Obtaining Groundwater Level over time

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MeL_rr
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Joined: Mon Mar 15, 2021 12:28 pm
Location: Netherlands

Obtaining Groundwater Level over time

Post by MeL_rr » Wed Mar 17, 2021 5:05 pm

Dear Jirka,

I am currently working on a 2D model of a peatland with two drainage ditches on either side. The material distribution of this domain is roughly as followed:
- A top layer of peat of 1,5 m thick
- A clay layer of 1 m thick
- And at the bottom a sandy layer of 2 m thick

At the bottom boundary I applied a constant head boundary. The sandy layer consequently has a constant pressure head in time. However, due to the limited flow through the thick clay layer (with low Ks) the pressure head in the top peat layer is not cosntant in time. Here the pressure head, and the groundwater level, fluctuates in time based on atmospheric input.

I was wondering if it is possible to extract groundwater level data in time from HYDRUS. So for example daily groundwaterlevel (location in depht where pressure head = 0) at the middle point of the domain for each day of my simulation. I read on the forum that this could be done by extracting hBot and substracting the lenght of the domain. However, since I applied a constant head boundary this would not give me the fluctuating groundwater level in the peat. This since the pressure head remains constant in time in the sand layer but not the peat layer.

Additionally, I read on the forum that the groundwater level could be obtained by placing a observation node below the groundwater level. Is this however possible when there are no hydrostatic conditions? The amtospheric conditions and the two ditches on either side constantly influence the system and hydrostatic conditions do not occur in my domain. Is it then still possible to obtain the groundwater level through an observation node? And how would one, roughly, go about doing that?

Or is there perhaps another way to obtain a graph of groundwater level (at the middle of the domain) in time?

Thank you for your time and effort

Sincerely

Jirka
Posts: 5268
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

Re: Obtaining Groundwater Level over time

Post by Jirka » Thu Mar 18, 2021 4:13 am

You can always see the exact location of the groundwater table at print times (see the h=0 contour).

Between print times, you should get the position of the GWT from the observation nodes (located in the vicinity of and little below the GWT). You can always compare these values with the h=0 contour at print times), and from that see whether or not you have hydrostatic conditions in the saturated zone along a vertical. Since we do not consider the specific yield in the Richards equation, there should be either hydrostatic equilibrium (if there is no vertical flow) or some easily defined gradient (if there is vertical flow), from which you could derive the position of the GWT.

J.

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