## Optimal depth of the drain

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drisya86
Posts: 8
Joined: Fri Jan 04, 2013 10:51 am
Location: India

### Optimal depth of the drain

Sir,
I am using H-2D to determine optimal depth,based on soil type and salinity.....
The simulation I want to do will not have irrigation or evaporation but with inital saturation condition. A uniform soil profile(general type with drain as seepage face) was created.... What i want to know is how much water is leaving the top BC on a daily basis, and when does it reaches a steady state, for pre-fixed depth of water table.....For this, what should be the boundary conditions i should use and how to calculate seepage flux?Is there any worked out example or article for the same? whether Hooghoudts equation can be used for drainage flux calculation?

Jirka
Posts: 5226
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

### Re: Optimal depth of the drain

Look at the Furrow example that comes with the installation and has a tile drain in the profile. You can also look at Drain examples that are posted at http://www.pc-progress.com/en/Default.a ... plications.

I you specify no irrigation and evaporation as the upper BC then there will be no flux across the upper boundary and all water will leave through the drains. The steady condition will be a horizontal water table at the depth of drains.

HYDRUS solves the Richards equation and not a Hooghoudt's equation, which a simplified equation with a lot of assumption. You can obviously compare the results, with HYDRUS giving a correct solution, and Hooghoudt giving an approximate solution. Seepage flux (through drains) is one of the output fluxes.
J.

drisya86
Posts: 8
Joined: Fri Jan 04, 2013 10:51 am
Location: India

### Re: Optimal depth of the drain

thank u sir for the example mentioned and clue to the boundary conditions..it helped me...But again, would you mind telling me how to specify a water table height say 0.5m for a soil column of length 5m in H2D general discretizations.?

Jirka
Posts: 5226
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

### Re: Optimal depth of the drain

I do not have H2D installed on my PC anymore. In the newer HYDRUS 2D/3D, one would do the following. Select the entire domain, choose the option Equilibrium From the Lowest Located Nodal Point, and specify such value so that at depth of 0.5 the pressure head is zero.

J.

drisya86
Posts: 8
Joined: Fri Jan 04, 2013 10:51 am
Location: India

### Re: Optimal depth of the drain

Sir,
Hope that we dont want to create any additional surface for this, which is not possible in H2D. Im trying to solve it, but error is showing as "inital condition is either smaller than residual water content or greater than saturated water content",although its not so....I used eqn 2.48 in soil physics with hydrus for the matric potential calculation.? and set all other BC as no flux,only top BC equivalent to residual water content..where shall be the possible error??

Jirka
Posts: 5226
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

### Re: Optimal depth of the drain

You likely set that you want to specify the initial condition in terms of water contents. You need to set in Iteration Criteria window that you want to specify Initial Condition "In the Pressure Head". Then you could not get this message. J.

drisya86
Posts: 8
Joined: Fri Jan 04, 2013 10:51 am
Location: India

### Re: Optimal depth of the drain

Sir,
thank you for the reply...i have checked everythng.but couldnt trace any error.but anyway when i made it rectangular geometry, with all the other items same the problem solved. the program runs and the outputs are got. But now, i want to analyse the number of days for it took to saturate from the output files.which output file shall i take for this?..

Jirka
Posts: 5226
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

### Re: Optimal depth of the drain

I do not understand your question. You use drains to drain the profile and then ask about how long it takes to saturate the profile (which is just the opposite)? Please, try to formulate your questions more clearly and also try to use the proper grammar so that we can understand what you are saying. Anyway, you should check pressure heads. J.

drisya86
Posts: 8
Joined: Fri Jan 04, 2013 10:51 am
Location: India

### Re: Optimal depth of the drain

What I want to know is how much water is leaving the top BC on a daily basis, and when does it reaches a steady state, for pre-fixed depth of water table.? i am not using any drain for this specific problem..what file should i take for analysis?

Jirka
Posts: 5226
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

### Re: Optimal depth of the drain

v_mean.out and CumQ.out. See there description in the Technical manual. J.

drisya86
Posts: 8
Joined: Fri Jan 04, 2013 10:51 am
Location: India

### Re: Optimal depth of the drain

Sir,
This is a general doubt not related to the drain problem. Whether saturated conditions can be described as a steady state condition?

Jirka
Posts: 5226
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

### Re: Optimal depth of the drain

Steady-state conditions are when all boundary fluxes and conditions in the transport domain are constant, invariable of time. I do not see any reason why such conditions could not occur under both saturated or unsaturated conditions. J.

drisya86
Posts: 8
Joined: Fri Jan 04, 2013 10:51 am
Location: India

### Re: Optimal depth of the drain

Thanks for the reply sir..it may be due to the variability nature of the influential parameters....

I understood the possiblity of importing the output of the models as inital conditions of the next run.
But,may i know, is it possible to use the option in the following objective:

To make a saturated soil column first, and then asking the same to leave entire water content(upto residual water content)

I liked the software setup very much and am interested to explore the same. I am also pleasure enough to say that i liked your way of answering in all forums.

Jirka
Posts: 5226
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

### Re: Optimal depth of the drain

Drains will never drain the profile down to the residual water content. Drains can only lower the pressure head down to zero at their horizontal level (i.e., to full saturation) and then equilibrium pressure head distribution above them. Thus you can get residual saturation only at infinite height above drains. J.

drisya86
Posts: 8
Joined: Fri Jan 04, 2013 10:51 am
Location: India

### Re: Optimal depth of the drain

Sir,
If we are selecting the reference level at the base of the column and doing for the output flux analyis, whether the negative fluxes indicate the upflow in the column?