Appendix 4 171
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–– We did not model roads as rigorous spatial networks, so travel times along roads could not be considered, only straight-line distances to existing urban areas. –– We did not consider differences in road quality. Urban area –– The choice of what constitutes “urban” involves some subjectivity when doing digitization from imagery, as well as choosing which satellite settlements to include as part of the named urban area. Barriers –– Barriers to the movement of people, such as a large river, were not incorporated in the model, as the primary consideration was proximity, not how a cell would be accessed. Water –– The landcover data used is very coarse, and may miss smaller water bodies that are nonetheless important for constraining urbanization. Zoning –– Land use conversions are highly dependent on regulatory regimes such as land use zoning, which could not be captured here because of inadequate, comparable data. Other exclusions –– Due to a lack of consistent and readily available data, we did not include swamps or airports in these simulations.
Room for Improvement As this modeling effort was a quick pass to illustrate the potential impact that future urbanization could have on nearby water resources, there are a number of ways that the approach and results could be improved. Particular options include: • Sensitivity analysis and more rigorous parameterization would improve the model. • As a last step of urbanization allocation, the lowest suitability zone to be urbanized has pixels assigned randomly, with no consideration of spatial neighborhood—it would be better to preference cells closer to the city for random allocation (that is, density decay grid). • Road quality, such as the difference between roads connecting secondary cities versus minor roads, may provide additional spatial information on the suitability of urban growth along these different types of roads.