# Error runing the EFDC water quality model

Hello,
I’m modeling a reservoir in the EFDC using the Temperature, Cohesive sediment, Dye and Water quality modules, i ran the same model before with different data and i didn’t observed any problems but when i want to analyse another period of time (changing the flow, temperatures, winds, pressure, cohesives, water quality and atmospheric time series) the model shows me the following message: “WQ concentration time series Maximun iterations exceeded in external solution”,
what could it be?
Thanks for any help

Scardon9,

I guess this is probably due to the time step. Try lowering your time step to see if it solves your problem.

Regards,
John

Hi John,
As you said lowering the time step solves the poblem, but now i have a question. In wich way it helps to solve the problem, i mean in which way it helps to reduce the iterations made by the water quality module?
Regards,
Sebastian

Sebastian,

I am sorry it took a while to get back to you. EFDC model uses the explicit scheme to solve for the governing equations and it is a feature of the explicit solutions that you need to comply with the CFL (Courant Freidrich Levy) conditions for your solutions to converge. Previously, when you had large time step, you were probably violating the CFL condition. You can find more details about CFL condition from Wikipedia. http://en.wikipedia.org/wiki/Courant–Friedrichs–Lewy_condition

Regards,
Janesh

Hello
I’m modeling a deltic sea with the EFDC model,and two problems caught me.Firstly,the model stops ,showing negtive depths at I,J…but if i set the depth a constant value ,the modeljust runs well.How could it happen? Secondly,the model shows error at 83 of the calsed subrutine,with the x1**x2,x1<0 and x2/=0.By checking the calsed ,the x1 represent sed(l,k).
What could be done to make them corrected?
Thanks for any help

xingguopan wrote: Hello
I'm modeling a deltic sea with the EFDC model,and two problems caught me.Firstly,the model stops ,showing negtive depths at I,J....but if i set the depth a constant value ,the modeljust runs well.How could it happen? Secondly,the model shows error at 83 of the _calsed_ subrutine,with the x1**x2,x1<0 and x2/=0.By checking the calsed ,the x1 represent sed(l,k).
What could be done to make them corrected?
Thanks for any help

Hi Xingguapan,
It seems that your first problem with the bathymetry caused by some strange cells, for example, the elevation is sudden increase or decrease. These are useful make the model unstable.