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 50%, restrictions on water supply will be imposed for a period of time. Restrictions are usually lifted when dam water volumes exceed another dam volume higher than the volume at which the restriction was applied. In other words a particular Restriction is not lifted until there is some significant recovery in stored water volume.
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; 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 0.7 or above the Restriction Level returns to 0.
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 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) from Level 1 to Level 2 occurs at 45% storage as storage volumes are getting lower and then returns from Level 2 to Level 1 at 55% 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.