Select Timestep and Run Range

The timestep and run range for Optimization are set in the Run Control dialog, just as for Simulation models. The same timesteps are available for Optimization as for Simulation; however due to the nature of Optimization, some additional issues should be considered when setting the timestep and run range for Optimization.

During an optimization run a linear program (LP) is created which represents the physical and policy constraints for the system. The time required to solve the LP is related to the size of the model, including the number of objects, the number of timesteps in the run, and the number of constraints. Thus to permit a simulation that runs in an acceptable time, one might need to consider increasing the timestep length and/or shortening the run duration. It is important to realize that the run time for Optimization does not increase linearly with the number of timesteps. The run time tends to increase with something between the square and the cube of the number of timesteps.

Another reason for limiting the duration of an optimization run is that long optimization runs can exhibit perfect foresight. That is, since each LP solution optimizes over the full run duration, it can incorporate behavior that would not be plausible for operators who at any point in time do not have perfect knowledge of future events. Thus as a rule of thumb, optimization models should have a run duration no longer than the time frame for which accurate forecasts are likely to exist for the river basin operations.

Caution: Although all timestep lengths are available for Optimization, it is not recommended that a 1 Month timestep be used with Optimization. Optimization does not have a concept of current timestep. Thus a 30 day month is used for all calculations and constraints that incorporate the timestep length, for example, conversions between volume and flow and between power and energy. As such, the mass balance in the Optimization solution will not match the mass balance of the Post-Optimization Rulebased Simulation for a 1 Month timestep.

Revised: 08/02/2021