Scenario Inheritance

An important concept in using scenarios is the idea of scenario inheritance. In WEAP's Data View, you create mathematical expressions that define the data values of each branch/variable combination in your analysis. Scenario inheritance allows you to create hierarchies of scenarios that inherit default expressions from their parent scenario. Initially, you create expressions for the Current Accounts. These can either be constant expressions, or expressions that generate a time-series of values. Then, you can create additional scenarios, with expressions that either simply inherit the Current Accounts expressions, or override these for particular branches and variables. So, for example, you might create a scenario that examines an irrigation efficiency program, that inherits most of its expression from a baseline "business as usual" scenario. Because the efficiency scenario inherits from the baseline scenario, when initially created it will contain exactly the same expressions as the baseline scenario, and hence will yield exactly the same results. To fully define the scenario you only need to type in expressions to reflect the branches and variables affected by the irrigation efficiency program. The inherited expressions for all other branches stay the same. You can define any number of levels of inheritance. So for example you could make an irrigation efficiency scenario that inherits from the first, with slightly revised assumptions. This approach makes it very easy to edit and organize scenarios, since a) they can be created with a minimum of data entry and b) common assumptions in families of scenarios can be edited by just editing the parent scenario. The ability to establish scenario inheritance is demonstrated in Manage Scenarios.

When editing scenario data in WEAP's Data View, the expression fields in data entry tables are color coded to show which expressions have been entered explicitly in the current scenario, and which are inherited either from a parent scenario or from the data specified for Current Accounts. Red text indicates a value entered explicitly in the current scenario, while black text indicates an inherited value (or data entered in Current Accounts).