Fish Stocking Model

[VTHokie]

Hello All,

I am working on a grass carp stocking model which simulates grass carp herbivory on aquatic plants over the long term (20 years). My question is: Is there a function in vensim PLE+ that I can use to alternate the number of fish stocked annually (inputs into the model)? For example, I need to program 6000 fish stocked in year 1, 3000 in year 2, and 1500 in year 3, with each stocking occurring in April. The modulo function works for a constant stocking rate, however I was hoping someone may provide insight into handling different levels of stocking.
Thanks!

[Administrator]

You could use a lookup table combined with modulo.

Code: Select all

fish restock F ((1,6000),(2,3000),(3,1500))

fish restock = fish restock F (time)

[VTHokie]

Oh yes, of course. Thank you!

[tomfid]

Another option would be lookup + PULSE TRAIN as the trigger. PULSE TRAIN may be a little simpler to use than IF THEN ELSE( MODULO(Time, ... )), as you don't have to worry about roundoff and time step boundaries.

[tomfid]

An example ...

fish stocking.mdl

(3.84 KiB) Downloaded 243 times

[VTHokie]

Great, that will work perfect for inputting the grass carp stocking regime into my model. As a newbie to Vensim and systems modeling, I really appreciate your suggestions.

[VTHokie]

Hello Everyone,

I have another vensim programming question. Is there a way to program a lookup to repeat at the same time-steps annually? I plan to model grass carp increases in weight over the growing season to accurately portray aquatic plant consumption (which is a function of grass carp body weight). I have calculated weight at each interval over the growing season for all age classes in my model (1-16). My model runs on a montly time step where grass carp advance to the next age group after 12 months. I planned to use a lookup to model increases in weight, for example at age-1 monthly weights (kg) for grass carp are ((5,.25),(6,.5),(7,1),(8,1.6),(9,2.2),(10,3),(11,0)) where growth only occurs between May and October when plants are present, and the 0 in November resets it for the next stocking to come in. While this works for one year, I plant to simulate 20 years of stocking, and repeating this lookup 20 times doesn't appear feasible. I'm guessing there's a much easier way to do this, but I haven't been able to find it in the vensim documentation. I appreciate any insight you can provide.

[tomfid]

Right - rather than using

carp growth lookup(time)

just use

carp growth lookup( MODULO( time, months per year ) )

with months per year = 12.