A decision support system for the analysis and use of stage-discharge rating curves

A decision support system for fitting stage-discharge relations is being developed by the University of Manitoba, Facility for Intelligent Decision Support, for Environment Canada's hydrometric program. The process of developing and applying stage-discharge relationships has been documented, the decision support system framework established, and a number of expert 'rules' developed and evaluated for stable and non-stable channels. The system categorizes stage-discharge rating curves as stable if they deviate within a predetermined accuracy (e.g. +/- 5% of discharge), or if one of a suite of curves can be applied within the predetermined accuracy specifications for given changes in the energy slope caused by weed growth, channel ice and other conditions affecting backwater. Non-stable channels are defined as channels that experience evolution or abrupt changes in stage-discharge relationships resulting from changes in channel form. Rating curve stability is determined based on deviations in the stage-discharge relationship, utilizing specific gauge, absolute differences between sequential streamflow measurements and an analysis of residuals. Periods of instability, such as a meander cut-off changing the hydraulic control, are often repetitive in hydraulic response, such that a pattern of instability is recognizable as events occur. In addition, periods of instability are interspersed with periods of stability. The repetition in pattern allows knowledge of correctly identified historic events to be applied to contemporary hydraulic conditions. A suite of rating curves can be developed for specific periods (e.g. summer weed growth) and stages. These curves can be automatically applied based on changing streamflow conditions such that streamflow estimates can be generated in a rigorous and accurate manner with minimal hydrographer intervention.