You can create Key Assumptions and Other Assumptions, which you can reference in expressions elsewhere in WEAP. It is very useful to create variables here for all your major modeling assumptions, especially those that will vary from scenario to scenario, as it will organize and highlight the most important parts of your model, help ensure that consistent assumptions are used throughout your model, and make it convenient to view and edit them. (Less important intermediate variables should go in the Other Assumptions section. Note: You can change the name of the Other Assumptions branch.) You may create a multilevel, hierarchical structure for your Key Assumption and Other Assumption branches, grouping together related concepts. For example, the categories of Key Assumptions in the Weaping River Basin dataset are Drivers, Monthly Variation, and Elasticity. You can create any number of levels of grouping.
Although Key Assumptions can be thought of as variables, they are actually added as new branches to the Data View Tree under the Key Assumptions and Other Assumptions sections. Alternatively, you can also add your own User-Defined Variables under the sections for Demand Sites, Supply and Resources, and Water Quality. As opposed to Key Assumptions, which are added as new branches to the tree, User-Defined Variables are added as new variable tabs to existing branches. Key Assumptions are best if you are using the same expression in multiple places in your model, or to highlight a major modeling assumption. User-defined variables are better if you want to create a variable whose values will vary by demand site (or groundwater node, or reservoir, etc.). See the Customizing Data Variables section for more information about them.
To create a Key Assumption branch, right click on the branch under which you want to add the new branch (on or under the Key Assumptions and Other Assumptions top level branches), and choose Add from the context menu. A new branch will appear, with the name "New Branch" highlighted for you to type a new name. To create a multilevel structure, first create the branch that will hold the other branches (e.g., "Drivers"), then create the other branches underneath it (e.g., "GDP," "Price of Water," and "Technical Innovation").
For example, suppose you wanted to model the population of your demand sites by using a simple growth rate. If the growth rate was the same for all municipal demand sites, it would be best to create a single Key Assumption branch called Population Growth Rate, and then reference this in the Annual Activity Level expression for the municipal demand sites (e.g., Growth(\Key\Population Growth Rate). However, if you wanted to use a different growth rate for each demand site, to do this with Key Assumptions would require adding a different Key Assumption branch for each demand site. It would be better to create a single user-defined variable called Population Growth Rate in the Demand Sites section of the tree. After entering the growth rate value for each demand site, the expression for Annual Activity Level would be Growth(Population Growth Rate).
Because the values of Key Assumption variables can be displayed in the Results View, these variables can also be used to create new result variables or alternative indicators. For example, you could perform a social cost-benefit analysis by creating indicators such as the social cost of unmet demand (shortages), or the environmental cost of low river flows. The shortage cost key assumption could refer to the unmet demand at one or more demand sites (using the PrevTSValue function), with an increasing cost associated with increasing unmet demands. A complete "triple bottom line" analysis could be done, involving financial, social and environmental costs. You could create a key assumption group for each of these types of costs, with individual cost items under each. (The financial costs Key Assumption Variables would reference the Financial Results variables using PrevTSValue.)
Entered on: Data View, Branches: Key Assumptions, Other Assumptions