2.5 LAND USE CONVERSION SEQUENCES
Not all land use changes are possible and some land use changes are very unlikely
(e.g., arable land cannot be converted into primary rain forest). Many land use
conversions follow a certain sequence or cycle, e.g. fallow land and forest regrowth often
follow shifting cultivation. Figure 15 indicates a number of possible land use trajectories
identified on Sibuyan island.
33
Figure 15. Possible land use trajectories on Sibuyan island.
The conversions that are possible and impossible are specified in a land use conversion
matrix. For each land use type it is indicated in what other land use types it can be
converted during the next time step. Figure 16 provides a simplified example of a land
use transition sequence. Forest can be converted in either agricultural land or grassland,
while it is impossible to obtain new (primary) forest through the conversion of agricultural
land or grassland directly. The figure also illustrates the translation of these conversion
sequences into a land use conversion matrix, which can be used by the model.
Depending on the definition of this conversion matrix and the time-steps chosen,
complex land use sequences are possible.
Figure 16. Land use transition sequence
The land use conversion matrix can be edited by clicking ‘edit files’ and select ‘change
matrix’. It is also possible to use a text editor (e.g. Notepad) to edit the file ‘allow.txt’ in
the installation directory. The rows of this matrix indicate the land use types during time
step t and the columns indicate the land use types in time step t+1. If the value of a cell
is 1 the conversion is allowed while a 0 indicates that the conversion is not possible. The
rows and columns follow the number code of the land use types.
34
Example: in the matrix below all conversion are possible except the conversion from
coconut plantation into rice fields.
1 1 1 1 1
1 1 1 0 1
1 1 1 1 1
1 1 1 1 1
1 1 1 1 1
Code Land use type
0 Forest
1 Coconut plantations
2 Grassland
3 Rice fields
4 Others
Run the baseline scenario for Sibuyan island with a different setting of the conversion
matrix (keeping all other settings equal) and analyse the differences in outcome with
ArcView. We suggest to compare a model run that allows all changes with a model run
in which the conversion of grassland into agricultural land (coconut plantation and rice
fields) is no longer possible due to soil degradation. Compare the results.
Note: Some land use conversion settings will have no effect because they are overruled by the
conversion elasticity and land requirement settings. In the baseline scenario we have assumed
that the ‘others’ land use type is not changing and forest cannot ‘re-grow’ from other land use
types as long as its total land area is decreasing. Changing the conversion settings for these land
use types in the conversion matrix will have no effect on the simulation results.
Each simulation, the model will overwrite the results of a previous simulation. If you want to save
the results, rename the output files or move the output files to another directory.