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Green-Ampt Infiltration Parameters: Difference between revisions

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== <font color="red">'''PAGE UNDER CONSTRUCTION'''</font> ==
 
== Introduction ==
ThereTUFLOW areprovides a number ofseveral methods availablefor withinmodelling TUFLOW to infiltrate waterinfiltration onfrom the 2D surface into the sub-surface., These areincluding Green-Ampt, Horton, and Initial Loss/Continuing Loss. These The modelsmethods are used to representsimulate hydrological losses, particularly when direct rainfall is applied directly to the 2D surface and runoff is generated. As such, the infiltration module used, and the parameters selected, are important calibration parameters which should be used to calibrate simulated flows to observed flows.<br>
 
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_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.<br>
 
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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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:h<sub>0</sub> is the depth of ponded water<br>
:F(t) is the cumulative infiltration calculated from:<br>
[[File:Accumulative_infil.png|350px]]<br><br>
 
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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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Table 2 presents summary statistics for the Green-Ampt USDA Parameters typesand typical values. This provides a good indication of the typical ranges of the Green-Ampt parameter values. <br>
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'''Table 2 USDA Summary Statistics for all Soil types for the Green-Ampt Infiltration Method'''<br>
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In order to help those undertaking real world calibration of TUFLOW models to observed data, a sensitivity analysis of the various parameters have been undertaken to show the effect of each Green-Ampt parameter in isolation. The comparison has been undertaken on a real-world whole catchment model of the Plynlimon catchment in mid-Wales. The model was run with a real rainfall event from 2015 with a temporal resolution of 30 minutes as shown in the Figure 2.<br>
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[[File:Fig2 GWY RF.png|600px|Figure 2: Plynlimon Rainfall|border]]<br>
 
'''Figure 2: Plynlimon Rainfall'''<br>
Within the Plynlimon catchment, the soil types were distributed within the catchment as shown in the map below although a single soil type has been selected for the comparison. Therefore, the effect seen is a consequence of changing a single soil parameter value.<br>
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For the purposes of this sensitivity analysis of the parameters, a single soil type was used representing the general clay soil types that are present.<br>▼
[[File:Fig3 GWY SoilMap.png|600px|Figure 3: Plynlimon Gwy Catchment Soil Types]]<br>
 
'''Figure 3: Plynlimon Gwy Catchment Soil Types'''<br>
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▲For the purposes of this sensitivity analysis of the parameters a single soil type was used representing the general clay soil types that are present.<br>
 
== 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 and max ponding depth. What follows is a description of each parameter and the sensitivity to a 10%low, medium and 20%high increasevalue andbased decrease inon the defaultUSDA soil type summary values.<br>
 
=== Capillary Suction Head ===
The suction head, represented in millimeters, is the capillary attraction on the soil voids. It is large for fine grain soils such as clays and smaller for sandy soils. To test the sensitivity of the simulated runoff at a gauged location, a low (49.5mm), mid representing the mean (184.4mm) and high (316.3mm) value of the suction head parameter were used with other parameters representing a clay soil (soil type 1).<br>
The larger the value of the capillary suction head value, the more capillary action that is achieved and the amount of infiltration that takes place. This is shown by the increase in cumulative infiltration in the graph below with a greater cumulative infiltration for the 20% increase in the suction head.<br>▼
To test the sensitivity of the simulated runoff at three gauged locations, as shown in figure 2, a low 49.5), mid representing the mean (184.4) and high (316.3) value of the suction head parameter was used with other parameters representing a clay soil (soil type 1).<br>
▲The larger the value the capillary suction head value, the more capillary action that is achieved and the amount of infiltration that takes place. This is shown by the increase in cumulative infiltration in the graph below with a greater cumulative infiltration for the 20% increase in the suction head.<br>
 
[[File:Fig4 sens to suction.png|600px|Figure 43: Sensitivity of cumulative infiltration in the Plynlimon Gwy catchment to the Capillary Suction Head parameter in the Green-Ampt infiltration model.|border]]<br>
 
'''Figure 43: Sensitivity of cumulative infiltration in the Plynlimon Gwy catchment to the Capillary Suction Head parameter in the Green-Ampt infiltration model.'''<br>
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As a consequence of this, there is a less runoff generated as shown in Figure 54. TheAs can be seen, the model is not particularly sensitive to the suction head parameter and this fits with observations made within the literature from other similar studies.<br>
<br>
 
[[File:Fig5Suction suctionheadHead.pngjpg|600pxborder|760x760px|Figure 54: Sensitivity of simulated flow at the Cefn-Brwn gauge location in the Plynlimon Gwy catchment to the Suction head parameter in the Green-Ampt infiltration model.]]<br>
 
