What PLE could do with cummulative values???

[yanet288]

Hi all
My question is related to whether or not I can modelling with the Free version of vensim (Vensim PLE) using a value that will feed later on the same variable

Here is the BIG question:
Is it possible to use one variable (stock - level) on Vensim PLE that could store the previous value of the variable and use it to estimate the next value, when I have already declared an initial value, for instance, the equation inside the stock - level is as follow:

Variable name: Aquifer Pollutant Load
Equation: =INTEG(Aquifer Pollutant Load - Pollutant Load OUT + New pollutant Load IN)
- initial value = 100

In order to get the second value of this variable "aquifer pollutant load", which in an excel spread sheet it will be = previous value of the variable "aquifer pollutant load" + new pollutant load IN - pollutant load OUT, which function/type of variable/equation/trick could allow me to accumulate always the previous value of the "aquifer pollutant load" in order to use it for the next simulation?

I have done it on Excel but it seems it could not be reproduced in vensim PLE. I have tried with the CUMULATE function but it indicates this function is not supported by vensim PLE.

Any suggestion/comments/advise are more than welcome!

[Administrator]

DELAY FIXED will do what you need. Just delay by one time step (or whatever time you need the previous value from).

[tomfid]

Typically you want to avoid this kind of discrete time formulation though. Much better to free yourself of the limits of Excel and rethink your model in continuous terms.

[yanet288]

DELAY FIXED will do what you need. Just delay by one time step (or whatever time you need the previous value from).

Thank you for your advise
I have used the function DELAY FIXED as follow:
Aquifer concentration (aquifer accumulate, TIME STEP, INITIAL TIME)
but I do get the following error
ERROR: Floating point error computing - AQUIFER Concentration - at time = 14.000000.
Trying to save the results anyway
do you have any idea what I am doing wrong?

[Administrator]

Have a look at the numerical values using the table tool and see what is going wrong.

Most likely your levels are going too big.

[tomfid]

For an aquifer, it would be more typical to model it continuously, e.g.

water in aquifer = INTEG( recharge rate - withdrawal rate, initial water in aquifer )