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Faculty of Agricultural Engineering Technion-Israel Institute of Technology Haifa 32000, ISRAEL
agvictor{at}tx.technion.ac.il
I thank Dr. White for his interest in my work. His only objection consists in the assertion: "... random cracking is not natural in soils". For confirmation, he gives a brief review of his works dedicated to ped structure development. Before replying, I would like to emphasize the following three points: (i) in fact Dr. White's objection relates not only to the discussed work (Chertkov, 2000), but also to the exposition of a basic model (Chertkov and Ravina, 1998); (ii) the treatment of cracking in the works to be cited by Dr. White is behavioral and qualitative. Unlike that of Chertkov and Ravina (1998) and Chertkov (2000) which proposed and validated the model capable of a quantitative prediction; (iii) in my opinion there are no contradictions between the model and Dr. White's major assertions.
Dr. White asserts that "... random cracking is not natural in soils". However, random cracking was observed in an intact clay (Konrad and Ayad, 1997) and in coal mine tailing (Morris et al., 1992) undergoing desiccation and shrinkage. Random distributions of crack spacing, width (at the surface), and depth were measured in a number of works (Zein el Abedine and Robinson, 1971; Yaalon and Kalmar, 1984; Dasog et al., 1988). Even though one considers that shrinkage cracks, developing in a dry season, totally follow ped boundaries and "... can be deduced from soil structure characteristics", this does not mean that random cracking is not natural in soils. Indeed, the cracks themselves, forming ped boundaries, sometime appeared in an intact clay soil under the actions of desiccation and shrinkage. In any case, the distribution of the intervals between the intersections of the cracks with an arbitrary line is random from the viewpoint of an observer (Scott et al., 1986; Zein el Abedine and Robinson, 1971; Yaalon and Kalmar, 1984; Dasog et al., 1988). In the frame of the discussed model, the distribution of vertical cracks is connected to horizontal shrinkage as a function of soil depth (Chertkov and Ravina, 1998). The distribution of horizontal cracks is connected to vertical soil subsidence as a function of depth and additional drying of thin layers at the walls of the vertical cracks (Chertkov and Ravina, 1999).
Two major points are worth noting. Dr. White writes: "their [peds] size tends to increase with depth". In connection with this, see a quantitative prediction in Chertkov and Ravina's (1998) Fig. 5. Further, Dr. White writes: "... the width of the upper part of the crack is a function of the depth of drying". In connection with this, see a quantitative prediction in Chertkov and Ravina's (1998) Fig. 7. The figures illustrate an absence of a contradiction between Dr. White's observations and the model. The observations virtually give additional validation to the model.
Complications mentioned by Dr. White, concerning a gilgai surface or influence of plant roots on cracking, were not considered in the discussed work (Chertkov, 2000) or in the previous work (Chertkov and Ravina, 1998).
Finally, I did not study hydraulic conductivity (both saturated and unsaturated) in the discussed work (Chertkov, 2000) unlike Dr. White's assertion at the end of his comment. Again, I thank Dr. White very much for his interest in my paper.
Received for publication February 19, 2001.
REFERENCES
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