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.
The demand for a month (m) equals that month's fraction (specified as data under Demand\Monthly Variation) of the adjusted annual demand.
MonthlyDemandDS,m = MonthlyVariationFractionDS,m x AdjustedAnnualDemandDS
The monthly demand represents the amount of water needed each month by the demand site for its use, while the supply requirement is the actual amount needed from the supply sources. The supply requirement takes the demand and adjusts it to account for internal reuse, demand side management strategies for reducing demand, and internal losses. These three adjustment fractions are entered as data--see Demand\Loss and Reuse and Demand\Demand Side Management.
MonthlySupplyRequirementDS,m = (MonthlyDemandDS,m x (1 - ReuseRateDS) x (1 - DSMSavingsDS)) / (1 - LossRateDS)