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Basic Model Behavior with Stella.

Learning Objectives:  Start learning: 1) how to use Stella II software; 2) some basicsof model construction; and fundamentals of Stocks and Flows.

Open Stella II software and get into the odel !indow "see left tab).

#uild each simple model described in this document "drag and drop icons and connectthem together appropriatel$) and run each for 2% da$s "&un menu select &un Specs tochange the length of the run). 'he model is simulating the content "in acre(feet) of water 

in a lake. 'he flows are in acre(ft per da$. Set up a graph of and either inflow "oroutflow) for each simulation. *o this b$ clicking and dragging the graph pad icon to anempt$ location on the model window. +e,t double click on the graph pad and selectitems for graphing.

'his -ou'ube ideo is a /uick guide to getting started. http:00www.$outube.com0watch-c$345t#67 

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&esae this document and modif$ it b$ including $our answers to all /uestions and pasting in graphs where appropriate. Submit this document when $ou are finished.

Linear Growth/decrease

Stella odel 1 'ank flow Stella odel 2 'ank flowC

inflow

"t) "t ( dt) 8 "inflow) 9 dtI+I' %

I+F3O!S:inflow

C

inflow

"t) "t ( dt) 8 "inflow) 9 dtI+I' %

I+F3O!S:inflow "note : the inflow was changedfrom uniflow to biflow. 'o do this doubleclick on the inflow when $ou are in themodel screen. )

Stella odel 'ank flow Stella odel < 'ank flowC

outflow

"t) "t ( dt) 8 "( outflow) 9 dtI+I' %

O='F3O!S:outflow

C

inflow"t) "t ( dt) 8 "inflow) 9 dtI+I' %

I+F3O!S:inflow - "note : the inflow waschanged from uniflow to biflow. 'o do thisdouble click on the inflow when $ou are inthe model screen. )

Question1: >ow does the behaior of model 2 compare to that of model 1

?ns1:

Insert a graph here of s time and inflow s time "both on same a,es) for model 2.@lease see the -ou'ube ideo regarding pasting from Stella into !ord. It discusses howto aoid huge electronic file si4es. http:00www.$outube.com0watch2c%,<#-dhfs 

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Question : >ow does the behaior of model compare to that of model <?ns2:

Question !:  *ouble click on the icon for and check or uncheck the non(negatiecheck(bo,. &e(run the model for 2% da$s. ompare the output of model < when thenon(negatie check(bo, is checked to the output when the non(negatie check(bo, is +otchecked. !hat does the non(negatie check bo, do?ns:

@aste graphs of checked and unchecked below. &esi4e so the$ sit side b$ side.

hange the units of time to $ears. 'hereAs nothing reall$ that $ou hae to do e,ceptchange $our mental focus and double click on the graph to change the labelcorresponding to units.

Stella odel  "#ank #alance)

$

Monthly increase

Initial $

yearly growth Rate%B"t) B"t ( dt) 8 "onthl$Cincrease) 9 dtI+I' B InitialCB

I+F3O!S:onthl$Cincrease B9$earl$CgrowthC&ateD01%%InitialCB 1%%$earl$CgrowthC&ateD 2%

Euestion <: >ow much is in the account after 2% $ear ?ns <:

Insert a graph of B s. time here for $our 2% $ear simulation.

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hange the units of time to minutes. 'hereAs nothing reall$ that $ou hae to do e,ceptchange $our mental focus and double click on the graph to change the labelcorresponding to units. below stands for tank content.

Stella odel 5 "3eak$ 'ank)

"t) "t ( dt) 8 "( +onameC1) 9 dtI+I' initialCcontent

O='F3O!S:Outflow 0holeCparameter holeCparameter 1%initialCcontent 1%%

'he reason for adding the initial content as a separate conerter here is that this can then be linked to a slider input in the interface window. 'his allows the model user to easil$change the input of a model.

#uild odel 5 and set up a graph of s time in the interface window. !hen setting upthe graph check the comparatie graph check bo,. ?dd slider inputs for both initialcontent and hole parameter.

ake < runs of the model and present a graph showing all of these runs.&un 1: Initial content 1%% ; hole parameter1%&un 2: Initial content 2%% ; hole parameter1%&un : Initial content 1%% ; hole parameter2%&un <: Initial content 2%% ; hole parameter2%

Euestion : as the hole parameter increases the tank becomes "more or less leak$) ?ns:

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Insert a graph of s. time here for $our four 2% minute simulations.

hange the units of time to $ears. 'hereAs nothing reall$ that $ou hae to do e,ceptchange $our mental focus and double click on the graph to change the labelcorresponding to units. 'he stock population is the number of organisms of ah$pothetical population.

S Sha"ed Growth

Stella odel Shaped Growthpopulation

births

birth rate

deaths

death rate

 population"t) population"t ( dt) 8 "births ( deaths) 9dtI+I' population 1

I+F3O!S: births population9birthCrateO='F3O!S:deaths population9deathCrate8.%%<9populationH2 birthCrate .2deathCrate .%1

Euestion : !hat is the carr$ing capacit$ of this population I.e. the e/uilibrium leel.

Insert a graph of population s. time here for a % $ear simulation.