Practice note: Modelling urban demand

This practice note is one of a set developed to provide consistency and transparency of river system models being used within the Murray-Darling Basin. The notes cover modelling practices, such as naming conventions for folder structures, to model methods, such as for flow routing and residual inflow estimation, and have been developed through a collaboration between the MDBA, the Basin States and CSIRO.

Produced in collaboration with:

This practice note, 'Modelling urban demand' describes the agreed general principles and practices for modelling of urban water demands in eWater Source river system models.

Background

Diversions for urban, stock and domestic and industrial uses account for ~4% of the total diversion on the Murray and Lower Darling Rivers. It is important that the different entitlement types described in the States' water sharing plans (including urban water which includes the critical human water needs explicitly defined in the Basin Plan) are explicitly represented in their Source river system models. Historically within the Murray and Lower Darling Rivers, these diversions have been lumped with irrigation diversions and modelled using regression methods.

This practice note presents the method to explicitly model urban water demands that was developed during the development of the River Murray and Lower Darling Source model as the recommended practice.

General principles

Urban demands should be modelled separately from irrigation demands, to allow the explicit representation of the different entitlement types described in the States' water sharing plans
Urban demands include: critical human water need + additional variable component use for domestic, stock, and other users (industry)
Modelling should consider the impact of:
1. climate on water use
2. water restrictions on water use
Critical human water need (CHWN) should be included as a minimum water requirement
Annual CHWN requirements per state should be based on the values in the Basin Plan and should be disaggregated into the individual demand models based on population
Annual CHWN requirement per demand model should be disaggregated into daily values based on the pattern of average monthly water use.

Recommended high-level method (based on Bethune(2015) )

Urban demand models should include:

Critical human water needs (CHWN)
Season variation in use in response to climate drivers
Variations in use in response to water restrictions. Urban Demand = Requirement_CHWN + Requirement_Variable

Requirement – Critical Human Water Needs (CHWN)

These are defined in the Basin Plan as an annual requirement at the State level based on a fixed consumption rate per capita with an allowance for distribution losses.
This annual requirement per state is disaggregated into different demand models based on the population being represented.
For each demand model, the annual CHWN is disaggregated to a daily value based on the average historical monthly pattern of urban water (see Figure 1 for monthly values for three states).
The same pattern is used for all demand models in a State, as investigation showed that individual monthly use patterns for each town were similar and did not materially impact on the modelled water use.

where:
P_month = pattern of average monthly use (Figure 1)
D_month = days in month
CHWN_A = Annual critical human water need for town/demand model

Figure 1: Pattern of average monthly use for disaggregation of annual CHWN and average annual variable use for New South Wales, Victoria and South Australia

Requirement – Variable

This term includes a representation of the average annual variable use (Use_{Av car}) which is a constant and is calculated as the difference between average annual use and annual CHWN for population represented by each demand model.

The Use_{Av car} is disaggregated based on the same average historical monthly pattern (Figure 1).

This is then adjusted for climate and water restrictions,

where,
P_month = pattern of average monthly use

D_month = days in month

Use_{Av car} = Historical Average Annual Water Use - CHWN_A

f(climate) defines the impact of climate on daily urban use

f(restriction) defines the reduction in urban water use resulting from lower water allocations

Average Deficit is the long-term average deficit and is the average of E_t – R_t for the last seven days, where

E_t = Evaporation on day t

R_t = Rainfall on day t

f(restrictions) - based on a simple relationship between available water and restriction level (Figure 2)

Figure 2: Relationship used to determine the factor for restrictions

References

Bethune M (2015) Urban and stock and domestic demands in the Murray and Lower Darling Rivers. Murray Darling Basin Authority, Canberra.

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