How to get infiltration rate from HYDRUS 2D/3D

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Lili Yao
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Location: USA

How to get infiltration rate from HYDRUS 2D/3D

Post by Lili Yao » Mon Oct 02, 2017 2:28 am

Hi,
I am trying to simulate infiltration into soil column in HYDRUS 2D/3D. I built a model with a 100cm by 500cm(xz) domain. I use no flux boundary condition at the bottom and constant flux boundary condition at the top because I want to simulate precipitation with constant intensity at the soil surface. I do not know how to get the infiltration rate from simulation result. In “Results-Other Information-Boundary Fluxes-Infiltration”, there is not plot. I really want to know how to get the infiltration flux from the model. Could you help me? Thank you very much.

Jirka
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Location: Riverside, CA

Re: How to get infiltration rate from HYDRUS 2D/3D

Post by Jirka » Tue Oct 03, 2017 12:12 am

If you select a constant boundary condition, then infiltration flux is equal to the specified boundary flux., since this BC is a system independent BC and thus the specified boundary flux is always enforced. If you want to consider a system dependent BC, when infiltration flux depends on the status of the system (its saturation status and its hydraulic properties), you need to use an atmospheric boundary condition. Only for this BC, the code displays separately infiltration and evaporation rates (since they may be different from specified potential fluxes). J.

Lili Yao
Posts: 8
Joined: Fri Sep 29, 2017 6:31 pm
Location: USA

Re: How to get infiltration rate from HYDRUS 2D/3D

Post by Lili Yao » Tue Oct 03, 2017 4:36 am

Thank you very much for you reply. So, I use the atmospheric boundary condition at the top surface now. I don’t know how to add constant precipitation rate to the atmospheric boundary, but according to User Manual, when the boundary is time variable, I can apply precipitation rate to the atmospheric boundary. So, I set my constant precipitation rate at the time variable boundary conditions table. The simulation time is 10 days. After running the model. I found the infiltrate rate is always equal to the precipitation rate (3 cm/day). But the saturated hydraulic conductivity of the soil Ks=1.44cm/day. So, after ponding(and it occurs ponding during the simulation time), the infiltration rate will decrease and should be smaller than precipitation rate. So, I don’t know why the infiltration rate will be a constant in my model? Could you help me with this problem? Thank you very much.

Lili Yao

Jirka
Posts: 4759
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

Re: How to get infiltration rate from HYDRUS 2D/3D

Post by Jirka » Tue Oct 03, 2017 12:04 pm

HYDRUS will certainly calculate the lower infiltration rate when the surface ponding is reached. When using the atmospheric BC, the model applies the flux BC as long as the boundary is unsaturated and the pressure head (equal to zero) BC once the boundary becomes saturated (see the technical manual for details on system dependent boundary conditions). In the output, you should see the potential atmospheric flux (your applied flux) and the actual atmospheric flux (reduced flux due to ponding, if ponding is reached). J.

Lili Yao
Posts: 8
Joined: Fri Sep 29, 2017 6:31 pm
Location: USA

Re: How to get infiltration rate from HYDRUS 2D/3D

Post by Lili Yao » Tue Oct 03, 2017 3:44 pm

Thank you so much for your help. I finally get the infiltration rate from the “actual atmospheric flux“ in the output and the numbers seem reasonable. But in the “v_mean.out” file, there is a value named “infiltration”. According to User Manual, it should be the infiltration fluxes for the atmospheric boundary, but in my model this value remains as the flux I applied to the atmospheric boundary. I don't know why. So, I have uploaded my model and the v_mean.out file. Could you help me to find out what goes wrong? Thank you very much.

Lili Yao
Attachments
Infiltration_test_Lili Yao.rar
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Jirka
Posts: 4759
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

Re: How to get infiltration rate from HYDRUS 2D/3D

Post by Jirka » Tue Oct 03, 2017 4:47 pm

Set the bottom two nodes as Free Drainage BC so that water has where to go and everything will be fine. J.

Lili Yao
Posts: 8
Joined: Fri Sep 29, 2017 6:31 pm
Location: USA

Re: How to get infiltration rate from HYDRUS 2D/3D

Post by Lili Yao » Tue Oct 03, 2017 5:42 pm

I am sorry, I only change the bottom boundary from no flux BC to free drainage BC, and other parameters remains the same, but the model cannot run successfully now since it has not converged in 10 consecutive time steps. Do you know what goes wrong this time?

Lili Yao
Attachments
Infiltration_test_Free drainage BC.rar
(42.62 KiB) Downloaded 79 times

Jirka
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Location: USA
Location: Riverside, CA

Re: How to get infiltration rate from HYDRUS 2D/3D

Post by Jirka » Tue Oct 03, 2017 6:03 pm

I have just noticed that you are using the Brooks & Corey model for soil hydraulic properties. For this model, you will need to have the initial pressure head, hi at the free drainage BC below the air-entry pressure head value (i.e., hi<-1/alpha). For the van Genuchten model, there would not be such constraint as that model does not have an air-entry value. J.

