Measurement:Unit Identifiers

• Part of: Measurement:Main_Page
• Related to: Measurement:Units

AOWIS uses the canonical SI-based engineering units specified in Units. UCUM is the normative syntax and code system for the machine-readable unit identifiers.

AOWIS defines a canonical set of UCUM unit identifiers used across all system interfaces, storage formats, APIs, and inter-system data exchanges.

Unit identifiers are machine-readable tokens and are not intended to represent typographic or human-readable unit notation.

General Rules

• All unit identifiers MUST conform to UCUM.
• Each measurement quantity MUST be assigned exactly one UCUM unit identifier as its canonical unit identifier.
• Canonical unit identifiers MUST be used consistently across all AOWIS-compliant systems, interfaces, storage formats, APIs, and inter-system data exchanges.
• Canonical unit identifiers MUST be interpreted independently of locale, display format, implementation, and system context.
• Canonical unit identifiers MUST remain stable across AOWIS-compliant systems and data formats.

Requirements

REQ-MEAS-UNI-001: All unit identifiers MUST conform to UCUM.

REQ-MEAS-UNI-002: Each measurement quantity MUST be assigned exactly one UCUM unit identifier as its canonical unit identifier.

REQ-MEAS-UNI-003: The canonical unit identifier assigned to a measurement quantity MUST be used consistently across all AOWIS-compliant systems, interfaces, storage formats, APIs, and inter-system data exchanges.

REQ-MEAS-UNI-004: The measurement quantity identifier defines what is measured, while the unit identifier defines the unit in which it is expressed.

Canonical Measurement Quantities and Units

The canonical UCUM unit identifier assigned to a measurement quantity is authoritative whenever a unit identifier is represented in serialization or data exchange.

The AOWIS identifier suffix is an ASCII-safe representation used in field names, property names, database columns, and other technical identifiers. Division is represented by _per_, multiplication by _, and upper case is normalized to lower case.

For fixed-schema measurement fields, the measurement quantity and canonical unit MAY be defined normatively by the schema and represented through the field name and its AOWIS identifier suffix. Such fields do not require a separate unit-identifier property.

When a unit identifier is represented explicitly, it MUST use the canonical UCUM identifier assigned to the measurement quantity. The AOWIS identifier suffix is a technical field-naming convention and is not itself a UCUM unit identifier.

Generic or dynamically typed measurement values MUST explicitly identify their measurement quantity and canonical UCUM unit and MUST NOT rely solely on a field-name suffix.

Each identifier suffix is explicitly defined in the following tables.

Length and Distance Quantities

Measurement Quantity Identifier	Definition	Canonical Unit	Canonical UCUM Unit Identifier	AOWIS Identifier Suffix	AOWIS Model / EPANET Mapping
length	Linear spatial extent along an object or path	metre (m)	m	m	Pipe and link length
elevation	Vertical height relative to an engineering or hydraulic datum	metre (m)	m	m	Junction, tank, reservoir, and terrain elevation
altitude	Vertical position reported by a geographic positioning source	metre (m)	m	m	Geographic altitude; not necessarily suitable as hydraulic elevation
distance	Spatial separation between positions	metre (m)	m	m	Derived geometric distance
vertical_offset	Vertical difference between referenced elevations or heads	metre (m)	m	m	Elevation, tank-bottom, and reservoir-head offsets
link_diameter	Internal or nominal diameter of a pipe or valve	millimetre (mm)	mm	mm	EPANET pipe and valve diameter
tank_diameter	Internal diameter of a cylindrical storage tank	metre (m)	m	m	EPANET tank diameter

Geographic Coordinate Quantities

Measurement Quantity Identifier	Definition	Canonical Unit	Canonical UCUM Unit Identifier	AOWIS Identifier Suffix	AOWIS Model / EPANET Mapping
latitude	Angular position north or south of the equator	degree (°)	deg	deg	WGS 84 latitude
longitude	Angular position east or west of the prime meridian	degree (°)	deg	deg	WGS 84 longitude
projected_easting	Eastward coordinate within a projected coordinate reference system	metre (m)	m	m	UTM, UPS, and other projected-coordinate eastings
projected_northing	Northward coordinate within a projected coordinate reference system	metre (m)	m	m	UTM, UPS, and other projected-coordinate northings

