The monthly demand node is suitable for representing demands when these can be assumed to follow a fixed monthly pattern. A typical application might be to represent the demand pattern for a town water supply; another application might be to represent the demand pattern for irrigated paddy rice in a wet tropical environment. It can be used when modelling both unregulated and regulated river systems.
Implemented at the site scale.
Providing a pattern of monthly demands as input data is a basic, and well established, approach in river and water supply system, and storage behaviour modelling.
Source version v4.3
A Water User node is required by Source to access the monthly demand pattern node.
The monthly demand pattern is input as a set of twelve values of monthly volumes expected to be delivered at the demand node (see). When a model time-step of less than one month is being used (eg. daily) the value for a given month is distributed evenly across all time-steps in that month.
The Monthly Demand node provides a demand value to the Water User node at every model time-step. When modelling a regulated system, the Water User node supplies the Monthly Demand node with a value of maximum travel time (expressed in numbers of model time-steps). At a given model time-step, the demand value provided is the one for the appropriate number of time-steps in advance of the current time-step. When modelling an unregulated system, the demand value provided is the one that is applicable to the current model time-step. More information on how this value is further processed is available in the Water user node section of this Scientific Reference Guide.
Assumptions and limitations
The key assumption is that the demands to be modelled can be adequately represented by a fixed monthly pattern. Amongst other things, while seasonal variations are represented, this assumes the demand in any given month does not change from year to year due to climate variability.
A fixed pattern of 12 monthly values is input. More information on this and other input data is given in Parameters or settings, below. Full details are provided in the Source User Guide.
Parameters or settings
Model parameters are summarised in.
Table 1. Model parameters
Use Annual Total
There is a Boolean to enable the use of an annual total
True / False
|Volume of annual total||ML||0.0||Positive number|
Order per month (12 expected)
|A value for each moth of what the demand will be|
|Percentage of Annual Total|
When "Use Annual Total" is true, it defines the percentage of the annual Total for each month
|%||1/12 * Annual total||Positive number|
Return Flow (see note below)
Percentage of volume of water supplied (which may differ from demand)
A fixed volume
A volume defined using a function
Output comprises a time series of flows.
Note that when the return flow is being calculated no mass balance check is carried out. While there should be no potential for a mass balance problem when the return flow is evaluated as a percentage of the water supplied, when either a fixed volume or a volume defined by a function is used there is the potential to cause mass balance problems if an inappropriate value of the fixed volume or an inappropriate function is used.