Green-Ampt Infiltration Parameters: Difference between revisions
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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
[[File:Fig_1_GA_Model.png|300px|Figure 1 Green-Ampt Model Concept]]<br>
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'''Figure 3: 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
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=== 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
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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
[[File:Init moisture F10.png|600px|Figure 9: Sensitivity of cumulative infiltration in the Plynlimon Gwy catchment to the initial moisture parameter in the Green-Ampt infiltration model.|border]]<br>
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[[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
'''Figure
=== Max Ponding Depth ===
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