1. World Erosion Map in F. Fournier, Climat et érosion, Presses Universitaires.
2. “European Soil Map,” text, fao, 1967.
3. J. Dorst
Avant que nature meure, Paris, 1965, p. 149.
4. J. Snel, “L’eau souterraine dans l’economie regionale de l’éau,” ex. Technique de Veau, p. 255, 256 and 258, 1968.
5. A. Noirfalise, Forest management: ecological consequences of intensive pine cultivation in deciduous forest areas of temperate Europe, Council of Europe, 1968.