Overview - ISO 25377:2020 (Hydrometric Uncertainty Guidance, HUG)
ISO 25377:2020, titled Hydrometric uncertainty guidance (HUG), is an ISO standard that explains the nature and treatment of measurement uncertainty for hydrometry. It is applicable to flow measurements in natural and man-made channels (rivers, canals, engineered channels). The standard clarifies how to apply the internationally accepted Guide to the Expression of Uncertainty in Measurement (GUM / ISO/IEC Guide 98-3) to hydrometric practice. Rainfall measurements are excluded from this document.
Key technical topics and requirements
- Foundations from the GUM: Basic definitions, probability models, and the law of propagation of uncertainties applied to hydrometry.
- Random and systematic effects: Identification and separation of random vs systematic contributions to measurement uncertainty.
- Open-channel flow methods: Detailed guidance for the velocity–area method, including mean velocity determination, velocity measurement techniques, integration uncertainties (vertical/horizontal scanning) and perimeter flow uncertainties.
- Critical-depth and structure-based methods: Approach to head/geometry determination, iterative calculation and uncertainty evaluation for critical-depth methods and flow measurement structures.
- Dilution and tracer methods: Support for continuous-feed and transient-mass dilution techniques and associated uncertainty considerations.
- Hydrometric instrumentation: Performance specifications, validity of manufacturer uncertainty claims, and a performance guide for hydrometric equipment.
- Uncertainty reporting and drafting: Guidance for drafting uncertainty clauses in hydrometric standards and expressing results consistently with the GUM.
- Examples and worked cases: Practical worked examples (water level sensors, pressure transmitters, stage–discharge relations, current meters) to illustrate uncertainty estimation.
- Informative annexes: Introduction to hydrometric uncertainty, Monte Carlo Simulation (MCS), Interpolated Variance Estimation (IVE), and stage–discharge uncertainty analysis.
Practical applications and users
ISO 25377:2020 is intended for professionals who need robust, comparable flow data and uncertainty statements:
- Hydrometrists and field technicians performing river and channel gauging
- Water resources and environmental engineers (flood management, water allocation, irrigation)
- Regulators and policymakers requiring traceable measurement quality for compliance and reporting
- Instrument manufacturers and laboratories validating performance specifications and uncertainty claims
- Standards writers and quality managers drafting uncertainty clauses for technical standards and contracts
By aligning hydrometric practice with the GUM, ISO 25377:2020 enables consistent, defensible uncertainty estimates for flow measurements used in environmental monitoring, regulatory compliance, and engineering design.
Related standards
- ISO/IEC Guide 98-3 (GUM) - the foundational guide for expressing measurement uncertainty, which HUG adapts specifically for hydrometry.
