Assigning Downstream Boundary Conditions

I’m modeling the water quality process in a reservoir using boundary conditions of flow/volume , but comparing the water levels results with the expected ones i realized that the model is storing more volume that should be so that means the series don’t match. In the forum i read that the phenomenon occurs when this kind of boundary conditions is used, thats why i need to know if there is any option to assigning water levels instead of flow/volume boundaries conditions.

Hi scardon9,

The increase in decrease in model might occur from missing leakage or ungaged flows in the model. Which boundary condition are you using on downstream ? Are you using flow (discharge boundary) ? You might try using the water surface elevation boundary. If you are using the flow boundary then you should try to do mass balance in the model.

I hope it helps.


Hi John,

Thanks for your answer, in deed i’m using on downsteam of the model a flow discharge boundary, but as i wrote in the beggining of this forum i read that using the same type of condition up and downstream of the model it’s not advisable, that’s why i’m interesting in use the surface elevation boundary but i dont know how to do it. I read the user’s manual of the EFDC explorer 7 but i didn’t find any thing usefull where can i find something that can help me using this kind of boundary condition?

thanks again

Hi scardon9,

While assigning the water surface elevation boundary condition you need to be consistent with the datum you used for the bathymetry. The commonly used datum are MLLW (mean lower low water), MSL (mean sea level) or NAVD etc. When you know the datum you are using for your model then you can define the water surface elevation boundary. To define such boundary you need to define boundary either in East direction, west direction , south or north depending upon the grid orientation.

To create a new water surface elevation boundary follow the following steps:
1) Go to View Plan in EFDC Explorer.
2) Go to Viewing opt’s and choose Boundary C’s
3) Click on enable edit on the middle right part of the window.
4) Then right click on the cell where you want to define the boundary condition and choose new.
5) Enter the boundary group id. This is the arbitrary name that you can give so that you keep track of boundary conditions. You can write whatever you want.
6) IN the next step, you need to choose the boundary conditions/ forcing functions. There are total of 7 options and it might be different depending upon the version of EFDC explorer you are using. I am referring to EFDC Explorer version 6. For the water surface elevation boundary you have 4 options ; Open BC: South , west, north or east. If you want to define the boundary from the south face then click on 3 or if you want to define from the west face then type 5 and so on . After that you are all set.
you have now defined the boundary condition.
7) You can then edit your boundary condition. This time you will have to define pressure series instead of flow series when you defined discharge boundary.

I hope this helps.



Hi John,
Thanks for the infomation it was very usefull, finally i solved the problem with the model.

Hi Scardon9,

It’s good to hear you figured out your issue and I am glad I was of some help.

Good luck for your study.