Traditionally water availability for a particular property or license is governed by the total resource available in any particular season and the allocation attached to that property or license.
An allocation does not entitle a property or license to an absolute quantity of water; rather it defines the share of the total water available in any season.
The allocation structures are well established. In the Murray Valley the structures date back to 1964 and 1967 with a modification introduced upon the completion of the Dartmouth dam in the 1980's. In the Murrumbidgee Valley the development of the current structures commenced in the late 1970's and the allocation scheme was subsequently formally introduced in 1982/83.
In the past, water management agencies calculated seasonal allocations according to knowledge of storage levels and inflows (historical minimal or probable) combined with expectations of actual irrigation usage relative to announced levels of availability. Any residual was assigned to the river or "the environment". Generally agencies could be confident that not all of the water announced for the season would actually be used, but if actual usage was underestimated, the shortfall was offset (at least partially) by reducing water for the environment.
Other elements in the seasonal equation included access to off-allocation (that is un-regulated flow) water, the possibility of borrowing against the following season's allocation and in more recent years the opportunity to purchase water through temporary transfers.
Such allocation systems have now been in place for at least 15 years (35 in the Murray Valley districts) and agencies and irrigators have been well versed in understanding the meaning of announcements about allocations.
