Updates to the Ground Infiltration Module in InfoWorks ICM 8.0

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1 Updates to the Ground Infiltration Module in InfoWorks ICM 8.0 Introduction The release of InfoWorks ICM version 8.0 has seen a couple of additional features added to the Ground Infiltration module to improve efficiencies and assist with calibration to observed data. These are:- New constant evapotranspiration loss type, New evapotranspiration depth at which evapotranspiration losses from the soil store cease, Monthly evapotranspiration factors, Option whereby the ground infiltration depends on the surcharge state in the pipe, Ability to apply a ground infiltration event profiles to multiple subcatchments. Evapotranpiration losses can be set as linear or constant Prior to InfoWorks ICM version 8.0, the simulation engine linearly interpolated the evaporative loss rate from the soil store of the groundwater infiltration model from zero at the soil depth to the evaporation rate, as specified in the rainfall event, at ground level. In version 8.0, it is possible to define a separate soil depth that is the lower limit for evaporative losses. This is to represent the letup in evapotranspiration at a wilting point if for example there is heavy vegetation. By defining an evapotranspiration depth we can represent the point that evapotranspiration stops even if the soil store is not empty. There is also the possibility to allow the evaporative loss to vary linearly with depth (as previously) or to be constant. This also works in conjunction with the soil depth value, so that when the loss type is constant, the loss applies above the depth for evaporative loss and is zero below that depth. This provides two new parameters in the ground infiltration object properties as shown in figure 1. Figure 1: New Ground Infiltration Fields

2 To demonstrate this a subcatchment has been setup with the parameters shown in figure 1. This setup creates a soil store with no losses to percolation or groundwater and is nearly completely porous (it s not possible to be 100% porous). The example is completed with zero rainfall, constant evaporation of 5mm/day and an initially full soil store (initial soil saturation=99.999%). Figure 2 shows the difference in soil store depth between the existing, linear evapotranspiration type and the new constant evapotranspiration type. In the case of the constant evapotranspiration type, the soil store depth is reduced by the constant evaporation rate of 5mm/day (as shown by the green line). Where the evapotranspiration type is linearly interpolated depending on soil store depth. The evaporative losses rate in the soil store depth reduce as the soil store empties (as shown by the blue line) as the evaporative losses are linearly interpolated between the ground level and a soil store depth of zero. Figure 2: The Effect of the Evapotranspiration Type on Soil Store Depth, the blue line represents the original linearly interpolated evaporative losses, the green line the new optional constant evaporative losses. We can then introduce the evaportranspiration depth, below which the evaporative losses will cease. In this example we ve specified the evapotranspiration depth to be 0.050m with 2 models with linear and constant evapotranspiration types respectively. It can be seen from figure 3, that once the evapotranspiration depth is achieved that the losses from the soil store stop.

3 Figure 3: The effect of the evaporative depth on the soil store depth. Again, the blue line represents the original linearly interpolated evaporative losses, the green line the new optional constant evaporative losses. Comparing the linear evapotranspiration type with and without the evapotranspiration depth shows that the evaporative losses are linearly interpolated to the soil store depth or the evapotranspiration depth depending on whether one is specified (see Figure 4). Figure 4: The effect of the evapotranspiration depth on the soil store depth. The blue line represents linear evapotranspiration type interpolated between the ground level and the soil store depth, the green line the linear evapotranspiration type interpolated between the ground level and the evapotranspiration level.

4 Monthly Evaporation factors When undertaking long term simulation modelling using the ground infiltration module, it is currently possible to assign a time-varying evaporation profile within the rainfall event file to represent the variation of evaporation during a particular time period. With the release of InfoWorks ICM version 8.0, allows monthly evaporation patterns to be determined which work in conjunction with the evaporation profile. The monthly evaporation factors are again provided in the ground infiltration properties for a subcatchment (as shown in Figure 5). Figure 5: The new monthly evapotranspiration factors in the ground infiltration properties. To demonstrate the application of this, the same model as previously described was used, again with no losses to percolation or groundwater. The model was run with an evapotranspiration type of constant and the same evaporation rate of 5mm/day. The model was run for a whole year, one with no monthly evapotranspiration factors (so default of 1) and one with the same evapotranspiration factors as shown in figure 5. The results are shown in figure 6 and clearly show the changing rate of the soil store depth, indicative of the evaporative losses, on a monthly basis.

