Green-Ampt Infiltration Parameters: Difference between revisions
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== Introduction ==
The choice of infiltration method and its parameters is an important calibration factor and should be adjusted to match observed flow data. This is especially relevant for whole of catchment modelling, where infiltration is the main way hydrological losses are represented. This page describes the Green-Ampt infiltration parameters and their sensitivity.
== Green-Ampt Infiltration ==
The Green-Ampt approach varies the rate of infiltration over time based on the soil’s hydraulic conductivity, suction, porosity and initial moisture content. The method assumes that as water begins to infiltrate the soil, a line is developed differentiating between the ‘dry’ soil with moisture content θ<sub>i</sub> and the ‘wet’ soil (with moisture content equal to the porosity of the soil η). As the infiltrated water continues to move through the soil profile in a vertical direction, the soil moisture changes instantly from the initial content to a saturated state. This concept is shown schematically in Figure 1.
Note: The Green-Ampt approach is appropriate for simulating single rainfall events where evapotranspiration and gravity-driven drainage are not significant. The 2023-03 release introduced functionality in TUFLOW HPC to allow for horizontal movement of soil water, enabling long-term simulations with multiple rainfall events. To support this, a change was implemented in the Green-Ampt equation to account for changing initial soil moisture and cumulative infiltration over time. For further details, see Section 7.3.7.1.1 Green-Ampt (GA) in the [https://docs.tuflow.com/classic-hpc/manual/2025.1/TwoD-Domains-1.html#GA-5 <u>TUFLOW Manual</u>].▼
[[File:Fig_1_GA_Model.png|300px|Figure 1 Green-Ampt Model Concept]]<br>
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:F(t) is the cumulative infiltration calculated from:<br>
[[File:Accumulative_infil.png|350px]]<br><br>
▲Note: The Green-Ampt approach is appropriate for simulating single rainfall events where evapotranspiration and gravity-driven drainage are not significant. The 2023-03 release introduced functionality in TUFLOW HPC to allow for horizontal movement of soil water, enabling long-term simulations with multiple rainfall events. To support this, a change was implemented in the Green-Ampt equation to account for changing initial soil moisture and cumulative infiltration over time. For further details, see Section 7.3.7.1.1 Green-Ampt (GA) in the [https://docs.tuflow.com/classic-hpc/manual/2025.1/TwoD-Domains-1.html#GA-5 TUFLOW Manual].
United States Department of Agriculture (USDA) soil types have been hardwired into TUFLOW and are presented in Table 1 along with the soil parameters. Alternatively, it is possible to define a customised soil type by specifying user defined values within the tsoilf.<br>
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== Green-Ampt Infiltration: User Parameters ==
Where the inbuilt USDA soil types are not used, the user can specify their own values for the Suction, Hydraulic Conductivity, Porosity and Initial Soil Moisture. What follows is a description of each parameter and the sensitivity to a low, medium and high value based on the USDA soil type summary values.
=== Capillary Suction Head ===
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It can also be seen that the higher the suction head value that the longer it takes the hydrograph to start rising, with the high suction head scenario less responsive to the rainfall.
=== Saturated Hydraulic Conductivity ===
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[[File:Fig7 hydroconduct.png|600px|Figure 6: Sensitivity of cumulative infiltration in the Plynlimon Gwy catchment to the Saturated Hydraulic Conductivity parameter in the Green-Ampt infiltration model.|border]]<br>
'''Figure 6: Sensitivity of cumulative infiltration in the Plynlimon Gwy catchment to the Saturated Hydraulic Conductivity parameter in the Green-Ampt infiltration model.'''
=== Porosity ===
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'''Figure 8: Sensitivity of cumulative infiltration in the Plynlimon Gwy catchment to the porosity parameter in the Green-Ampt infiltration model.'''<br>
=== Initial Moisture ===
The initial moisture value represents the fraction of the soil that is initially wet. As both initial moisture and porosity are expressed as fractions, the soil capacity is defined as the difference between them both. As such, the initial moisture should not exceed the porosity otherwise soil capacity will be set to zero with no infiltration occurring for that soil type. A [[TUFLOW Message 2508 |
As you increase initial moisture at the beginning of your simulation, you experience less infiltration (as you are closer to the soil capacity), therefore have more run-off and a quicker response. Figure 9 shows the degree of change to cumulative infiltration with varying initial moisture and the effect on the catchment can be seen in Figure 10. As the event progresses, soils become more saturated and the influence of the initial moisture parameter becomes less significant.
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'''Figure 9: Sensitivity of cumulative infiltration in the Plynlimon Gwy catchment to the initial moisture parameter in the Green-Ampt infiltration model.'''<br>
[[File:Init moisture F11 catchment.png|600px|Figure 10: Sensitivity of simulated flow at the Cefn-Brwn gauge location in the Plynlimon Gwy catchment to the initial moisture parameter in the Green-Ampt infiltration model.|border]]<br>
'''Figure 10: Sensitivity of simulated flow at the Cefn-Brwn gauge location in the Plynlimon Gwy catchment to the initial moisture parameter in the Green-Ampt infiltration model.'''
=== Max Ponding Depth ===
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[[File:Fig10 GA soils.png|600px|Figure 11: Sensitivity of simulated flow at the Cefn-Brwn gauge location in the Plynlimon Gwy catchment to the USDA soil type parameter in the Green-Ampt infiltration model.|border]]<br>
'''Figure 11: Sensitivity of simulated flow at the Cefn-Brwn gauge location in the Plynlimon Gwy catchment to the USDA soil type parameter in the Green-Ampt infiltration model.'''
== Summary ==
The Green-Ampt infiltration model is one of the infiltration methods available within TUFLOW. There is extensive literature on its application, including suggested parameter values for various soil types, though these are mostly based on soils in the United States.
Three main Green-Ampt parameters have been tested to assess the sensitivity of model outputs to parameter values and variations in initial soil moisture. The results show that the model is relatively insensitive to the porosity and suction head parameters. However, outputs show significant variations in runoff volume in response to changes in hydraulic conductivity.
As part of any calibration process, it is recommended that hydraulic conductivity and initial moisture content be prioritised during calibration. Hydraulic conductivity influences runoff volume throughout the event, while initial soil moisture mainly affects the early part of the simulation until soils become saturated and results converge.
==Acknowledgements==
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