ISO 18320:2020

Overview

ISO 18320:2020 - Hydrometry: Measurement of liquid flow in open channels - Determination of the stage–discharge relationship provides standardized methods for developing and maintaining a reliable relationship (rating curve) between water stage (gauge height) and discharge (flow) at gauging stations. The standard replaces ISO 1100‑2:2010 and specifies accuracy criteria, required numbers of discharge measurements with matching stage readings, and guidance for both stable and unstable channels. It also addresses effects such as the transition from inbank to overbank flows, shifting controls, variable backwater and hysteresis.

Key topics and technical requirements

• Principle of stage–discharge relationships: definitions, controls (section vs channel control), and governing hydraulic formulae.
• Calibration and preparation: compiling discharge measurement lists, plotting (arithmetic and logarithmic), and preparing rating‑curve shapes.
• Curve fitting methods: hydraulic‑formula curves and mathematical rating curves; guidance on software tools to fit ratings.
• Complex behaviour: handling combination controls, shifting controls, inbank/overbank transitions, downstream backwater and hysteresis (loop ratings).
• Extrapolation and testing: procedures for extrapolating ratings beyond measured ranges and methods to test rating performance.
• Uncertainty analysis: statistical analysis of the rating, standard error of estimate, standard and expanded uncertainty of predicted discharge.
• Informative annexes: types of control, complexities, software packages, worked examples, extrapolation and uncertainty worked examples.

Note: ISO 18320 does not provide detailed methods for twin‑gauge stations or ultrasonic velocity‑measurement stations; these are covered in other ISO documents listed in the bibliography.

Applications and users

ISO 18320 is practical and essential for:

• Hydrologists and river gauging teams developing continuous discharge records from stage data.
• Water resources and flood‑management engineers calibrating gauging stations and validating flow records.
• Environmental agencies, water utilities and researchers needing standardized, traceable methods for flow estimation and uncertainty reporting.
• Developers of hydrometric software and decision‑support tools that implement rating‑curve fitting and uncertainty analysis.

Practical benefits include improved comparability of flow time series, better flood forecasting inputs, and documented uncertainty for water‑resource decision making.

Related standards

• ISO 748 (current-meter or float methods for open channels)
• ISO 772 (hydrometry vocabulary and symbols)
• Other ISO Technical Specifications and Reports covering twin‑gauge and ultrasonic velocity systems (see the ISO 18320 bibliography)

Keywords: ISO 18320:2020, hydrometry, stage–discharge relationship, rating curve, gauging stations, uncertainty analysis, inbank overbank, shifting controls, hydraulic calibration.