5 Figure 6: The effect of the evapotranspiration monthly factors on the soil store depth representing the differing evaporative loss rates. The blue line represents a constant evapotranspiration type with no monthly evapotranspiration factors (default=1), the green line a constant evapotranspiration type interpolated with the monthly evapotranspiration factors defined in figure 5. Ground Infiltration takes into account surcharge state of the conduit it drains to. Another change to the ground infiltration module is an option to address a limitation with the general approach of the GI module which meant that the level in the destination node for groundwater infiltration had no influence on the groundwater flow into that node. Clearly the level in the node would affect the amount of infiltration which could occur, although previously this was unaccounted for in InfoWorks ICM or legacy InfoWorks CS. This has been changed in InfoWorks ICM version 8.0 with the new simulation parameters node level affects groundwater infiltration. This will be the default for new networks and will be unselected for existing networks:-

6 Figure 7: New option in the Simulation parameter for ground infiltration to account for surcharge state. The use of this option will mean that flow into the pipe network follows the standard orifice equation with infiltration being calculated as follows:- Q gi = ((H H t2 ) H node ) 1/2 k 3 1 Where H is the level (m AD) of the groundwater reservoir, H t2 is the infiltration threshold level (m AD), H node is the depth (node level-floor level_ in the groundwater destination node, K 3 is a constant that determines flow into the pipe. Without the option selected, the behaviour reverts to the default prior to version 8.0 which is based on the following:- Q gi = ((H H t2 )) 1/2 k 3 1 The graphs below shows the effect of the option on a model with a single subcatchment with ground infiltration model compared to the same simulation without the option selected:-

7 Figure 8: The influence of the optional behaviour of the ground infiltration where the ground infiltration is influenced by the node levels (the green line considers the equation with the node water depth and the blue line is the original equation). As can be seen the groundwater inflow into the node is reduced as water depth increases. This is not taken into account using the previous approach which allows infiltration regardless of the water depth in the node. Ground Infiltration Event Profiles can be applied to multiple subcatchments. The final improvement that has been implemented in InfoWorks ICM version 8.0 is an option which allows the user to apply groundwater profiles to a Ground Infiltration ID rather than individual subcatchment IDs. It is still possible to specify the groundwater profiles on a subcatchment by subcatchment basis but it is now possible to specify on a ground infiltration ID basis. Any subcatchments using the ground infiltration ID will have the groundwater level set by the profile. It would be advisable to keep subcatchment IDs and ground infiltrations IDs unique but where the subcatchment ID and ground infiltration ID are the same, the profile will be assumed to refer to the

8 subcatchment. If a profile is specified to a subcatchment and the ground infiltration ID used by that subcatchment, the subcatchment profile will take precedence. The implementation fo this can be seen below. The following is the ground infiltration event applied to the Ground1 infiltration ID. Figure 9: The ground store level in the ground infiltration event editor applied to Ground1 Infiltration ID. Assessment of the results for subcatchment 10_000 shows that the ground store level follows that defined in the ground infiltration event. Figure 10: The application of the Ground1 ground store level profile to the 10_000 subcatchment. In a model with multiple subcatchments this allows us to specify one ground infiltration event but apply to a number of subcatchments. This is significantly improves efficiency when faced with 100s if not 1000s of subcatchments. In conclusion, there are several improvements to the Ground Infiltration module in InfoWorks ICM version 8.0 that might help you building the model and also calibrating to observed flows.