'''Figure 54: Sensitivity of simulated flow at the Cefn-Brwn gauge location in the Plynlimon Gwy catchment to the Suction head parameter in the Green-Ampt infiltration model.'''<br>
<br>
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.
''<font color="red">add comment on rising lim''</font><br>
 
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=== Saturated Hydraulic Conductivity ===
The saturated hydraulic conductivity, measured in mmmillimetres per hour, represents the easehow thateasily water can travel through the soil whilstwhen it isfully saturated. In Thethe saturatedHorton hydraulicinfiltration conductivitymodel, isthis thevalue equivalentcorresponds ofto the limiting infiltration rate in the Horton infiltration model. The hydraulicHydraulic conductivity is typically high for sandy soils butand low for compact clays. Again, the sensitivity was conducted by varying the value for clays, which itself is relatively by low, with three scenarios, low (0.3), mid representing the mean (15.86) and high (117.8). The results shown in figure 6 show that the parameter is very
 
sensitive to the changes in the hydraulic conductivity with the mid and high values providing a lot of infiltration and no runoff at the downstream gauge. As expected, the higher the hydraulic conductivity, then the more infiltration that occurs and the less runoff that is seen at the gauge.<br>
In the sensitivity testing, the focus was on clay soils, which generally have low conductivity values. Three scenarios were initially tested: low (0.3 mm/hr), mid (15.86 mm/hr), and high (117.8 mm/hr). However, the mid and high values resulted in such high infiltration that no surface runoff was produced, leading to zero simulated flow at the downstream gauge location. As expected, increasing hydraulic conductivity leads to more infiltration and less runoff, a relationship well documented in Green-Ampt-based infiltration modelling.
 
To improve the analysis, it is recommended that the model be rerun using a refined range of lower conductivity values that still allow some runoff, such as 0.3, 1.0, 3.4, and 7.6 mm/hr. This would provide a more meaningful understanding of the parameter sensitivity while preserving realistic surface runoff behaviour.<br>
<br>
 
[[File:Fig6Hydraulic hydroconductConductivity.pngjpg|600pxborder|760x760px|Figure 65: Sensitivity of simulated flow at the Cefn-Brwn gauge location in the Plynlimon Gwy catchment to the Saturated Hydraulic Conductivity parameter in the Green-Ampt infiltration model when using a clay soil type.]]<br>
 
'''Figure 65: Sensitivity of simulated flow at the Cefn-Brwn gauge location in the Plynlimon Gwy catchment to the Saturated Hydraulic Conductivity parameter in the Green-Ampt infiltration model when using a clay soil type.'''<br>
<br>
 
[[File:Fig7 hydroconduct.png|600px|Figure 76: Sensitivity of cumulative infiltration in the Plynlimon Gwy catchment to the Saturated Hydraulic Conductivity parameter in the Green-Ampt infiltration model.|border]]<br>
 
'''Figure 76: Sensitivity of cumulative infiltration in the Plynlimon Gwy catchment to the Saturated Hydraulic Conductivity parameter in the Green-Ampt infiltration model.'''<br>
 
=== Porosity ===
The porosity value represents the volume of dry voids per volume of soil and provides the maximum moisture deficit that is available, the difference between the moisture content at saturation and at the start of the simulation. Sandy soils tend to have lower porosities than clay soils, but drain to lower moisture contents between rainfall events because water is not held as strongly in the soil pores. Therefore, values of porosity tend to be higher for sandy soils when compared to clay soils. As shown in figure 7, the higher the porosity value, then the less runoff that is generated due to increased infiltration although the model is not particular sensitive to the porosity value.<br>
Sandy soils tend to have lower porosities than clay soils, but drain to lower moisture contents between rainfall events because water is not held as strongly in the soil pores. Therefore, values of porosity tend to be higher for sandy soils when compared to clay soils.<br>
As shown in figure 8, the higher the porosity value, then the less runoff occurs due to increased infiltration although the model is not particular sensitive to the porosity value.<br>
<br>
 
[[File:Fig7 porosity sensPorosity.pngjpg|border|600px760x760px|Figure 87: Sensitivity of simulated flow at the Cefn-Brwn gauge location in the Plynlimon Gwy catchment to the porosity parameter in the Green-Ampt infiltration model.]]<br>
 
'''Figure 87: Sensitivity of simulated flow at the Cefn-Brwn gauge location in the Plynlimon Gwy catchment to the porosity parameter in the Green-Ampt infiltration model.'''<br>
<br>
 
[[File:Fig8 porosity sens.png|600px|Figure 98: Sensitivity of cumulative infiltration in the Plynlimon Gwy catchment to the porosity parameter in the Green-Ampt infiltration model.|border]]<br>
 
'''Figure 98: 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 |2508 WARNING]] is issued if this is the case.<br>
 
In pre-2023 releases of TUFLOW, a single variable storage capacity was calculated by subtracting the initial moisture fraction from the porosity, in order to reduce memory requirements. However, in TUFLOW releases 2023 and onwards, the soil porosity and initial moisture must be stored separately to allow the soil to drain correctly when using the interflow functionality. This updated approach requires that a soil thickness be specified to calculate the soil depth. If a soil thickness is not specified when using the updated approach, an infinite soil depth is assumed for each layer and therefore different initial moisture fractions no longer have an effect on modelled results. These two approaches can generate different results when using the Green-Ampt method.
 
