of making a complex volume shared system more transparent

This paper describes principles for the apportionment of water allocations to users of a multi-reservoir water supply system utilising a volume shared entitlement and allocation framework. The challenge of this problem is that volume shared systems determine the available water for allocation based on a total system approach. The subsequent operational challenge is to then apportion this total volume of available water to specific reservoirs to meet individual user requirements. This is an important problem as entitlement and allocation frameworks usually have the water resource assessment process and high-level water sharing principles enshrined in a set of legally binding orders and instruments. However, some systems still have a subsequent apportionment of allocation problem, not codified in any binding document, where decisions need to be made around how much allocation should be made available from particular reservoirs for the various stakeholders or user groups. In shared systems where contests over water is common, or access to allocation may vary over time, it is desirable that the agency responsible for making the resource decisions uses an objective, fair and equitable method of allocating water. To work through this problem and present the set of principles for apportionment, the Wimmera-Glenelg System located in western Victoria, Australia, is used as a case study. The Wimmera-Glenelg System is a complex water resource system with multiple reservoirs and many different user groups and stakeholders. The region is also subject to a highly variable climate with frequent dry periods and water rationing, creating periods of time where the equitable apportionment of allocation becomes incredibly important. Concepts of capacity sharing have been used to help with the development of the apportionment principles to help maximise the transparency in decision making to stakeholders and because the system does have an emerging water market where commercial and economic certainty is becoming paramount. However, capacity sharing for systems with multiple reservoirs is not common, and so even this has limitations in use. The principles described can be universally applied to reservoir systems of varying complexity, where there are multiple users, and is compatible with both capacity shared systems and newer continuous sharing or continuous accounting systems. Results are shown for the Wimmera-Glenelg System.