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Methods of Developing Rating Curves

According to the International Commission on Water Resources Systems, a rating curve is a graph showing the relationship between the water level of a river -- the "stage" -- to the discharge, or amount of water flowing away. The length of time of a river's low flow can thus be worked out. This is important information, especially for water supply management purposes.

  1. Using a Flow Meter

    • Many different types of flow meter are available for streams, rivers, sewers, mining channels and other open channels. However, whilst a flow meter can yield much valuable information, according to hydrologists E. L. Upp and Paul J. LaNasa, great care must be taken in placing the device. The water must first be examined in terms of its PVT, or pressure, volume and temperature. It must have a flow pattern that is reasonably constant and that can be anticipated. The water must contain no unacceptable foreign material and not be swirling at the point where the meter is placed. Only then will the data collected be good enough to generate a useful rating curve.

    Using Historical Data Sources

    • According to the International Association of Hydrological Sciences (IAHS), rating curves can be generated from historical data sources, such as data on suspended sediment concentration in water. The data referred to in the IAHS case study is from the Murrumbidgee River catchment in Australia. However, a note of caution is sounded. Whilst generating the rating curve is fairly straightforward, since it is a simply plotted x-axis/y-axis graph, the provenance of the data needs to be established. In this study, a mixture of sampling strategies was used in the compilation of the data. Further, the exact purpose of the sampling was unknown, as were the precise sampling dates. All of these things serve to make the generated ratings curves less accurate and much less useful.

    Using Mathematical Formulae

    • Ideally, onsite measurements should be used to generate rating curves. However, according to urban hydrologists A. Osman Akan and Robert J. Houghtalen, mathematical formulae can be used. Average values may be needed in the working out of the formula used, which will have an effect on the integrity of the rating curve. However, for some purposes, such as teaching the methods of water flow analysis, using mathematical formulae is quite adequate.

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