Within Source, "Restriction Curves" are an approach to controlling water supply within a model and are used to reduce the water volumes released to users when the volume in the storage is declining. For instance, if dam water volumes drop below 60%, restrictions on water supply may be imposed for a period of time to reduce the use of a declining resource. Restrictions are usually lifted when dam water volumes exceed another dam volume higher than the volume at which the restriction was applied. In this way a particular restriction is not lifted until there is some significant recovery in stored water volume above the volume at which the restriction was applied.
The “Restriction Curves” approach uses Restriction Levels, for example:
- 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 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; when stored water volumes are declining and the proportion of full supply volume becomes equal to or less than 0.6, Restriction Level 0 changes to Restriction Level 1 (blue dotted line). Subsequently, when stored water volume rises to 0.7 or above the Restriction Level returns to 0 (solid red line). Hence, a "falling" Restriction Curve is used to indicate when the associated Restriction Level will be put in place if Restriction Level the day before was above the curve (i.e. it had a lower Level number and a less severe restriction). A "rising" Restriction Curve indicates when the associated Restriction Level will be put in place when Restriction Level on the preceding day was below the curve (higher Level number, more severe restriction). 'Rising' and 'falling' refer to the change in the stored water volume not the Restriction Level. The falling Restriction Curve for Restriction Level 0 is redundant and is not used (there is no less severe restriction than Level 0).
The figure above also shows that the critical values of stored water volume for changing Restriction Levels are allowed to change on a monthly basis. For example, lower levels in a water storage might be tolerated at the end of a dry season because of water use through this season – and an upcoming wet season is expected to provide inflows to the water storage and reduction in water demand.
An example consistent with the restrictions above is provided in the figure below. This figure shows that the change in Restriction Level (shown on the y axis on the right) goes from Level 0 to Level 1 at 60% storage and from Level 1 to Level 2 at 45% storage as storage volumes are getting lower and then returns from Level 2 to Level 1 at 55% storage and Level 1 to Level 0 at 70% storage.
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.
Restriction Curves provide water management rules that aim to avoid the situation where all water in the storage is used up and aim to extend the period when a certain minimum supply can be ensured. Users can be informed about these rules in advance so that all stakeholders know what to expect under the range of possible conditions of water supply. Restriction levels are best estimated using a long historical record of daily inflow rates and water storage levels and volumes.