simultaneous equations involving?

[lookingfor17]

when i was trying to run my model, vensim showed that model has errors and cannot be simulated. The reason is simultaneous equations involving. How to solve this problem?

thank you.
Joy

[Administrator]

Have you done a search of previous posts on the forum?

Simultaneous equations errors crop up quite often on the forum.

[tomfid]

This model comes from an out-of-date non-unicode version of Vensim, so I can't read anything - you may need to upgrade your copy of Vensim. http://www.vensim.com/buy.html

Judging by your diagram, the problem is that you have a feedback loop around the inflow to the stock (see attached image). In a dynamic model, every feedback loop needs to have a stock (level) in it.

You need to consider the function of this loop, and what real-world level would permit it to exist. Or, in the rare case that there's effectively an instantaneous or simultaneous process involved, solve it explicitly with the SIMULTANEOUS or FIND ZERO function.

Tom

[lookingfor17]

I have correct my model and get a satisfied result due to your help.
Thank you very much!

Joy

[alirezagohari]

Hi,
I have a SD model. Because the nature of variables in my model, all of them are Auxiliary.
after running this model i see "Simultaneous equation involving". I send a picture of my loop.
Please help me in this matter.

[Administrator]

See the following for where you have gone wrong.
http://www.vensim.com/documentation/ind ... ations.htm

You could try using the SIMULTANEOUS function if you really believe that there are no delays in the loop causing you the problem.
http://www.vensim.com/documentation/ind ... rative.htm

Hope this helps.

Tony.

[alirezagohari]

In my loop, there isn't a converge value and iterate to it. The simultaneous function ( SIMULTANEOUS(A,B)) is an iterative solution, but I want to order the variables.
Of course, i used Simultaneous function in my loop, but the loop haven't been converged. As i mentioned you above, This is a real loop for Agriculture that doesn't have a converged value, in different time step, the results are different.
When i cut the relation between "Expected wheat cultivated land area" and "Net benefit from whet", i don't see this error.

[tomfid]

As this simultaneous loop is depicted, it's a violation of causality.

The solution is to recognize that Wheat Area Cultivated (and possibly other variables) is a stock that cannot change instantaneously.

You might reformulate the loop with:

desired area cultivated = a function of expectations, equivalent to your current formulation, with units of hectares (or equivalent)

area cultivated = INTEG( cultivation change rate, initial area cultivated) ~ hectares

cultivation change rate = (desired area cultivated - area cultivated)/time to adjust cultivated area ~ hectares/year

time to adjust cultivated area = ... ~ years

[alirezagohari]

I used your comment. I can run my model . Thanks alot
I have a question about the rate variables: Is it possible to use a rate variable as a function of 2 or more the other variable?
I have some rate variables that they have this characteristic but after running, only one of input variable value have been used. I send a picture of one of my models rate.

[Administrator]

You can just add up the rates, is that what you want to do?

Total incoming surface flow = incoming transferred flow + returned surface flow + natural incoming flow.

Just make sure the units check correctly when adding the rates.