Infiltration / undergound water storage values Water Balance

thies

Dear Forum users,

 

I am using the water balance of the rainfall overlay. However, there is one thing that draws my attention. When I create a rather heavy rainfall event, I often see that there is water stored on the land surface of the project, and in the underground unsaturated zone. However, there is almost no water stored in the project saturated zone. For me mee this is rather strange, so I decided to investigate this situation.

For this, I started a complete empty project, so that I could easily control all parameters which could possibly influence the results of the water balance. I put the evaporation to 0mm/day, I excluded sewer systems and I created a project consisting of only 1 surface and underground terrain type. I also excluded a the dry period after the rain event.

I compared the water balance values with manual computations which follow the formulas related to Surface infiltration, Underground infiltration, and Groundwater level, as described on the Tygron Wiki (*1).

 

I looked at 4 different types of rain event:

A.    60mm in 60 min (60mm/h) -> more rain than could infiltrate to the unsaturated zone

B.    30mm in 60 min (30mm/h) -> less rain than could infiltrate to the unsaturated zone

C.     120mm in 2h (60mm/h) -> more rain than could infiltrate to the unsaturated zone

D.    60mm in 2h (30mm/h) -> less rain than could infiltrate to the unsaturated zone

 

What turns out:

1.     It turns out that in all situations the amount of storage in the unsaturated zone is almost 0, where you would expect rather high storage amounts according to manual calculations.

2.     According to the water balance, the water storage in the saturated zone does not change as a result of the precipitation amount. For every rain event, this is a fixed value in the water balance. For rain event A, the underground saturated storage in the water balance (almost) equals the manually calculated storage.  For event B, I would expect less water stored in the underground saturated zone based on the formulas. That does not seem to be the case according to the water balance.

For event C and D I would expect more water storage in the underground. The water balance shows an increase in storage, but not as much as would follow from the formulas/manual calculation.

3.     In situation C & D, the storages in both the saturated and unsaturated zone does not match the calculated values.

4.     Based on the formulas described on the Tygron Wiki (*1), I would expect that the saturated zone storage volumes would change when changing the initial groundwater table. This because the underground infiltration is dependent on the height of the saturated zone, which is determined by the terrain surface high and the water table. However, if I change the groundwater levels (higher or lower), the storage volumes do not change.

5.     When water infiltrate to the underground, the water table should rise. This is in all 4 situations observed. However, only in situation A the water level rise in the simulation corresponds to the calculated water level rise. In situation B, I would expect less water infiltration to the underground (point 2), and thus also a lower increase of the water table. But when measuring the water table, the water table is equally high as in situation 1.

The same happens when looking at situation C and D. In both situations, the groundwater table is at an equal height. I think the water table do not change since the amount of stored water in the underground does not change.

 

In the attached file a have placed materials to elaborate on my findings:

-       A table summarizing the 4 analyzed rain events,

-       Used default settings

-       The calculation method I used.

 

I am wondering if somebody could explain these findings.

*1

- https://support.tygron.com/wiki/Surface_infiltration_formula_(Water_Overlay)

- https://support.tygron.com/wiki/Underground_infiltration_formula_(Water_Overlay)

- https://support.tygron.com/wiki/Groundwater_level_formula_(Water_Overlay)

https://community.tygron.com/forum/uploads/732/PEWSCZT23SSV.pdf

Hansje

Hello @thies ,

Thank you for your post! I think it has to do with which groundwater mode you chose in the wizard. Could that be the case?

https://previewsupport.tygron.com/wiki/Ground_water_(Water_Overlay)

By default, I believe it is set to infiltration only and then the water does not go to the saturated zone as stated on this page: https://support.tygron.com/wiki/Infiltration_model_(Water_Overlay)

thies

Dear Hansje, thanks a lot for your reply.

I have double chaked it, in the analysis I did, I had the groundwater mode on “complete”. (Picture below).

If I would put it on “none”, also the saturated zone remains empty as well.

When I put the groundwater mode on “infiltration only”, exactly the volume previously stored in the saturated zone is then put in the unsaturated zone. But still the storage is much lower as expected based on the equations.

 

So I think I have the groundwater setting correct, but I do not observe any water in the unsaturated zone.

 

To be clear, I observed this problem in a real 3D project. Therefore I investigated it into detail in a empty project.

 

Hope to find a solution/explanation for this situation!

 

Kind regards,

 

Thies Eeltink

 

 

Hansje

Hello @thies , We are looking into your question but it will take a little bit longer because I want to asl my colleague about this as well.

I hope you have some patience and I will get back to you with answers as soon as possible.

thies

Dear Hansje,

Thanks you so much for looking into it! If there is something unclear about what I did, please let me know, I am happy to clarify things. 

Kind regards,

Thies

Godelief

Hi @thies ,

We have looked at your question and found a bug in the calculation of the unsaturated storage in the water balance. This bug is now fixed and the water balance should be as expected. Please let us know if you have any other questions!

thies

Hello @Godelief ,

Thanks a lot for looking in to it and reflecting to me the fix you did!

I already discovered in the past days that some things has changed. great!

I will let you know when I have more questions about it.

Kind regards, Thies