In Algeria, urbanization has clearly taken place in the most fertile regions, as half of the urban pixels are located in the areas with the highest average growing season PSN shown for the country, between 20 and 40g C/m^sup 2^ (Figure 4c). It is interesting to observe that Algeria is able to concentrate most of its 30 million population on a surface slightly larger than the one of Greece, which hosts only 10 million people, and still maintain a quite high value of PSNgs for half of the urbanized pixels. However, the pressure on the small vegetation cover of Algeria country is expected to increase if the predictions of demographic growth will be realized for this part of the Mediterranean. The countries of the Southern and Eastern Mediterranean rim are expected to grow from 235 million to 327 million by 2025 (Attane and Courbage, 2001). If population density increases in Algeria, the distributions in Figure 4 will move towards left and center on lower values of PSN, almost certainly compromising the well being of this water deprived country. A lowering of vegetation productivity and shifting towards left of the curves in Figure 4 can also be expected in the case of an intensification of the agricultural practices to satisfy a growing population. The analysis of the impact of human settlements on vegetation productivity in countries that like Algeria are at the limits of extensive desert regions becomes critical for their long term environmental management.
Conclusion
The results presented here indicate that application of the weekly vegetation photosynthesis product from MODIS in conjunction with the night time DMSP-- OLS data can be used as an effective tool to compare the impact of human settlements on vegetation productivity. The nighttime lights offer the advantage of higher spatial accuracy than the traditional censuses, offering also the possibility to account not only demographic growth but also changes in land use, as they reflect the spatial extent of built material. Repeated use of this methodology can be used as an effective tool for regional monitoring of urban sprawl and its impact on environment. The methodology is easily implemented since it relies only on remote sensing data. Even though this methodology is no substitute for ground monitoring and field analysis, its scale of applicability makes it particularly interesting for regional planning purposes and to increase public and political perception of the problems connected with unregulated development.