Mastering “What-If” Modeling in WaterGEMS: An Introduction to Scenarios and Alternatives
In the world of hydraulic modeling, one of the most powerful features you’ll encounter is the ability to simulate different "what-if" situations. In WaterGEMS, this is made possible through a structured approach called Scenarios and Alternatives. While it may sound complicated at first, don't worry — this topic, though advanced, will gradually make sense as you practice and build real-world experience.
Let’s walk through this concept together in a friendly, easy-to-understand way.
What Are Scenarios and Alternatives?
Imagine you’re working on a water distribution project for a hillside town. You want to test how the system performs in three different situations:
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Normal operation
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Peak demand in the morning
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One pump failure during night-time hours
Instead of creating three separate models, you can use WaterGEMS’ Scenarios and Alternatives feature to manage all three cases inside one model — efficiently and clearly.
Here’s how:
Scenarios are like different “versions” of your network’s story. Each version uses a set of Alternatives, which are the specific chapters or pieces that define parts of that story.
Physical Alternatives: These define the infrastructure — things that don't often change.
Example: You can have one alternative with 150mm pipes and another with 200mm pipes to study pressure differences.-
Demand Alternatives: These define how much water is needed and where.
Example: A base alternative with standard daily demand, and a new one with doubled demand at public taps during festival season. -
Control Alternatives: These define how pumps, valves, and other controls behave.
Example: One alternative may keep a pump running continuously, while another turns it on only when the tank drops below 60%. -
Initial Conditions Alternatives: These define the starting condition of your network.
Example: A full tank at 6 a.m. vs. a half-full tank at 6 p.m., affecting how the system behaves during simulation. -
Active Topology Alternatives: These define which elements are active or inactive.
Example: A valve that’s closed for maintenance in one scenario but open in another.
Each Alternative can build upon a Base Alternative, and you can create Child Alternatives to tweak specific settings without rebuilding everything.
Creating and Using Scenarios
Let’s go back to our hillside town example. Suppose you want to simulate the effect of a burst pipe on water supply.
Here’s how you’d set that up:
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Create a Physical Alternative where the pipe is marked inactive (simulating a burst).
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Use your regular demand, control, and initial conditions alternatives.
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Combine them into a new Scenario — maybe named “Burst Pipe at J-15”.
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Run the simulation and compare it with the base scenario.
This helps you answer important questions: Will users still get enough water? Will the pressure drop dangerously? How should operators respond?
WaterGEMS allows you to batch-run scenarios, making comparisons easier. Visual tools help you understand pressure zones, pipe flows, tank behavior, and more — all under different conditions. The follow along can be found in the workbook.
A Word of Advice to New Learners
This is an advanced topic — and that’s perfectly okay.
- You’re doing great.
- It’s normal to feel overwhelmed.
- And yes, it gets better with practice.
Don’t try to master all types of alternatives in one go. Start simple. Create a basic scenario. Then try changing just one thing — like the pipe size or tank level — and see how it affects the results.
Even experienced engineers build their models piece by piece, learning from mistakes along the way.
Hands-On Practice Is Essential
Think of WaterGEMS modeling like cooking. Reading recipes (or manuals) helps — but until you actually cook a dish, taste it, and adjust the seasoning, it’s all theory.
In the same way, the real learning happens when you simulate, get odd results, ask why, and adjust your setup.
Try these hands-on tasks:
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Clone your base scenario and increase demand by 25%. What happens to pressures?
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In another copy, add a second pump. Is service improved?
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Create a control alternative where pumps only work at night. How do tanks respond?
With each run, you’ll gain more confidence.
To all beginners reading this: take a deep breath. You’re on the right path.
These advanced features are not here to intimidate you — they’re here to help you solve real-life water supply problems more effectively.
Every expert modeler you meet has been confused at some point by Scenarios and Alternatives. What made them experts is consistent practice and real-world testing.
So keep modeling. Keep experimenting. Keep asking questions.
Because you’re not just learning software — you’re learning how to make water systems safer, smarter, and stronger for communities.
Stay curious. Practice often. And soon, managing scenarios in WaterGEMS will feel just like adjusting your own playlist — trying different mixes until you find what works best.
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