Jump to content
GMS, SMS, and WMS User Forum

Observation Coverage Data Problem


Hydrology
 Share

Recommended Posts

I have a model where I have been comparing the head in the grid to the head displayed in the observation coverage time series plot, and I have noticed that the two do not match up at all. My observation coverage is for layer one, and shows a value of 31.04 at time step 0.25, but the value for head from putting my cursor over the grid shows a value of 35.87. That is quite a big difference. The grid value is correct, because I am starting the transient model from a background elevation of 36. and it should only be depressed slightly by 0.25 days. By day 15, the grid shows a value of 15, which is correct, but the observation coverage shows 30.83 which is definitly wrong. The other observation points are incorrect as well, but the pattern of the error is different.

How is the value for the observation coverage calculated? Is it supposed to be the value in the grid where it is placed, or is something else going on?

Link to comment
Share on other sites

The values in the observation coverage come from the data that you enter at the observation points. If the observation data is transient, when you plot the computed vs observed it linearly interpolates the observed head from the transient observed data that was given to the observation point to the time step date. Observation points are used to calibrate the model by match known observations with the model. The computed vs observed chart plot the given heads at the observation point vs the computed head made by the model at the selected time step to see how the model accurately represents real world situations.

Link to comment
Share on other sites

I have a model where I have been comparing the head in the grid to the head displayed in the observation coverage time series plot, and I have noticed that the two do not match up at all. My observation coverage is for layer one, and shows a value of 31.04 at time step 0.25, but the value for head from putting my cursor over the grid shows a value of 35.87. That is quite a big difference. The grid value is correct, because I am starting the transient model from a background elevation of 36. and it should only be depressed slightly by 0.25 days. By day 15, the grid shows a value of 15, which is correct, but the observation coverage shows 30.83 which is definitly wrong. The other observation points are incorrect as well, but the pattern of the error is different.

How is the value for the observation coverage calculated? Is it supposed to be the value in the grid where it is placed, or is something else going on?

I assume you are talking about the value computed at the observation point and the value computed at the grid cell center. These two values can be different if the values of head in the adjacent cells are different than the cell you are looking at. MODFLOW computes a head value at the observation point. You can look at the MODFLOW 2000 documentation to see how those calculations are performed.

Without seeing your model I am not sure what is going on. If you think it is a bug in GMS then contact Aquaveo support and we will look at it.

Link to comment
Share on other sites

  • 1 month later...

I have a similar problem with the transient observation data. When I compare the values of an observation point using the time series graph and the same grid cell using the active data set time series graph, the points are shifted one time step. It looks like the time series graphs relate the computed head to the beginning of the time step and the active data set time series relates the heads to the end of the time step. How do I get these two types of graphs to agree?

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
 Share

×
×
  • Create New...