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Dynamic changes in the proportion of vegetation in different states - Model catalogue -

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Dynamic changes in the proportion of vegetation in different states

Model : vegproportions1
Simile version : 3.1+
Date added : 2003-06-25
Keywords : Vegetation dynamics ; function prev ; function last ;


This is actually two models in one! The idea is to work out how the proportion of vegetation in different states changes over time, knowing the initial proportions and the transition proportions from one state to the other.

One method (the 'proportions' array) uses the prev( ) function for looking up the proportions in the previous time step. This funtion returns the previous value of the same variable.

The other method (the 'new proportions'/'current proportions' combination) uses the last( ) function. This function returns the previous value of another variable.

Each method demonstrates how the concept of 'state' can be handled in a Simile model, without explicitly using a state variable (the compartment symbol).

Note that both versions represent a deterministic approach to modelling changes in the proportion of different types of vegetation (or, equivalently, the proportions of land in different land-use classes. This contrasts with the use of stochastic (probabilistic, Markov) models, in which the proportions are interpreted as probabilities, representingthe probability that one patch of land will undergo a transition from one state to another during one time step. See the 'Related models' section below.


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The following graph shows the changes of proportions of vegetation in various states, using the 'proportion' array.