The rating curve is described by the discharge (i.e. outflow) and storage relationship. Note that the stage or water depth variable is taken out of the input, since both discharge and storage are a function of stage. The Stage-Storage and Stage-Discharge rating curves are essentially combined into one Discharge-Storage curve. An example of a Discharge-Storage Curve is as follows:

Discharge (m³/s) Storage (ha-m)
0.00 0.00
0.06 0.34
0.21 0.48
0.37 0.60
0.66 0.83
0.94 1.00

Designing a Discharge-Storage curve, at the watershed or sub-watershed planning level, involves determining each storage ordinate for every given discharge ordinate. Discharge ordinates are usually known or can readily be determined. They may represent allowable flows or release rates that when combined with other flows are the allowable flows at key locations. Storage ordinates are what the modeler is trying to calculate in order to meet the discharge targets.

For single event analysis the Discharge-Storage curve is built from the smallest to largest values, which corresponds to the smallest to largest rainfall events. For example, the above Discharge-Storage curve was based on the following design storm events.

Discharge (m³/s) Storage (ha-m) Design Storm
0.06 0.34 25 mm
0.21 0.48 2 year
0.37 0.60 5 year
0.66 0.83 25 year
0.94 1.00 100 year

When building a curve, the storms must be run from smallest to largest and the storage iterated until the pond outflow matches that of the target value in the Discharge-Storage curve. Only then can the modeler move onto the next largest storm. The proper pond sizing methodology is therefore:

The modeler enters the first 2 sets of points on the curve, (0,0) and the first target flows (e.g. 0.06). The modeler guesses a storage value and then runs the model with the storm that corresponds to the target flows.

The modeler checks the outflow and compares it with the target. If the outflow is too high, then the modeler must increase the storage. If the outflow is too low, then the modeler must decrease the storage. Note that if the storage curve has been exceeded then the outflow may be erroneous. It is better to iterate from a large storage value to the correct storage than from a small storage value.

The modeler iterates step 2 until the calculates outflow matches (or is slightly less) than the target outflow. At this point the calculated storage should also match the storage in the input table.

The modeler then enters the next discharge ordinate for the next largest storm, guesses a new storage and runs the model.

Steps 2 and 3 are repeated until the outflow and storage are matched.

Step 4 is repeated with the next largest storm until the final storm is reached.
Once the last storm is iterated then the Discharge-Storage curve is complete. (e.g. when the (0.94,1.00) point is determined in the above example curve).