Introduction & background

The last twenty-five years have seen great advances in the instrumentation available for hydrometry and in the procedures and methods for the measurement of discharge. Due to several reasons, this progress has not seen a parallel development of guidance material and standards.

At its Twelfth Session (Geneva, October 2004), the Commission for Hydrology of WMO, having reached a general consensus that information on the appropriate use of this instrumentation and methodologies is direly needed, identified under the priority thematic area "Basic systems (hydrometry and hydraulics)" an activity to develop a proposal and implement a project to assess the performance of flow measurement instruments and techniques against WMO standards. [Read more]

Project activities - Progress achieved in the 2013-2016 period

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Project activities - Progress achieved in the 2009-2012 period

The project’s Work Plan continues to evolve, with the most up-to-date version being available on the project’s working website. The project currently comprises 7 core project activities.

The first two project outputs , which have now been completed, comprised surveys directed to National Hydrological Services. They were pertaining to:

The results of the first project output have been published here*, while the results of the second output on standards have led to the discharge techniques database. The results of the survey on use and popularity of instrumentation and techniques have been used to help guide and prioritize the development of other project outputs and their products.

The third project output focused on developing a framework of the assessment of uncertainty in discharge measurements and guidelines for its implementation, which are available here. In February 2009, the AWG, on behalf of CHy, adopted the uncertainty analysis framework for flow measurements as developed by the project, which is based on the Guide to the Expression of Uncertainties in Measurements (GUM, 1993).

The adopted report describes in detail the approaches to estimating uncertainty and outlines step-by-step procedures for its practical implementation. A suite of three detailed examples have been prepared to illustrate the implementation of the GUM framework to hydrometry. The examples included:

1. *The weighing and timing measurement of discharge;
2. *The measurement of discharge using point velocity meters and the velocity area method ;and
3. *The measurement of discharge in a sewer pipe flowing with free surface.

Given the complexity and extensive computational effort involved in the application of GUM to hydrometry, the project output 3 explored the development of a decision aid tool to facilitate its implementation by NHSs.  For this purpose, several available off-the-shelf GUM-based software packages were first critically evaluated and reported upon (available here). One commercially available software package was identified as being most promising and was subsequently applied to two typical flow measurement instruments and techniques, with documentation of the application being available here. The Project's Management Committee concluded that although the selected software is helpful in advancing the analysis of uncertainty, more effort is needed to customize the software to facilitate its implementation and use by NHSs.

The fourth project outcome focused on guidelines for conducting and reporting on the calibration and verification of the performance of discharge measurement instruments (draft report available here).  The report provides a number of examples of performance specifications for a small selection of popular hydrometric instruments and provides guidance on the reporting of instrument calibration and verification results. The latter aspect is to be further developed following the completion of report on protocols and specifications.

The fifth project outcome is the collection of test reports on the performance of instruments and techniques, with the creation of a website to make available the collected information. It is anticipated that this effort will commence following completion of project output four.

The sixth project outcome is focused on the uncertainty analysis of discharge determination via various techniques. The project outcome has been broken into three related initiatives.

• *The first is a literature review of the methods for estimating the uncertainty of the most frequently used technique as identified in the survey resulting from project outcome one, namely the stage-discharge relation.
• *The second is the identification and recommendation of a general methodology including terminology to be used in the estimation of the uncertainty of the determined discharge.
• The third aspect is the development of examples that illustrate the application of the proposed methodology for estimating the uncertainty of the determined discharge (available soon).

This project output when implemented should provide NHSs with approaches to estimate the uncertainty of continuously produced stream flow or related products over time such as daily mean flow or monthly flow.

Project output seven was developed to allow the provision of guidance and recommendations to NHSs, manufacturers, organizations and users of hydrometric data. It is anticipated that this effort will commence once project outcome 4 has been completed.

References

JCGM 100:2008, Evaluation of measurement data — Guide to the expression of uncertainty in measurement (GUM 1995 with minor corrections)

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