Overview
ISO 9123:2017 - Hydrometry - Stage-fall-discharge relationships - provides standardized methods for determining streamflow relationships in reaches affected by variable backwater (intermittent or continuous). The standard defines measurement arrangements, analysis approaches and uncertainty assessment procedures for cases where simple stage-discharge relations are not adequate. It is specifically aimed at steady, gradually varied flows and requires paired gauge measurements (a base reference gauge and an auxiliary gauge) together with a set of discharge measurements to calibrate ratings.
Key topics and requirements
- Scope and setup
- Applies when variable backwater influences gauge height for a given discharge.
- Requires two gauging stations: a base reference gauge and an auxiliary gauge for stage/fall measurements.
- Multiple discharge measurements are needed to calibrate the rating to the accuracy required.
- Methods covered
- Unit-fall method
- Constant-fall method
- Variable-fall methods, including normal-fall and limiting-fall
- Worked examples and computation procedures for each method are provided.
- Analysis and computation
- Procedures for computing discharge from stage and fall measurements.
- Statistical methods (including multiple least squares regression - Annex A) for parameter estimation.
- Uncertainty and quality
- Guidance on evaluating uncertainty consistent with GUM (Guide to the Expression of Uncertainty in Measurement) and hydrometric uncertainty guidance (HUG).
- Topics include propagation of uncertainty, prediction intervals, uncertainty in measured stage/fall and in rating curves.
- Operational considerations
- Periodic checking of stage-fall-discharge ratings, extrapolation cautions, and notes that detailed preparation of rating curves is referenced to other standards.
Applications and users
ISO 9123 is intended for professionals and organizations involved in streamflow measurement and water-resource monitoring where backwater effects complicate discharge estimation:
- Hydrologists and hydrometric technicians
- River and reservoir engineers
- Water-resource and environmental agencies
- Flood forecasting and modeling teams
- Survey and instrumentation contractors
Practical applications include gauging in tidal reaches, regulated rivers downstream of structures (dams, bridges), confluences with downstream tributaries, and any site where variable backwater changes the energy gradient and stage–discharge behavior.
Related standards
- ISO 1100-2 - Measurement of liquid flow in open channels - Determination of the stage-discharge relationship (for detailed rating-curve preparation)
- ISO 772 - Hydrometry - Vocabulary and symbols
- GUM / HUG - Guidance for uncertainty evaluation in hydrometry
Key keywords: ISO 9123, hydrometry, stage-fall-discharge, backwater, gauging stations, rating curves, uncertainty, GUM, HUG.