The three initial moisture sensitivity tests have been undertaken with the Green-Ampt method using both a pre-2023 release of TUFLOW and a post-2023 release of TUFLOW. Figure 9 shows how variations in the initial moisture affect the simulated cumulative infiltration, whereas Figures 10 and 11 show the effects of varying the initial moisture on flows at the catchment outlet when using the pre-2023 and post-2023 releases of TUFLOW. As the initial moisture is increased at the beginning of your simulation, there is less infiltration (as you are closer to soil capacity) and more runoff, causing the catchment outflows to exhibit a faster response to rainfall upstream. As the event progresses, soils become more saturated and the influence of the initial moisture parameter becomes less significant. In the examples shown here, the catchment outflows, as visible in Figures 10 and 12, show a higher responsiveness to variations in initial moisture at the beginning of the simulations, and attain higher peak values, when using the post-2023 TUFLOW releases.
 
[[File:Fig10Init GAmoisture soilsF10.png|600px|Figure 109: Sensitivity of simulatedcumulative flow at the Cefn-Brwn gauge locationinfiltration in the Plynlimon Gwy catchment to the USDAinitial soil typemoisture parameter in the Green-Ampt infiltration model.|border]]<br>▼
'''Figure 109: Sensitivity of simulatedcumulative flow at the Cefn-Brwn gauge locationsinfiltration in the Plynlimon Gwy catchment to the USDA soilinitial typemoisture parameter in the Green-Ampt infiltration model.'''<br>▼
[[File:Initial moisture.jpg|border|760x760px|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 in pre-2023 release of TUFLOW.]]<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 in pre-2023 release of TUFLOW.'''
[[File:Init moisture post 2023 v2.jpg.jpg|border|760x760px|Figure 11: 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 in post-2023 release of TUFLOW.]]<br>
'''Figure 11: 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 in post-2023 release of TUFLOW.'''
 
=== Max Ponding Depth ===
The max ponding depth value is an optional value that can be used, if desired, to set a limit for the depth of ponded water (h<sub>0</sub>) value used in the Green-Ampt equation. The minimum of the water depth and the max ponding depth value is used as the h<sub>0</sub> value. The default max ponding depth value is 0, to be consistent with the basic form of the Green-Ampt equation, as hydrology models do not necessarily have a depth calculated at cells.
 
This means, if using a max ponding depth (>0), infiltration rates will increase.
 
▲'''Figure 9: Sensitivity of cumulative infiltration in the Plynlimon Gwy catchment to the porosity parameter in the Green-Ampt infiltration model.'''<br>
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== In built USDA soil type ==
The model was also run with the default in-build USDA soil types. Figure 1012 shows the outputs. As expected the higher the soil type, then typically the more the infiltration and the lower the produced runoff. Soils 8-11, which represent sandy soils do not show any runoff in this example as the rainfall applied directly to the mesh is all infiltrated.<br>
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▲[[File:Fig10 GA soils.png|600px|Figure 10: 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.]]<br>
▲'''Figure 10: Sensitivity of simulated flow at the Cefn-Brwn gauge locations in the Plynlimon Gwy catchment to the USDA soil type parameter in the Green-Ampt infiltration model.'''
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[[File:Soil Type.jpg|border|760x760px|Figure 12: 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.]]<br>
'''Figure 12: 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.
The Green-Ampt infiltration model is one of the available infiltration models within TUFLOW. There is a wide range of literature on Green-Ampt applications and some suggested parameter values for particular soil types, albeit soil types from the US. The 3 main Green-Ampt parameters have been tested to show the sensitivity of model outputs to the values as well as the variation in initial moisture. The results show that the model is relatively insensitive to the porosity value and suction head parameter. However, the outputs do show variations in runoff volume due to variations in both hydraulic conductivity and initial soil moisture. As part of any calibration exercise it is suggested that these would be the parameters that are most focused on. The hydraulic conductivity appears to affect the runoff volume throughout the event whereas the initial soil moisture has a limited impact at the beginning of the event before soils become saturated and results converge.<br>
 
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==
 
The Plynlimon model contains data supplied by Natural Environment Research Council. The Plynlimon observed rain gauge and flow data was provided by the Centre of Hydrology, Bangor. The model uses LiDAR data which is public sector information licensed under the Open Government Licence v3.0.
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