This method is executed as part of the Flat Top Surcharge. After the Surcharge Release calculation has calculated the reservoir releases necessary for evacuating the surcharge pool over the forecast period, this method determines if this proposed release schedule would violate a Pool Elevation maximum duration constraint. If so, then the surcharge release is increased as necessary to avoid violating the constraint.

The additional flow is calculated as the volume of water which would need to be released to avoid violation of the constraint, and adding that volume to the release schedule in equal increments; that is, constant additional release. This calculation is performed for each row of the Elevation Maximum Duration Table. Each iteration may add water to the proposed release. See “Elevation Maximum Duration Table” for details.

Continuing with the example above, assume that at timestep t the reservoir computes the Surcharge Releases and that (given these releases) the reservoir's storage will have been between 800 and 900 m for 5 days at timestep t + 2. The difference in storage between the projected Pool Elevation at t + 2 and 1000 m is 300 m3. RiverWare will then add 100 m3 to the surcharge releases (and Outflow) for t, t + 1, and t + 2 to bring the elevation down to 1000m at t + 2.

This method sometimes produces results that are counter-intuitive. Following are some items to consider when looking at results.

This method determines the constant additional release required to meet each constraint. The actual additional flow that will be added to the surcharge release is the sum of the values for each constraint. Therefore, the final release schedule may not be a constant value if there are multiple constraints in effect.

The originally computed Surcharge Release schedule is not constant, so adding flows to the surcharge release will result in a non-constant release schedule.

The Surcharge Release method, including this method, is executed at each timestep and will update the values in the Surcharge Release and other slots throughout the forecast period. Therefore, at the end of the run, the results may not be obvious what is happening. If you really want to see the schedules used for a particular timestep, pause the run after that timestep has executed, then look at slot values or the special results in the model Run Analysis tool. See “Model Run Analysis—Special Results Details Dialog” in USACE‑SWD Modeling Techniques for details.

Surcharge Release operations do not include Evaporation or Precipitation in the computation. Similarly, reservoir diversions (withdrawals) are performed after surcharge operations. Once the other operations execute and the reservoir dispatches and solves, the resulting elevation may be slightly different than the values used in the Surcharge and elevation max duration computations. This will manifest in not exactly reaching the constraining elevations. Because diversions (and evaporation) remove water from the reservoir, typically the elevations will be slightly below the constraining elevations.

Maximum outflow constraints are applied after this additional release schedule is computed. Thus, even if the an additional amount is required, it may not be released and the target elevation may not be achieved. The proposed values are stored in the Surcharge Release slot, the actual maximums would be stored in the Outflow slot.

The algorithm looks at the pool elevation after forecasting and surcharge have executed. (This pre-constrained pool elevation is currently not available but looking at the forecasted inflows gives some indication of the values.) It then determines the additional flow to prevent violating the constraint at any time in the forecast period. If the elevation is already at the constraint, then the additional flow will likely keep the elevations at or even below the constraint. The method will add releases to the current timesteps (and future) timestep. There is no mechanism to delay releasing water to wait until the flood actually arrives. Thus, if it seems like the elevation should stay above the constraint, the algorithm will start releasing to make room for forecasted inflows.

For example, when a large flood is within the period of perfect knowledge, then the algorithm may then need to release a large volume to meet a constraint at some point in the forecast period. This may cause the reservoir to go below the constraint to make room for the incoming flood (that is, dig a hole for the incoming flood). But, the elevation should end up at the constraining elevation on the constraining timestep.

Only one violation per constraint is considered, though in theory there could be another violation later in the forecast period.