ISO 25377:2020

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.