WMO Recognizes Image Velocimetry as Core Hydrological Method in New Guide

Image velocimetry is a remote sensing technique that uses digital cameras and video analysis to estimate surface flow velocities in open water bodies such as rivers, canals, and streams.

The World Meteorological Organization (WMO) has taken a landmark step in modern hydrometry with the introduction of a new chapter on image velocimetry in its Guide to Hydrological Practice, Volume I: Hydrology – From Measurement to Hydrological Information (WMO-No. 168, 2025 edition). This addition marks the beginning of a broader update to the authoritative reference for hydrological services worldwide, aligning field practices with innovative and cost-effective methods for measuring rivers and streams.

What is Image Velocimetry?

Image velocimetry is a remote sensing technique that uses digital cameras and video analysis to estimate surface flow velocities in open water bodies such as rivers, canals, and streams. By tracking the movement of naturally occurring features—such as foam, leaves, or sediment—across video frames, flow velocity can be determined without direct contact with the water.

Compared with traditional in-situ methods, image velocimetry offers significant advantages:

• Safety – Technicians and hydrologists can avoid entering hazardous rivers, particularly during floods or extreme events.

• Cost-effectiveness – Minimal equipment and reduced field labor lower overall operational costs.

• Accessibility – Methods can be deployed in remote or hard-to-reach areas where conventional instruments are impractical.

• Scalability – Cameras can be installed permanently or used in mobile deployments, enabling both long-term monitoring and rapid-response flood assessments.

A New Chapter in the WMO Guide

The WMO Guide to Hydrological Practice has long been the international standard for hydrological procedures, offering practices, specifications, and protocols that help members achieve compliance and consistency. By incorporating image velocimetry into this framework, WMO sends a clear signal that these once-experimental methods are now part of the recognized global hydrometric toolkit.

The chapter provides:

• Technical guidance on best practices for deploying image velocimetry.

• Standards and specifications for ensuring data accuracy and comparability.

• Recommendations for integration with existing hydrological monitoring systems.

• Case studies showcasing successful applications in flood monitoring, river gauging, and real-time hydrological services.

From Innovation to Mainstream Practice

The inclusion of image velocimetry reflects more than a decade of innovation, testing, and collaboration across the hydrological community. What was once a niche research technique has now matured into a robust operational tool. Its recognition in the WMO Guide represents a turning point, signaling to practitioners, institutions, and policymakers that image-based methods are ready for mainstream adoption.

For national hydrological services, this development provides institutional confidence and helps overcome hesitancy around emerging technologies. It confirms that investing in image-based monitoring systems can strengthen observation networks, improve data availability, and enhance resilience against water-related challenges such as floods, droughts, and climate variability.

Benefits for Water Managers and Policymakers

The chapter is not just a technical update—it carries broader implications for decision-makers and managers:

• Risk reduction: By reducing field exposure during dangerous conditions, staff safety is prioritized.

• Data-driven governance: Reliable and continuous flow measurements support better planning, early warning systems, and climate adaptation strategies.

• Capacity building: The chapter encourages knowledge-sharing and skills development among hydrological institutions.

• Global harmonization: By standardizing practices, data from different regions becomes more comparable, supporting transboundary water management.

Looking Ahead

As climate change increases the frequency and severity of water-related hazards, the demand for robust and innovative hydrological monitoring tools continues to grow. By formally recognizing image velocimetry, WMO has reinforced the importance of digital, contactless methods in building resilient water information systems.

This milestone is expected to pave the way for further updates in the Guide, incorporating other emerging technologies such as drone-based hydrometry, satellite observations, and machine learning applications. For the global hydrological community, it underscores the shift towards safer, smarter, and more integrated water monitoring practices.