Multiyear freshes – variable success

About Multiyear freshes

The most common way to describe environmental water requirements is in terms of considering events described with the following parameters:

• Magnitude – flow volume(ML/d);
• Duration – how long should that volume occur for to achieved the desired outcome (days);
• Timing – what time of year should the event/s occur;
• Frequency – how many events are required to achieved the desired outcome
• Independence – how should those events be spaced (days)

This model covers any flow requirement which can be described using the above five parameters and also assess those parameters across multiple years.  This model captures the range of flow requirements described below (but is mostly used for overbank flows):

• Freshes: short-duration flow events that submerge the lower parts of the river channel. They are important for plants that grow low on the banks and provide opportunities for fish and other animals to move more easily along the river.
• High flows: larger and last longer than freshes. They usually fill at least half the river channel, and therefore are important for plants that grow higher up the bank. High flows also encourage some species of fish to move and breed.
• Bankfull flows: large enough to fill the river channel with little spill onto the floodplain. Bankfull flows have high energy; they move large amounts of sediment and help to shape the river channel.  They can also act as a trigger for breeding in some fish species.
• Overbank flows: exceed channel capacity and spill onto the floodplain. These flows improve floodplain productivity, fill wetlands that provide feeding and breeding opportunities for waterbirds, fish, frogs and turtles, allow animals to move between the river channel and floodplain and help disperse plant seeds and fragments, nutrients and carbon.

Water requirements

The basic approach of defining water requirements in terms of magnitude, duration, timing, frequency and independence is well established. This model adds two key additional functionality elements:

Prevailing climatic conditions

The prescribed environmental water requirements are different in periods of drought where the water provision will be focused on biological functions that support species survival than in times of high rainfall where the environmental watering requirements are focused on the provision of conditions suitable for recruitment.

Variable success

The prescription of water requirements as magnitude, duration, timing, frequency and independence allow for a binary assessment. That is, either all of the conditions were met or they were not.  The Variable success reporting allows each of the five parameters to be represented as continuous functions to reflect the level of importance of each parameter. For example, the preferred duration of an event may be 10 days, however, a 9 day event may still provide some environmental value.