# Annual Demand and Monthly Supply Requirement Calculations

## Annual Demand

A demand site's (DS) demand for water is calculated as the sum of the demands for all the demand site's bottom-level branches (Br). A bottom-level branch is one that has no branches below it. For example, in the structure shown at the right, Showers, Toilets, Washing and Other (and four others underneath Multifamily that are not shown) are the bottom-level branches for South City.

AnnualDemandDS = ( TotalActivityLevelBr x WaterUseRateBr )

The total activity level for a bottom-level branch is the product of the activity levels in all branches from the bottom branch back up to the demand site branch (where Br is the bottom-level branch, Br' is the parent of Br, Br'' is the grandparent of Br, etc.).

TotalActivityLevelBr = ActivityLevelBr x ActivityLevelBr' x ActivityLevelBr'' x ...

For the example above, this becomes

TotalActivityLevelShowers = ActivityLevelShowers x ActivityLevelSingleFamily x ActivityLevelSouthCity

= percent of people in single family homes who have showers x percent of people who live in single family homes x number of people in South City

The activity level for a branch, and the water use rate for a bottom-level branch, are entered as data. (See Demand\Annual Water Use\Activity Level and Demand\Annual Water Use\Water Use Rate.) Note that those branches marked as having "No data" (unit = N/A) are treated as having an activity level of 1.

## Monthly Demand

The demand for a month (m) equals that month's fraction (specified as data under Demand\Monthly Variation) of the adjusted annual demand.