Using simple estimates of uncertainty to set and monitor standards of hydrometric data

John Fenwick - NZ NIWA , 22 August, 2008

In New Zealand, standards for hydrometric data have focused on statements relating to the accuracy of stage and flow measurements and continuity of records. Australian agencies have tended to follow this philosophy, but often also provide information in the form of quality codes attached to data points that provide the informed data user with indications of the sensor accuracy and/or characteristics.

For flow gauging accuracy, many procedures have followed the recommendations for determining uncertainty as described in the ISO Standard 748. This requires that the individual measurements and calibrations that comprise the components of a current meter gauging each be assigned a statistical uncertainty, and then these are combined in an algorithm that can be applied to each individual gauging.

It is suggested that a similar process be applied to the components of a streamflow record so that flow data can be provided to users with a more meaningful estimate of uncertainty. A simplistic example is presented, using mainly the Type 2 method of estimating uncertainty (this principally uses subjective but experienced judgement). The application of the method is discussed in the context of setting uncertainty objectives and the practicality of measuring their achievement or otherwise.

The use of uncertainty by informed users may require some promulgation of the concept, but the concepts are described in the literature, particularly in the ISO domain, and it is argued that the concept is under-utilised in our industry. Although this approach is simplistic and does not address other issues, this review suggests one way of improving the standards we use to set standards of data quality and describe them to data users.