This Routing Method represents the reservoir or lake with a user-provided relationship between storage and discharge. The water surface in the reservoir is assumed to be level. The relationship between discharge and storage must be unique, which does not permit looped rating curves. Further, the relationship must be monotonically increasing with storage. The user must develop the storage relationship external to the program by considering all the possible outlets for water from the reservoir and combining them in a single storage relationship. The simulation is accomplished with the Modified Puls algorithm using one routing step.

Storage Method

There are four different options for specifying the storage relationship. The first option is the Storage-Discharge choice. The user must select a Storage-Discharge curve from the available curves in the Paired Data Manager. The second option is the Elevation-Storage-Discharge choice. The user must select both a Storage-Discharge curve and Elevation-Storage curve from the Paired Data Manager. The third option is the Elevation-Area-Discharge choice. In this case the user must select both an Elevation-Area curve and an Elevation-Discharge curve from the Paired Data Manager. With this choice, the program automatically transforms the elevation-area curve into an elevation-storage curve using the Conic Formula. The final option is the Elevation-Storage-Area-Discharge choice. In this case the user must select a Storage-Discharge, an Elevation-Storage curve, and an Elevation-Area curve from the Paired Data Manager. Regardless of which option is selected, the routing is always performed using only the storage-discharge curve. After the routing is complete using the storage-discharge curve, the program will compute the elevation and surface area for each time step, depending on the selected storage method.

Interpolation is used when the Elevation-Storage-Discharge, Elevation-Area-Discharge, and Elevation-Storage-Area-Discharge storage methods are used. This means that it is not necessary for the storage-discharge and elevation-storage/area curves used to contain matching independent variables. The curves do not need to have the same storage values in each curve, or even have the same number of rows. At compute time, the curves selected by the user are combined into a single routing table with rows for: Elevation, Storage, Area, and Discharge. The table is initially configured using the curve selected by the user as the Primary Curve. The remaining column(s) are interpolated from the curve not selected as the primary curve. Finally, the storage routing is completed from the combined table using the storage and outflow columns, and then elevation and area is calculated from the computed storages where possible. 

You must select appropriate functions to define the selected Storage Method. For example, if you select the Elevation-Storage-Discharge method, you must select an elevation-storage and a storage-discharge paired data function. Appropriate selection lists will be shown directly under the Storage Method selection list. Any necessary paired data functions must be defined in the Paired Data Manager before they can be used in the reservoir. Choose an appropriate function in each selection list. If you wish, you can use a chooser by clicking the Paired Data button next to the selection list. A chooser will open that shows all of the paired data functions of that type. Click on a function to view its description.

Initial Condition

The Initial Condition sets the amount of storage in the reservoir at the beginning of a simulation. Therefore, the simplest option is to specify the Storage as a volume of water in the reservoir. For convenience, other options are also provided. The Inflow=Outflow method takes the inflow to the reservoir at the beginning of the simulation, and uses the storage-discharge curve to determine the storage required to produce that same flowrate as the outflow from the reservoir. Some storage methods permit the specification of Elevation as the initial condition. In this case, the elevation provided by the user is used to interpolate a storage value from the elevation-storage curve. Other storage methods permit the specification of Discharge as the initial condition. In this case, the storage is interpolated from the storage-discharge curve. The initial condition options depend on the selected storage method and are shown in the table below.

Available initial condition options for different storage methods used with the outflow curve routing method

Storage Method

Available Initial Conditions


Discharge, storage, inflow = outflow


Discharge, storage, elevation, inflow = outflow


Discharge, elevation, inflow = outflow


Discharge, elevation, inflow = outflow, storage