Area and Volume

Measurement Quantity Identifier	Definition	Canonical Unit	Canonical UCUM Unit Identifier	AOWIS Identifier Suffix	AOWIS Model / EPANET Mapping
area	Two-dimensional spatial extent	square metre (m²)	m2	m2	Tank cross-sectional area and other geometric areas
volume	Three-dimensional spatial extent or storage capacity	cubic metre (m³)	m3	m3	Tank volume

Flow and Transport

Measurement Quantity Identifier	Definition	Canonical Unit	Canonical UCUM Unit Identifier	AOWIS Identifier Suffix	AOWIS Model / EPANET Mapping
volumetric_flow_rate	Volume transported per unit time	cubic metre per hour (m³/h)	m3/h	m3_per_h	Demand, link flow, leakage flow, emitter flow, and pump-curve flow
velocity	Rate of linear movement	metre per second (m/s)	m/s	m_per_s	Link flow velocity

Hydraulic Head, Pressure, and Stress

Measurement Quantity Identifier	Definition	Canonical Unit	Canonical UCUM Unit Identifier	AOWIS Identifier Suffix	AOWIS Model / EPANET Mapping
hydraulic_head	Total mechanical energy per unit weight of fluid expressed as an equivalent fluid-column height	metre (m)	m	m	Junction, reservoir, and tank hydraulic head
pressure_head	Fluid pressure expressed as an equivalent fluid-column height	metre (m)	m	m	EPANET node pressure, pressure-driven demand thresholds, and pressure-control valve settings
water_level	Height of a water surface relative to a defined local reference level	metre (m)	m	m	Initial, minimum, maximum, and simulated tank levels
head_gain	Increase in hydraulic head across an energy-adding component	metre (m)	m	m	Pump head curves and pump simulation results
head_loss	Reduction in hydraulic head across a hydraulic component	metre (m)	m	m	Total head loss across pipes, valves, and other energy-dissipating components
head_loss_gradient	Hydraulic head loss per unit pipe length	metre per kilometre (m/km)	m/km	m_per_km	EPANET pipe head-loss reporting
pressure	Normal force per unit area within a fluid system	kilopascal (kPa)	kPa	kpa	Physical pressure measurements, sensors, SCADA data, and external-system exchange
stress	Internal force per unit area within a material	megapascal (MPa)	MPa	mpa	Structural and material analysis; not currently represented in the hydraulic model

Time

Measurement Quantity Identifier	Definition	Canonical Unit	Canonical UCUM Unit Identifier	AOWIS Identifier Suffix	AOWIS Model / EPANET Mapping
elapsed_time	Time elapsed from a defined simulation or process origin	second (s)	s	s	Simulation elapsed time and time-based control triggers
duration	Length of a time interval	second (s)	s	s	Simulation duration, hydraulic timesteps, quality timesteps, reporting intervals, and rule intervals
time_of_day	Time elapsed since the start of the current day	second (s)	s	s	Simulation start time of day and clock-time controls

Dimensionless and Percentage Quantities

Measurement Quantity Identifier	Definition	Canonical Unit	Canonical UCUM Unit Identifier	AOWIS Identifier Suffix	AOWIS Model / EPANET Mapping
efficiency	Ratio of useful output to total input, expressed as a percentage	percent (%)	%	percent	Pump efficiency values and efficiency curves
relative_flow	Flow relative to a defined reference flow, expressed as a percentage	percent (%)	%	percent	Valve characteristic curves
valve_position	Valve opening position relative to its full operating range	percent (%)	%	percent	Position-control valves and valve characteristic curves
source_trace_percentage	Proportion of water originating from a specified source node	percent (%)	%	percent	EPANET source-trace analysis

Reaction and Leakage Quantities

Measurement Quantity Identifier	Definition	Canonical Unit	Canonical UCUM Unit Identifier	AOWIS Identifier Suffix	AOWIS Model / EPANET Mapping
first_order_bulk_reaction_coefficient	First-order reaction-rate coefficient for a substance within the bulk fluid	per day (d⁻¹)	/d	per_d	Global, pipe-specific, and tank-specific first-order bulk reaction coefficients
first_order_wall_reaction_coefficient	First-order reaction-rate coefficient for a substance at a pipe wall	metre per day (m/d)	m/d	m_per_d	Global and pipe-specific first-order wall reaction coefficients
leak_area_per_pipe_length	Effective leak area distributed over pipe length	square millimetre per metre (mm²/m)	mm2/m	mm2_per_m	Pipe leakage-area model
leak_area_expansion_per_pressure_head	Increase in effective leak area per unit pressure head	square millimetre per metre (mm²/m)	mm2/m	mm2_per_m	Pressure-dependent leak-area expansion

