19 Rating curves

Rating Curves Module Configuration

The ratings curve module is a standard file (i.e. it has a fixed name) which contains all the rating curves used in your system. It is referenced by the location id. I.e. each location has a specific rating curve for that point in the river.

The ratings are either "qhrelationtable" or "simpleratingcurve" refer to by the hydroMeteoFunction in the transformation module. An example of the reference from the transformation module is shown below:

<hydroMeteoFunction function="qhrelationtable" outputVariableId="Flow" useRatingCurve="true" ratingCurveType="LevelToFlow"/>
or
<hydroMeteoFunction function="simpleratingcurve" outputVariableId="Flow" useRatingCurve="true" ratingCurveType="LevelToFlow"/>

Rating curve

Figure 1: Overview of rating curve config file

location for which the rating is valid should be the same as in the locations.xml
ratingCurveType you can choose from either LevelToFlow or FlowToLevel
reversible if this option is set to "true" then both level to flow and flow to level calculations with be allowed
ValidPeriod allows you to enter a start and end date for which the rating is valid (i.e. a summer and winter rating)
correction see below
ratingCurveTable see below
ratingCurveEquation see below

correction

Figure 2: correction complex type

The correction complex type allows the user to specify a correction technique for unsteady flow (jones equation) or backwater (constant fall method or normal fall method).

jonesEquation the user must specify the minimum h for which the method is valid (h_min and the a, b and c parameters - see below)

The Jones Equation is of the form:

where:
Qm = unsteady discharge
Qc = steady discharge
S0 = energy slope for steady flow
vw = wave velocity
dh/dt = rate of charge of water level in time (m/day)

The adjustment factor 1/S0vw (day/m) varies with water level. This factor is fitted by a parabolic function of h:

for h>hmin

Twin gauge station fall-discharge methods

Stage-fall-discharge or twin gauge station fall-discharge methods are user to include backwater effects on stage-discharge ratings.

In these methods the fall F between the water level at the discharge measuring site and a downstream station is considered as an additional parameter, to account for the effect of water surface slope on discharge. Both the constant fall method and normal fall method are based on the following equation:

Where:
Qm = backwater affected discharge
Qr = reference discharge
Fm = measured fall
Fr = reference fall
p = power, with 0.4 < p < 0.6

constant fall method

In this method the reference fall Fr is taken as a constant. A special case of the constant-fall method is the unit-fall method, where Fr = 1m is applied. In the computational procedure a value for Fr is assumed. Then a rating curve is fitted to the values:

normal fall method

In this method the reference fall (Fr) is modelled as a funtion of the water level; Fr = f(h). This funtion is represented by a parabola:

In Fews you should specify the a, b and c parameters and also a value of hmin, below which the backwater correction is not valid.

Rating Curve Table

ratingCurveTable this allows you to simply enter the pairs of q and h values.

  </ratingCurve>
  <ratingCurve ratingCurveId="TheBigRiver">
    <location>
      <locationId>X1123</locationId>
    </location>
    <ratingCurveType>LevelToFlow</ratingCurveType>
    <reversible>true</reversible>
    <ratingCurveTable>
      <ratingCurveTableRecord flow="0.100" level="0.054"/>
      <ratingCurveTableRecord flow="0.500" level="0.155"/>
      <ratingCurveTableRecord flow="1.000" level="0.244"/>
      <ratingCurveTableRecord flow="1.479" level="0.317"/>
    </ratingCurveTable>
  </ratingCurve>

Rating Curve Equation

A rating curve equation can be defined per section of the rating curve using the lowerLevel and UpperLevel tags. The equation can be in the form of a power equation or a parabola. The form of the equation:

Power equation
Parabola

Here is an example:

  <ratingCurve ratingCurveId="Test location">
    <location>
      <locationId>234206</locationId>
    </location>
    <ratingCurveType>LevelToFlow</ratingCurveType>
    <reversible>true</reversible>
    <ratingCurveEquation>
      <lowerLevel>0</lowerLevel>
      <upperLevel>0.391</upperLevel>
      <equation>Power</equation>
      <a>11.9001</a>
      <b>0</b>
      <c>1.55067</c>
    </ratingCurveEquation>
    <ratingCurveEquation>
      <lowerLevel>0.391</lowerLevel>
      <upperLevel>0.807</upperLevel>
      <equation>Power</equation>
      <a>16.6258</a>
      <b>-0.1</b>
      <c>1.8564</c>
    </ratingCurveEquation>
  </ratingCurve>