Once a master plan (MPD) has been adopted, it becomes the baseline for measurement of future impacts (environmental and economic) due to changes in the MPD assumptions. Such changes include:

- Land Use
- Hydrologic Parameters (e.g., rainfall statistics update)
- Hydrologic Methods (e.g., change in hydrology criteria)
- Hydraulic System (e.g., new flood control channel, pump station, basin, etc.)
- Economic Factors (e.g., unit costs, interest rates, etc.)
- Other

To measure these impacts, YOU utilize the MAINTENANCE MODE in SIMS. YOU generally make edits to the model network in BUTTON 1, and possibly changes to criteria at any of the SIMS BUTTONs that control analysis tool sets. This MAINTENANCE work results in a new master plan called a "SCENARIO". Once YOU have completed the SIMS circuit in the MPD Manager, YOU will have a completed SCENARIO, which is an entirely new master plan, with all computations regenerated and results updated to reflect the new conditions at issue. YOU literally have reinvested all the efforts paid in generating the BASELINE master plan in developing a SCENARIO master plan.

And now YOU can estimate the environmental and economic impacts to the BASELINE master plan (i.e., the master plan that has been officially adopted for the community) should the SCENARIO master plan be adopted instead. The SIMS can now be used to develop a master plan of DIFFERENCES!

The DIFFERENCES data base is composed of the variations between the BASELINE and SCENARIO master plans. Any estimated changes in the network elements (e.g., pipe sizes, box sizes, etc.), water quality constituents, total costs, and so forth, are captured and stored by SIMS in the DIFFERENCES data base.

Now YOU have an estimate of master plan system impacts, cost impacts, and environmental impacts, as well as prioritization impacts (e.g., which element should be built first?). This information may be used to evaluate and mitigate for the effects of changes to the approved BASELINE master plan.

And if the SCENARIO is acceptable, YOU may then use SIMS to update the BASELINE master plan to the SCENARIO master plan conditions, by redefining the SCENARIO to be the new BASELINE. Thus, the BASELINE is an evolving plan with time.

The evolutionary steps that connect the first BASELINE to the current BASELINE are stored in the HISTORIAN data base that is available in SIMS.