Lili Yao
Posts: 8
Joined: Fri Sep 29, 2017 6:31 pm
Location: USA

Re: How to get infiltration rate from HYDRUS 2D/3D

Post by Lili Yao » Tue Oct 03, 2017 10:03 pm

Thank you very much. Because I want to check another code by comparing the infiltration rate results from that code and HYDRUS, and Brooks & Crook model is used in that code, so I’d like to use the same model for soil hydraulic properties in HYDRUS.

And you mean that if the bottom boundary is no flux boundary, the initial pressure head of the bottom could be zero even if I use Brooks & Corey model for soil hydraulic properties, right? Since the simulation time is not long enough for the wetting front arrives at the bottom, maybe there is no much difference whether I use no flux BC or free drainage BC at the bottom and the only difference is that I have to get the infiltration rate result from the “actual atmospheric flux” in the output, right?

I am sorry but a new problem comes up. When I use sand (Ks=23.56cm/hr) in the model and I also apply a constant flux to the atmospheric BC, the model often couldn’t run successfully since it has not converged in 10 consecutive time steps. When the constant flux=47.12cm/hr, the model could run successfully finally, but the “actual atmospheric flux” in the output seems unreasonable. It is supposed to be equal to Ks(23.56cm/hr) in the late stage, but it actually remains 37.7cm/hr after ponding in my model. What reasons could be for these two problems?

Thank you,
Lili Yao
Attachments
Infiltration_sand.rar
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Jirka
Posts: 4759
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

Re: How to get infiltration rate from HYDRUS 2D/3D

Post by Jirka » Tue Oct 03, 2017 11:48 pm

When you use B&C functions, you should either not used the internal interpolation tables or set the lower limit below the air-entry value (i.e., ha>1/alpha=7). Then you probably get what you want. If you use the internal interpolation tables with B&C functions, you likely do not have sufficient precision close to saturation if you starts them from zero (or ha=0.0001 m). J.

Lili Yao
Posts: 8
Joined: Fri Sep 29, 2017 6:31 pm
Location: USA

Re: How to get infiltration rate from HYDRUS 2D/3D

Post by Lili Yao » Wed Oct 04, 2017 5:35 pm

Thank you very much.

But I am sorry, I still don’t understand why “set the lower limit below the air-entry value((i.e., ha>1/alpha=7)”. Do you mean I should set the pressure of the bottom boundary smaller than positive 7 cm? The initial head of the bottom boundary I set in the model was 0. I am sorry, maybe I misunderstand your words, so could you please explain it a little bit more? And if I specific the lower limit below the air-entry value, can I use internal interpolation tables to get the initial condition for the soil column?

In addition, if I want to use water content to specify an initial condition and I only know that the bottom boundary is saturated since it’s a groundwater table. How to get the initial water content profile or how to get the initial condition in terms of water content in HYDRUS? By the way, Brooks-Corey model is used for soil hydraulic properties in my model.

Jirka
Posts: 4759
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

Re: How to get infiltration rate from HYDRUS 2D/3D

Post by Jirka » Wed Oct 04, 2017 6:45 pm

No, set the lower limit of the tension interval to 7 or 10 in the Iteration Criteria window. J.

Lili Yao
Posts: 8
Joined: Fri Sep 29, 2017 6:31 pm
Location: USA

Re: How to get infiltration rate from HYDRUS 2D/3D

Post by Lili Yao » Thu Oct 05, 2017 6:44 pm

Hi Jirka,

Thank you very much for your kind help and patience. If I want to use water content to specify an initial condition and I only know that the bottom boundary is saturated since it’s a groundwater table. How to get the initial water content profile or how to get the initial condition in terms of water content in HYDRUS? I think it’s not good idea to get initial water content distribution from the interpolation table. So, could you tell me how to do with it? By the way, Brooks-Corey model is used for soil hydraulic properties in my model.

Thank you,
Lili Yao

Jirka
Posts: 4759
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

Re: How to get infiltration rate from HYDRUS 2D/3D

Post by Jirka » Thu Oct 05, 2017 8:43 pm

Lili, I think you will have to resolve some issues yourself. I do not have time to guide you step by step. J.

Lili Yao
Posts: 8
Joined: Fri Sep 29, 2017 6:31 pm
Location: USA

Re: How to get infiltration rate from HYDRUS 2D/3D

Post by Lili Yao » Thu Oct 05, 2017 10:40 pm

Sorry for taking you so much time. I am sorry. But thank you very much, you help me a lot.

Thank you,
Lili

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