F. Rating Streams and Computing Discharge
A thorough understanding of the relationship between stage and discharge is the foundation of hydrometric work. This section details the steps involved in converting the field data to actual flow data set to Provincial Standards.
Discharge is the volume of water flowing through a given cross section of a watercourse during a given or implied time period. A measurement of discharge at a given point in time has limited value by itself because it does not allow determination of daily or monthly flows or other flow parameters which are required to understand the streamflow regime. But for any stream location there is a correlation between water level and discharge, that once established, allows one to record the water level, which is relatively easy to do on a continuous basis, and estimate discharge from this correlation (water level, when recorded against a fixed reference is referred to as gauge height or stage).
Daily discharges are rarely measured directly because of the effort required. On occasion, when the effort is justified, it can be approximated by near continuous metering.
This Section will describe the stage-discharge relation and attempt to portray its importance to the practice of hydrometric surveying.
Daily or continuous discharge data can not practically be obtained directly. It is however possible to obtain daily or continuous stage data and from that a continuous discharge record can be estimated based on this relationship of water level and flow. The result is a correlation called the stage-discharge relationship.
To develop this relationship, discharge measurements are obtained at the gauging station over the maximum range of gauge heights possible. A history of the relationship evolves over time, as each discharge measurement and corresponding stage is plotted, and a smooth curve is drawn that best represents these points (Figure F-1). This curve is converted to a table of discharge values for incremental gauge heights, which in turn is referred to as a stage-discharge table. Daily or continuous discharge can be derived from this table using a daily or continuous stage record.
The stage-discharge rating curve has historically been drawn by hand on standard arithmetical forms. This form has been adapted to the RIC Standards Program and is available as Form AQU-09 (Figure F-1).
Rating curve drawing can be performed mathematically by computer using standard graphics software as well as more specific applications. But few hydrometric sites can be well represented by this method and therefore computer fit curves are not acceptable for Provincial Standards. The reasons for the restriction against computer plots include:
Stream geometry changes with depth and requires more than one equation.
The relationship between plotted points is affected by chronological order. In a manual plot, dates are added and must be constantly referenced.
Confidence in data is not constant. Field personnel must give weight to the measurements based on experience.
The difference between an errant measurement and a shift in the stage-discharge curve is a grey area that is best resolved using skilled judgment when drawing the curve manually.
Figure F-1. Example of a Stage-Discharge Rating Curve RIC Form AQU-08, shown half size.
