AULOS software models interconnected open and closed channels

Calibration and Validation: Is there a methodology to deal with errors in calibration and validation at a gauging station?

Often modellers are asked to make an estimate of calibration and verification accuracy based on errors in matching the hydrographs measures at gauging sites. I suggest this starts at the wrong end in analysing these errors, because the modeller has to deal with a mixture of level errors (at the gauge) and flow errors (everywhere else).

If instead the gauge levels are specified as boundary conditions, the model will then be forced to estimate the flows at the gauging point, and these can be compared with any flow measurements at the same point. Then the model will exactly match the gauged levels and all errors will be flow errors. This method of calibration is called Residual Flow Analysis, because it examines the residual left after all measured and estimated flows have been taken into account.

For example, the computed model should indicate that at the same gauge level more flow is passing through on a rising flood than during a flood recession, and one significant error may be ignoring this loop rating effect. Another error which can be explored by this method is the choice of the wrong rain gauge for the rainfall-runoff model, as a sudden burst of rainfall should create a delayed response in the computed flow hydrograph.

Such elimination of gauge level errors effectively uses hydraulic modelling to produce a lateral inflow hydrograph for the modelled river reach, and this should match with the local catchment outflow hydrograph produced by hydrological rainfall/runoff modelling. If this match is poor, both reach inflow and catchment outflow must be reconsidered and adjusted to improve the match.

This approach provides a systematic methodology for measuring and improving the accuracy of fit between the lateral flow hydrographs produced by hydraulic and hydrological modelling.

More details can be read via a download of the paper Barnett A.G., C. Hellberg “Storm Lateral Runoff Calibration by Direct Hydraulic Balances” Stormwater 2013, Water New Zealand, Auckland, May 2013 - click here (This is an updated version of Barnett A.G., C. Hellberg “Residual Flow Modelling: An Advance in Channel Resistance Calibration” Paper 3370, Proc. 34th IAHR World Congress, Brisbane, Australia June-July 2011.)

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