Investigating the uncertainty in flow at gauging sites in Gippsland using Australian Standard 3778.2.3

Nurullah Ozbey, Phil Scanlon, Andrew Western - Thiess Services, University of Melbourne , 22 August, 2008

The methodology outlined in Australian Standard 3778.2.3 was used to estimate the uncertainty in the flow data reported for 80+ sites in Gippsland. The flow data is obtained by applying a rating table to monitored water levels. The uncertainty estimated in these flow data results from two sources: the uncertainty in the flow estimates due to uncertainty in the recorded water level; and the uncertainty in the flow estimates due to the uncertainty in the rating table.

A number of problems were encountered using the methodology, not all of which could be satisfactorily resolved and which had to be worked around. However, given that at present there are no uncertainty estimates for flow data routinely available, an indication of the range of uncertainty in the reported flow values was calculated from the data available. Results indicated the uncertainty in the annual flow at most monitoring sites in Gippsland ranged from +/- 5% to +/- 15% in 2005-2006. These results provide valuable guidance for decisions on changes in monitoring protocols to reduce uncertainty where desired and possible.

The process of estimating the uncertainty in the flow provided additional valuable information about the data collected at the monitoring sites. In addition, uncertainty calculations can contribute to auditing procedures by highlighting sites that require investigation. However, further work is required to enable the consistent calculation of uncertainty by different parties. Uncertainty in the flow should be reported as a routine part of data dissemination to enable the user to take into account the confidence level of hydrological data in decision making.