Groundwater Modelling Advice Draft

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Common Questions Answered (FAQ)

Why does changing the initial soil moisture in the Green-Ampt (GA) infiltration method not affect infiltration rates as expected?

In the 2023 release of TUFLOW, the treatment of initial soil moisture in the GA method was updated to accommodate horizontal soil water movement. This change allows soil moisture to flow between sub-surface cells and be removed via evapotranspiration, making it more dynamic over time.

Instead of remaining static in the infiltration equation, the initial soil moisture is now used as the cumulative infiltration (F) at the first timestep of the simulation. This means:

• The infiltration rate at the start of the simulation is influenced by the initial soil moisture, but subsequent infiltration behaviour depends on the balance of infiltration, drainage, and evapotranspiration.

• If no soil thickness is defined, the model assumes dZ = 0, meaning the initial moisture value is ignored altogether.

Workarounds: 1. Reverting to the pre-2023 method. Use the backward compatibility switch: Defaults == Pre 2023

Note: This changes multiple default settings, not just the GA method.

2. Adjusting the soil porosity in the .tsoil file. Instead of defining initial soil moisture separately, set: Adjusted Porosity = Porosity - Initial Moisture Example: If soil porosity is 0.385 and initial soil moisture is 0.200, set the porosity to 0.185.

TUFLOW is updating documentation to clarify this change and is working on a dedicated command to allow users to revert to the original GA method without affecting other default settings. Further refinements are also being considered for long-term simulations involving multiple wet/dry cycles.

If you have feedback or specific cases where this affects your modelling, please let contact TUFLOW Support via support@tuflow.com.

Can TUFLOW trap groundwater beneath impervious areas to prevent unrealistic exfiltration?

Currently, TUFLOW does not directly simulate pressurised groundwater flow beneath impervious surfaces. The existing groundwater model calculates exfiltration based on mass balance, meaning that groundwater can still migrate upward even in areas where the surface is defined as impervious.

However, we recognise that this could be an important feature for improving groundwater representation in urban modelling. We are considering incorporating a method to account for trapped groundwater as part of the 2026 development cycle.

In the meantime, users may mitigate this issue by:

• Removing impervious locations from the soil layer entirely.

• Setting horizontal hydraulic conductivity to zero for soil polygons beneath impervious areas.

• Adjusting the soil thickness and properties to reduce unrealistic seepage effects.

If this issue is recurring or if you have specific use cases where this feature would be beneficial, we encourage you to share your feedback via support@tuflow.com.

What is the recommended method for representing a railway ballast area in TUFLOW?

A railway ballast area in TUFLOW can be represented as a soil layer (specific soil ID) with high infiltration, suitable porosity, and high hydraulic conductivity in both horizontal and vertical directions.

A global soil thickness value can define the depth of this layer relative to the surface elevation. Similarly, a global soil base elevation value beneath the railway ballast area can set the absolute elevation of its bottom. Both parameters can also be varied spatially with GIS and/or grid layers.