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• At Restriction Level 0, water volume released for a water user = the original allocation or entitlement  (no change)
• At Restriction Level 1, water volume released to a user will be reduced to  65% of the original allocation or entitlement for example
• At Restriction Level 2, water volume released to a user will be reduced to 45% to 40% of the original allocation or entitlement  for example
• And so on … where increasing restriction levels are applied as the stored water volume decreases to lower and lower volumes

An example set of Restriction Curves is shown in the following figure as implemented in Source.  The stored water volume is expressed as a proportion of the stored volume at full supply level; in the figure, when stored water volumes are declining and this value becomes equal to or less than 0.6,  Restriction Level 0 changes to Restriction Level 1.  When stored water volume rises to 0.65 7 or above the Restriction Level returns to 0.    

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The figure also demonstrates that the critical values of stored water volume for changing Restriction Levels are allowed to change on a monthly basis.  For example, a water storage might be expected to have lower levels at the end of a dry season because of water use through this season – and lower levels can be tolerated when an upcoming wet season is anticipated providing expected to provide inflows to the water storage and reducing reduction in water demand. 

The Restriction Curves approach provides a look-up table for Restriction Level that is dependent on the antecedent stored water volume conditions.   When applying the Restriction Curves in a Source model the Restriction Level is used to adjust the time series demand for water supply.

The approach is particularly relevant to urban water supply but can also be used to trigger events such as releases from a certain storage, desalination plant supply rates or environmental flows based on salinity levels. The aim of the trigger/restriction curves is to provide a time series that tracks what level of restriction the system is in. This time series can then be used to modify some other variable such as demand or inflow (desalination) as well as being a measure of the standard of service delivered.       

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