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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.

Scale

Implemented at the site scale.

Principal developer

eWater CRC

Scientific provenance

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.

Version

Source version 2.17

Dependencies

A Water User node is required by Source to access the monthly demand pattern node.

Theory

The monthly demand pattern is input as a set of twelve values of monthly volumes expected to be delivered at the demand node (see Parameters and settings). 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.

Input data

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.

Table 1. Model parameters

Parameter

Description

Units

Default

Range

Travel time (only when modelling a regulated system)

Expected time for water to travel from nominated storage to the demand node

Days

0

Governed by maximum travel time in the system being modelled

Monthly demand values (12 expected)

A file of demand values

ML/month

0.0

Positive number

Information from an existing scenario

An expression defined by the modeller

Percentage of volume of water supplied (which may differ from demand)

%

A fixed volume

ML/month

Output data

Output comprises a time series of flows.

Image Modified

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 an expression is used there is the potential to cause mass balance problems if an inappropriate value of the fixed volume or an inappropriate expression is used.