Ventana software support forum

Dear members,

I have constructed a model to predict the water shortage rate and used simultaneous equations to study the effects of different regulatory policies on the water shortage rate. This control was achieved by setting f to a value within [0, 1]. An error "Unable to converge simultaneous loop at time T" (not all the time) occurred when I assumed that f1 =0.7, f2 =f3=0.

I don't get an error using the latest version of Vensim.

But are you sure you should be using the SIMULTANEOUS function? There has to be a delay between a shortage and any kind of adjustment to capacity.

You might also want to add units to the model. It's impossible for me to know what the model is actually calculating (eg, what is fish pond area measured in, is it m2 or km2 or something else?). It's also bad practice to embed constants in your equations (eg, what is "0.00748269" in the "water shortage rate"?).

Thank you for your reply.

I took into account the information transmission as "water shortage rate -> feedback rate a -> policies determining water demand and supply -> new water supply and water demand - water shortage rate after regulation". Water shortage rate was included in this cycle, so the simultaneous equation was used. And 0.00748269 was the predicted water shortage rate without policy regulation. The unit issue will be modified in the formal model.

"There has to be a delay between a shortage and any kind of adjustment to capacity." Are you saying that I should use delay functions instead of simultaneous equations?

Thanks and regards.

Kira Zhou wrote: ↑Mon Jun 15, 2020 8:34 amI took into account the information transmission as "water shortage rate -> feedback rate a -> policies determining water demand and supply -> new water supply and water demand - water shortage rate after regulation". Water shortage rate was included in this cycle, so the simultaneous equation was used. And 0.00748269 was the predicted water shortage rate without policy regulation. The unit issue will be modified in the formal model.

This implies that a change in water shortage gives an immediate change in policy. Is that correct? I'm sure there should be a delay in there somewhere.

Yes, in theory there should be delays! Your comments are very relevant, and I will further improve the model according to this idea.

Another issue here seems to be that supply and demand are completely insensitive to the water shortage index. For example, a1 is constant as parameterized.

Dear administrator, I hope to delete all posts under this topic due to the need of confidentiality, but I do not have permission. Could you please help me?

I deleted the post with the model. The rest is useful for others, if OK to leave it. If not, let us know.