Rating Tables and Curves
A rating table or curve is a relationship between stage and discharge at a cross section of a river. In most cases, data from stream gages are collected as stage data. In order to model the streams and rivers, the data needs to be expressed as stream flow using rating tables. Conversely, the output from a hydrologic model is a flow, which can then be expressed as stage for dissemination to the public.
The USGS develops and maintains most of the rating tables used by the National Weather Service. Ratings need to be updated periodically due to changes in the river channels (i.e. after a flood). It is important to keep ratings up to date and to ensure that all offices (RFC and WFO) are using the same rating for each gage so that forecasts are consistent and as accurate as possible.
Figure 1. Example of a Rating Curve
To develop a rating curve, the USGS makes a series of streamflow measurements using a current meter. These points are plotted versus the accompanying stage, and a smooth curve is drawn through the points. There can be significant scatter around this curve. Because of this, when using a rating curve, it is good to keep in mind that the discharge read from the curve is the most likely value, but it could be a little different from the measured value. Also, since rating curves are developed with few stage/discharge measurements, and measurements of high flows are rare, there can be significant errors in rating curves at high levels, especially around record level flows.
At times, the USGS will go out and take measurements during floods and provide these readings to the National Weather Service. These values can then be used to adjust the upper end of the rating curve.
Obtaining and updating rating tables:
The process to keep rating tables up to date is different from office to office. Most WFOs receive updates to USGS rating tables automatically from the RFC through Hydromet. Such a process ensures that all offices (WFOs and RFCs) are using the same ratings, which is essential to operating the hydrology program. For river locations rated by other agencies (counties, USBR, etc..), or for USGS ratings not updated by the RFC through Hydromet, WFOs must work in coordination with the RFCs to obtain the latest ratings from the concerned agencies, and must ensure everyone has been provided with this latest rating (neighboring WFOs, backup WFOs, RFCs, etc..).
Rating curves usually have a break point, which is around the stage at which the river spreads out of it's banks, or it could be at a lower stage if the river bed cross section changes dramatically. Above that stage, the river does not rise as fast, given that other conditions remain constant. This is illustrated by a change in slope in the rating curve. On this figure the break point appears to be around 6-7 feet.