Green-Ampt Infiltration Parameters: Difference between revisions
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== Introduction ==
There are a number of methods available within TUFLOW to infiltrate water on the 2D surface into the sub-surface. These are Green-Ampt, Horton and Initial Loss/Continuing Loss. The models are used to represent 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. This is particularly important for whole of catchment modelling. <br>
== Green-Ampt Infiltration ==
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In order to help calibration off TUFLOW models to observed data, a sensitivity analysis of the various parameters have been undertaken to show the effect of each 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 Figure 2.<br>
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== Green-Ampt Infiltration: User Parameters ==
Where the inbuilt USDA soil types
=== 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.<br>
To test the sensitivity of the simulated runoff at a gauged location, a low (49.
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 increase in the suction head.<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 a less runoff generated as shown in Figure 4.
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'''Figure 4: 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>
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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.
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=== Saturated Hydraulic Conductivity ===
The saturated hydraulic conductivity, measured in mm per hour, represents the ease that water can travel through the soil whilst it is saturated. The saturated hydraulic conductivity is the equivalent of the limiting infiltration rate in the Horton infiltration model. The hydraulic conductivity is high for sandy soils but 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.
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'''Figure 6: 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.<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 7, the higher the porosity value, then the less runoff
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== In built USDA soil type ==
The model was also run with the default in-build USDA soil types. Figure 9 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 9: 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>
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