For the Specified Release Routing Method, there are two different options for specifying the storage relationship. The first option is the Elevation-Storage choice. The user must select an elevation-storage curve from the available curves in the Paired Data Manager. After the routing is complete, the program will compute the elevation and storage for each time interval. The second option is the Elevation-Area choice, which requires the selection of an elevation-area curve from the available curves in the Paired Data Manager. With this choice, the program automatically transforms the elevation-area curve into an elevation-storage curve using the conic formula. After the routing is complete, the program will compute the elevation, surface area, and storage for each time interval.

You must select a discharge time-series gage as the outflow from the reservoir. The gage should record the discharge to use for each time interval of the simulation. If there is missing data in the record and the basin model options are set to replace missing data, a zero flow rate will be substituted for each missing data value. If the basin model is not set to replace missing data, any missing data will cause the simulation to stop and an error message will be displayed.

The maximum release setting is optional. It will cause a warning message during a simulation if the specified release exceeds the setting value. The specified release from the time-series gage record will always be discharged from the reservoir. However, the warning will occur when the specified release exceeds the optional maximum release value.

The maximum capacity setting is optional. It will cause a warning message during a simulation if the calculated storage exceeds the setting value. The storage is calculated for each time interval using conservation of mass. The storage at the end of the previous time interval, and the inflow volume and specified outflow volume for the current time interval, are used to calculate the storage at the end of the current time interval. The calculated storage will not be changed when it exceeds the setting value. However, the warning message will occur when the calculated storage exceeds the setting value.
Start with the conservation of mass.

$\begin{array}{l}\displaystyle \frac{S_{i+1}-S_i}{dt}=\frac{dS}{dt}=∑Q_{i+1}^{in} - \sum{Q_{i+1}^{out} }\end{array}$

The change in storage in a timestep is the difference between the inflows and the outflows. Adding the inflows is straightforward, but there can be many outflows. We assume the outflows can be calculated given the pool and tailwater. Typically, the determination of each individual outflow object is straightforward. Each outlet has its own piece in the equation which is calculated based on given pool and tailwater. The object that represents each outlet is captured and stored at the end of the time step.

This becomes a root-finding problem and can be solved for storage using standard numerical methods. The root-finding algorithm Brent's Method is used because it is a very dynamic solution that combines speed and reliability (Numerical Methods reference).

Applicability and Limitations of the Specified Release Method

The Specified Release Routing Method is best used for replicating events, forensic hydrology, or calibrating the channel network. Discharge is calculated based on the outlet elements that are specified, which can include outlets, spillways, gates, and dam top, but it cannot include evaporation. Maximum release and capacity settings offer warnings but do not stop the compute. After calibration with the Specified Release Routing Method, the model may be used with a different routing approach.