Water Quality Quantities

Measurement Quantity Identifier	Definition	Canonical Unit	Canonical UCUM Unit Identifier	AOWIS Identifier Suffix	AOWIS Model / EPANET Mapping
chemical_concentration	Mass concentration of a chemical substance in water	milligram per litre (mg/L)	mg/L	mg_per_l	EPANET chemical-quality input and simulation results
water_age	Time elapsed since water entered the modelled distribution system	hour (h)	h	h	EPANET water-age analysis

Power, Energy, and Electrical Quantities

Measurement Quantity Identifier	Definition	Canonical Unit	Canonical UCUM Unit Identifier	AOWIS Identifier Suffix	AOWIS Model / EPANET Mapping
power	Rate of energy transfer	kilowatt (kW)	kW	kw	Pump power and electrical power
energy	Work performed or energy transferred	kilowatt-hour (kWh)	kW.h	kw_h	Pump energy consumption and electrical energy
energy_intensity	Energy consumed per unit volume of transported water	kilowatt-hour per cubic metre (kWh/m³)	kW.h/m3	kw_h_per_m3	Pump energy-use reporting
electric_current	Rate of electric charge flow	ampere (A)	A	a	Electrical model
voltage	Electric potential difference	volt (V)	V	v	Electrical model
electrical_resistance	Opposition to electric current flow	ohm (Ω)	Ohm	ohm	Electrical model
capacitance	Ability to store electric charge	farad (F)	F	f	Electrical model

Context-Dependent Measurement Values

A numeric field whose measurement quantity or unit depends on another field, enum, object type, equation, or operating mode MUST NOT be assigned a single canonical unit or AOWIS identifier suffix.

Such values MUST be represented using quantity-specific fields or a discriminated data structure that identifies the applicable measurement quantity.

Current Model Concept	Context-Dependent Meaning	Required Resolution
Valve setting	Pressure head, volumetric flow rate, loss coefficient, valve position, or a curve reference, depending on valve type	Replace the generic numeric setting with quantity-specific fields or a discriminated valve-setting structure
Control action setting	The setting quantity supported by the controlled link type	Use the same quantity-specific representation as the target link
Control trigger level or pressure	Water level or pressure head, depending on the controlled node and control type	Represent water-level and pressure-head triggers separately
Rule premise value	Demand, hydraulic head, water level, pressure head, volumetric flow rate, power, elapsed time, time of day, fill time, drain time, or another quantity selected by the rule variable	Store the rule variable together with a quantity-specific value representation
Rule action setting	The setting quantity supported by the target link type	Use the same quantity-specific representation as the target link
Initial quality and simulation quality	Chemical concentration, water age, or source-trace percentage, depending on the quality-analysis mode	Represent each quality-analysis value using its defined measurement quantity
Quality-source strength	Chemical concentration or chemical mass flow rate, depending on the quality-source type	Represent source strength using a source-type-specific structure
Bulk and wall reaction coefficients	The dimensions of the coefficient depend on the configured reaction order	Use reaction-order-specific quantity fields or a discriminated reaction-coefficient structure
Emitter coefficient	Volumetric flow rate divided by pressure head raised to the configured emitter exponent	Store the coefficient together with its exponent in a dedicated structure
Generic curve coordinates	Coordinate quantities determined by the curve type	Use a typed curve structure whenever the coordinate quantities are known
Report thresholds	The measurement quantity of the corresponding report field	Associate each threshold with the report field's defined measurement quantity
EPANET map and backdrop coordinates	Coordinate units determined by the associated EPANET map or backdrop configuration	Convert to an explicitly identified geographic, projected, or local coordinate representation

Data Model

Concrete AOWIS data structures and field definitions are specified by the AOWIS Server Model.

Fixed-schema fields encode their canonical unit through the normative schema and AOWIS identifier suffix; generic or dynamically typed measurement values carry an explicit canonical UCUM unit identifier.

Measurement fields defined by the model MUST use the canonical units specified on this page.