Reservoir Discharge Controlled by Recession Constant

This GoldSim model demonstrates the dynamic simulation of reservoir discharge using a recession constant applied to the operational storage volume. It calculates outflow based on the volume of water stored above a specified target elevation. The core logic multiplies this calculated operational volume by a user-defined recession constant (e.g., in %/day or 1/day) to determine the discharge rate.

By dynamically linking the outflow rate directly to the available storage above a threshold, this model provides a simple yet effective method for representing certain discharge behaviors. It is particularly useful for conceptual modeling, simulating processes like natural baseflow recession from storage, or representing simplified operational rules tied to volume rather than head. This approach offers computational efficiency when detailed hydraulics are unnecessary or unknown.

Model Overview

The core of this model calculates the volume of water above a target level and applies a recession constant to determine the outflow rate. Key components typically include:

• Pool: Represents the reservoir, tracking the total stored water volume.
• Inflow/Evaporation: Placeholders representing other gains and losses to the reservoir volume.
• Elev_Vol_Table: Defines the elevation-volume relationship for the reservoir.
• Target_Elev: Defines the reference elevation determining the operational volume threshold. You could replace this with the tailwater elevation.
• Operating_Vol: Calculates the volume currently stored above the target volume threshold. This is often calculated as: max(Reservoir - Elev_Vol_Table(Target_Elev), 0 m3), where Reservoir is the volume in the Pool.
• Recession_Const: Defines the rate at which operational volume is discharged (units of 1/Time, e.g., %/day or 1/day).
• Discharge_Request: Computes the outflow rate by multiplying the operational volume by the recession constant: Recession_Const * Operating_Vol.
• Outflows: The Pool element has withdrawals for losses (e.g., Evaporation) and the primary Discharge_Rate.

Why Use a Recession Constant Approach?

Using a recession constant linked to operational volume provides a simplified way to model certain reservoir or storage discharge processes:

• Simplicity: Requires defining only the target level (or volume) and a single recession constant parameter, making it very easy to implement compared to detailed hydraulic calculations or complex rating curves.
• Conceptual Link to Storage: Directly relates the outflow rate to the amount of water available above a threshold. This can conceptually represent processes like groundwater baseflow receding from a saturated zone or simplified managed releases tied to storage bands.
• Computational Efficiency: The calculation is direct and very fast, avoiding iterative solvers or complex hydraulic formulas, which can be beneficial in large, complex models.
• Parameterization: The recession constant can sometimes be estimated from observed flow recessions (e.g., plotting log(Q) vs. time for baseflow periods) or used as a calibration parameter representing aggregate discharge behavior.

This method is less physically detailed than explicit hydraulics or head-based rating curves but can be effective for conceptual models, representing natural recession phenomena, or simulating high-level operational rules where outflow is primarily dictated by the available storage volume above a certain point.

Example Simulation Results

Below is an example time history chart showing the simulated reservoir water level compared to the target level, along with the resulting inflow and the discharge rate determined by the recession constant applied to the operational volume.

Figure 2: Example time history results showing water level, target level, inflow, and recession-based discharge.