Rating Curves Workshop – International Best Practices Explored in New Zealand

International Rating Curve Standards

Stage-discharge rating curves define a unique relation between water level and discharge, enabling continuous derivation of streamflow from water level record. This is important because water level (which is relatively easy to monitor) is only locally relevant whereas discharge (which is relatively difficult to measure directly) is the integral of all runoff processes upstream of the gauge. The vast majority of all streamflow data that has ever been produced is a derived result of a rating curve.

In other words, almost everything that we know (or rather that we think we know) about hydrology is a result of rating curves.

The problem with this paradigm is that there has never been global consensus on best practices for the development and management of rating curves. Assuming stable control features, steady flow, and an abundance of gaugings evenly distributed over the full range of stage and time, then regional differences in methods for rating curve development might be interesting but would probably not be very impactful. However, it is rarely the case that any rating curve problem is fully constrained to reveal a unique solution regardless of approach.

Given that ratings are managed in the real world – not some idealized world of fully predictable hydraulic conditions – it’s curious that there has, historically, been very little interest in the dependency of trillions of dollars’ worth of water resource decisions on the truthfulness of rating curves.

It’s almost like a Stockholm syndrome phenomenon.

Hydrologists, engineers, water resource managers, along with the many other users of flow data are held hostage by their dependence on rating curves. They are grateful to get any data at all and loathe to reject data when there really isn’t any realistic alternative to the status quo.

It is therefore up to the hydrometric community to step up to the challenge of setting standards and developing best practices to ensure that the results of rating curve derivation are, at the very least, consistently meaningful.

It’s not the case that all derived discharge results need to have equally low uncertainty.

However, it is the case that if confidence in the derived result is low that there should be some way of communicating that, under the circumstances, an explicit determination of discharge is not possible. Even when the quantity of flow is uncertain the data may be extremely useful for determining the timing, routing, and relative magnitude of peak events and should certainly inform where some change in the monitoring plan is required to produce better data in the future.

The New Zealand Hydrological Society is showing leadership in developing a path forward. A ratings workshop in Christchurch on the 11th and 12th of April was motivated, in part, by the publication of the National Environmental Monitoring Standard (NEMS) for the construction of stage-discharge and velocity-index ratings and, in part, by discussions with respect to the International Streamflow Rating Curve Project (ISRCP).

The workshop started with an international perspective of how ratings are managed in the United States, Canada, France, Australia, and New Zealand. The rest of the workshop explored many of the issues that need to be well understood to guide development of a best practice approach. All workshop presentations are available online on the NZHS web site.

This may be the first, but it definitely should not be the last international workshop on ratings.

There is interest in holding a similar workshop in Australia and I will be promoting the idea of North American Stream Hydrographers (NASH) hosting a ratings workshop at the CWRA conference in Lethbridge Alberta in 2017. It would be best if we can also inspire similar workshops in Europe, Asia, South America, and Africa. Please let me know if you would like to be involved with organizing a workshop in your region.


Free Whitepaper: 5 Best Practices for Building Better Stage-Discharge Rating Curves

A reliable rating curve is one that is credible, defensible, and minimizes re-work. This paper outlines 5 modern best practices used by highly effective hydrographers. Read whitepaper here.

4 responses to “Rating Curves Workshop – International Best Practices Explored in New Zealand”

  1. Ferdinand Quiñones April 24, 2016 at 6:22 am

    The USGS has had standards for the development of rating curves for more than 100 years. It has published several handbooks in its series on “Techniques on Water Resources Investigations” describing the best methods and procedures for most types of field conditions. These reports are readily available in the USGS web page. You are again discovering the wheel…..

    • Hi Ferdinand,
      I had surmised that the historic lack of interest in rating curve development and management was due to the Stockholm syndrome but I think you have highlighted a more plausible explanation. The lack of interest is because it is widely perceived to be a ‘solved problem’. The prevailing attitude is that developing and managing rating curves is no more challenging than baking a cake. You just need a recipe book such as USGS WSP 2175 (Rantz et al. 1982), USGS TWRI 3-A13 (Kennedy, 1983), USGS WRIR 01-4044 (Sauer, 2002) or WMO 1044 vII (WMO,2010).

      It is certainly true that it is a ‘best practice’ for the development and management of rating curves to be conformant with a trusted standard (see, for example, my whitepaper on best practices for rating curves). Conformance with a known standard ensures that the process is the same but it does not ensure the result is the same. If you give the same set of data to 10 different hydrographers you will likely get 10 different results even if they all follow the same procedural guidelines.

      The distinction between a’standard procedure’ approach and a ‘best practice’ is that the objective for a best practice is not conformance but rather convergence. Specifically, convergence on the truth. Whereas there are many standards, and more are being written all the time, there can only be one practice that is ‘best’. If only we could find out what that is.

      In our hypothetical 10 different hydrographs – produced by 10 different hydrographers working from the same set of information – the ‘truth’ will not be close to the average of the results, the truth will be closest to the results produced by the most experienced hydrographers with the greatest familiarity with the hydrology, hydraulics, biology, geomorphology, and operations of the gauge. Experience is not something that can easily be written into a standard.

      There has never been a truly global dialog about what it is that the most experienced hydrographers around the globe do similarly to each other (regardless of which standard they choose to follow) and what differentiates them from naive hydrographers such that these similarities and differences enable them to reliably produce truthful derived discharge results. The investigation of best practices is a systematic attempt to capture the knowledge and experience of the ‘best’ into ‘practical’ guidance.

      In our hypothetical example of 10 cooks baking cakes using the same ingredients and the same recipe, it doesn’t matter if the cakes don’t all taste the same. If one cake tastes bad you can just spit it out. This is not the case with discharge data production. If even one hydrographer produces mis-informative discharge record then all discharge records are placed under suspicion of having a similarly low quality. Furthermore, trillions of dollars of water resource decisions are made annually based on hydrometric data. Every error in the data has economic, social and environmental repercussions.

  2. Jagat K Bhusal April 25, 2016 at 4:24 pm

    Hi Hamilton
    I appreciate your idea of sharing best practices of Rating Curves thru Workshops. The society of hydrologists and Meteorologists Nepal would like to host similar workshop in Nepal so that hydrologists of SAARC region could be benefited. Plz advise us (bhusaljagat@gmail.com) how to proceed to get grant if available to organize the workshop.

  3. Sylvand M Kamugisha May 3, 2016 at 5:52 am

    Dear Stu Hamilton
    Your contribution as always been supper. Rating curve development or validation is still a challenge in certain part of the globe – including my country Tanzania. It may take time to brake through as the new tools being developed are not easily available due to cost implication; you may get them through donors, but you fail to maintain them – sustainability issue; investment in this area is not “stable as our river station controls” are. While stage data collection may be easy but discharge measurements to cover the entire range has been a challenge. The best practices that will ensure the developer of the rating curve get “good” flow measurements at higher stages may add value in the process and motivate more hydrographers in the practice, the profession is attracting few people. Experience, interest, knowledge of the area and the subject, appropriate tools and proper documentation of the past will contribute heavily in sharing the best practices. If this type of workshop happen to be held in East Africa it would be great as all development designs related to water depend on the rating curves we are developing in a challenging situation.

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