--This article is applicable to GoldSim 11.0 and later--
What happens if my Reservoir element has a moving Lower Bound that becomes larger than the volume currently stored in the Reservoir?
An Upper and a Lower bound can be specified for a Reservoir element. If a Lower Bound is specified for a Reservoir, this value provides the lower limit on the amount that can be “stored” in the reservoir. The value for the Lower Bound can be specified in the Properties dialog as shown in Figure 1. In this figure, the Lower Bound is set to a constant value of 0.0 m3.
Figure 1: Reservoir element Properties dialog
When a Reservoir element has a specified Lower Bound, the Reservoir element will limit the withdrawals from the Reservoir so that the volume does not go below the Lower Bound. From the GoldSim User’s Guide (TFM), “If the Lower Bound is constant, the Current Value in the Reservoir will never fall below the Lower Bound. However, if the Lower Bound is changing with time, under some circumstances, it is possible for the Current Value of the Reservoir to fall below the Lower Bound. The primary purpose of the Lower Bound is to accurately compute the Withdrawal_Rate. If the Lower Bound exceeds the amount in the Reservoir (because the Lower Bound has changed during a time step), the Reservoir will not permit any withdrawals until it returns to the Lower Bound (via additions).” The User’s Guide describes the “Current Value” of the Reservoir because Reservoirs can accept any units and are not limited to units which are length cubed. GoldSim will write a warning to the run log if the Current Value in a Reservoir is below the specified Lower Bound.
When a Reservoir element has a moving Lower Bound, the Current Value in the Reservoir can be below the Lower Bound. As an example, the properties and data for a Time Series element which provides a Lower Bound for a Reservoir element are shown in Figure 2. Note that this Time Series represents values as “Constant value over the next time interval”. If the Time Series from Figure 2 is then used in a simple 10 day simulation, using a 1 day time-step, of a Reservoir (“Reservoir1”) which starts the simulation with a volume of 10 m3 (i.e. starts the simulation above the Lower Bound) and has no other inflows or outflows, the results from this simulation are provided in Figure 4. The simulation settings of a 10 day duration and a Basic Step duration of 1 day are shown in Figure 3.
Figure 2: An example time series for a changing Lower Bound
Figure 3: Example simulation time settings including a "Basic Step" duration of 1 day
In Figure 4, the Reservoir volume is below the specified Lower Bound from 2.0 days to 7.0 days. During this period, no withdrawals are allowed from the Reservoir. In this simple example case, no withdrawals are requested and as a result the limitation on withdrawals when the Current Value is below the Lower Bound does not need to be enforced. The Current Value, or volume in this case, remains below the specified Lower Bound until additions to the Reservoir bring the Current Value above the Lower Bound or until the Lower Bound moves below the Current Value. Warning messages are written to the run log which tell the user the Reservoir Current Value, the Lower Bound value, and that the Reservoir Current Value is below the Lower Bound.
Figure 4: Moving Lower Bound simulatin with a 1 day "Basic Step" duration