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Effects of cropping and puddling practices on the cracking patterns in paddy fields

Abstract Shrinkage cracks in clayey paddy fields are major pathways for the drainage of excess water through tile drainage. In this study, the factors that affect the cracking pattern were investigated in a paddy field where paddy-rice was planted in rows. The factors examined included 1) the existence of transpiration, 2) row spacing, and 3) the puddling intensity. The experiment was designed according to the Taguchi method of experimental design. Shrinkage cracks were induced by drying the field after the release of the ponding water in mid-summer. The cracking patterns between the rows were recorded on transparent sheets, and their geometry was quantified using the crack direction index (CDI), which represents the direction of cracks relative to the rows of rice, as well as the equivalent width (EW) and compactness (CP) which represent the average width of the cracks and the complexity of the cracking patterns, respectively. Variations in the degree of desiccation affected the cracking patterns; these effects were taken into account in the analyses. ANOVA was performed on the variables CDI, EW, and CP, and the following conclusions were drawn: 1) transpiration from the crops planted in rows induces significant straight cracks along the rows; 2) row spacing exerts a significant effect on the EW of cracks, for example, when the spacing doubled from 30 cm (the conventional spacing in Japan), the cleavage became more concentrated and the cracks became wider; 3) repeated puddling also induces wider straight cracks along the rows and makes the cracking patterns simpler.