Tanks vs. Reservoirs in WaterGEMS

The Fundamental Difference: Volume and Head

The primary distinction between a reservoir and a tank in hydraulic modeling comes down to two concepts: storage volume and the stability of the water level (or hydraulic grade).

Let's start with a foundational reading that defines these two elements.

Now, let's watch a brief video from Bentley that explains this distinction from the software's perspective.

Based on these resources, we can summarize the key differences in a table:

Practical Applications: When to Use Each Element

Understanding the definition is the first step. Knowing when to apply each element is what makes you an effective modeler.

Use Cases for a Reservoir

You should model a boundary as a reservoir when it represents a water source so large that its level is not significantly affected by the demands of your modeled system. The reading from Advanced Water Distribution Modeling and Management gave some excellent examples:

• Large natural sources like lakes.
• Engineered sources like clearwells at a water treatment plant.
• Groundwater wells where the pump maintains a near-constant head.
• A connection to a much larger municipal water system. This is a very common scenario in freelance projects where you are modeling a smaller subsystem or a new development. The connection point to the city's large-diameter main is treated as an infinite source with a known pressure (which you convert to an HGL elevation).

Use Cases for a Tank

You should use a tank to model a storage facility that is part of the distribution network itself. Its purpose is to handle fluctuations in demand.

• It stores water during periods of low demand (e.g., overnight) when the source pumps might be running.
• It supplies water during periods of high demand (e.g., morning and evening peaks) to supplement the pumps and prevent pressures from dropping.
• It provides emergency storage for events like firefighting or power outages.

Elevated water towers and ground-level storage tanks are the classic examples.

The network schematic below illustrates a typical configuration where both are used.

In this diagram, the Reservoir (R-1) represents the ultimate source (like a lake or treatment plant clearwell). The Tank (T-1) is an in-system facility that helps meet daily peak demands and maintain adequate pressure in the upper parts of the network.

How Simulation Type Affects Behavior

The most significant difference in the hydraulic function between a tank and a reservoir appears when you run an Extended Period Simulation (EPS), which models the system over time.

In a Steady-State Simulation

A steady-state simulation is a snapshot of the system at a single moment (e.g., the moment of peak demand). In this mode:

• The water level in a tank is fixed at its Initial Elevation.
• Because its water level is not changing, the tank provides a constant HGL, just like a reservoir.
• Therefore, for a steady-state run, a tank is hydraulically identical to a reservoir set at the tank's initial water level.

The model will calculate whether water is flowing into or out of the tank to satisfy the system's demands and balance its energy, but the tank's level itself does not change during the single calculation.

In an Extended Period Simulation (EPS)

An EPS models the system over a duration (e.g., 24 hours) with calculations at regular time steps. This is where the true hydraulic function of a tank becomes visible.

• The Reservoir's HGL remains constant throughout the entire simulation.
• The Tank's HGL changes at each time step. The software calculates the net flow into or out of the tank over a time step, and then uses the tank's geometry to determine how much the water level rises or falls.

This dynamic behavior is the core function of a tank. Let's watch this in action. The following clips are from a workshop where a system is analyzed over time.

Now, let's see what happens when demand increases to its peak.

These videos clearly demonstrate the dynamic role of a tank in an EPS: it absorbs water when supply exceeds demand and provides water when demand exceeds supply. A reservoir cannot perform this function; it can only act as a constant source or endpoint.

Conclusion

In this lesson, we have clearly distinguished between the hydraulic functions of tanks and reservoirs in WaterGEMS. Understanding this difference is not just theoretical; it directly impacts how you build your models and whether they accurately represent the real-world systems your clients present to you.

Key Takeaways:

• Reservoirs model infinite sources with a constant hydraulic grade (HGL). They are used to represent large water bodies or connections to external systems.
• Tanks model finite storage facilities with a variable HGL. They are used to balance supply and demand within a system, stabilize pressure, and provide emergency storage.
• While a tank behaves like a reservoir in a steady-state simulation, its true dynamic function of filling and draining is only revealed in an Extended Period Simulation (EPS).

Now that we understand how tanks are used to handle fluctuations in system demand, our next lesson will focus on how to model those demand fluctuations themselves. We will explore the concept of diurnal demand patterns and how to apply them to different types of water users in your model.