I´m sorry to ask for probably a very simple question, but I cant solve my vensim problems by myself.
I need to create a model with 2 competing prey populations and 1 preadtor which preadates also other aquatic organism (not completly dependent). My problem is that I cant get higher initial values for the population sizes than 1. I would like to have real sizes like (A=100,B=100,X=100 and P=10 or something).
Could someone help me please?
Vensim Beginner cant solve problems
Vensim Beginner cant solve problems
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karankhosla
- Senior Member
- Posts: 107
- Joined: Wed Nov 26, 2008 6:12 am
Re: Vensim Beginner cant solve problems
Hi! a Units Check (Ctrl U) shows that many formulations don't make sense; I'm not sure what these assumptions are based on, so I can't say that they are wrong; however, for example:
Your variable "Birth rate A" should have units "1/Month", so that when it multiplies with the Population of "Plecoptera A" the units for "Births A" result in something like "Plecoptera A/Month"; at the moment the units work out to "Plecoptera A*Plecoptera B".
Similarly, "Meeting A-P" seems to have units, ["Cinclidae P"*"Plecoptera A"], I presume that if this is a contact rate, this should be a ratio.
You are, in fact, able to put any initial value into the stocks, however, your current formulations are creating simulation errors. Also, go to "Model" -> "Settings" and reduce to your Time Step to around "0.0625"
I suggest starting the model from scratch and working backwards from the flows into and out of the stock, by formulating the equations on the basis of dimensional or units consistency.
Best Wishes,
Karan
Your variable "Birth rate A" should have units "1/Month", so that when it multiplies with the Population of "Plecoptera A" the units for "Births A" result in something like "Plecoptera A/Month"; at the moment the units work out to "Plecoptera A*Plecoptera B".
Similarly, "Meeting A-P" seems to have units, ["Cinclidae P"*"Plecoptera A"], I presume that if this is a contact rate, this should be a ratio.
You are, in fact, able to put any initial value into the stocks, however, your current formulations are creating simulation errors. Also, go to "Model" -> "Settings" and reduce to your Time Step to around "0.0625"
I suggest starting the model from scratch and working backwards from the flows into and out of the stock, by formulating the equations on the basis of dimensional or units consistency.
Best Wishes,
Karan