Propagation and closure law of desiccation cracks of loamy clay during cyclic drying-wetting process

Abstract: To investigate the propagation and closure law of farmland soil desiccation cracks during cyclic drying-wetting process, laboratory simulation tests were carried out to quantitatively analyze the geometric and morphological characteristics of desiccation crack network of loamy clay with the application of digital image processing techniques. The results showed that the change of gravimetric moisture content during the drying process experienced 3 steps over experiment time, and could be fitted to a linear equation, a quadratic equation and a power equation, respectively, whose determination coefficients were larger than 0.95. During the wetting process, the closure of cracks could be divided into 3 stages and when the moisture content increased to 45%, the cracks were closed completely. When the moisture content was lower than 12%, both crack area ratio and area perimeter ratio decreased slowly with the increasing of the moisture content, and decreased rapidly once the moisture content reached 30%, while keeping stable under the moisture content of between 12% and 30%. Crack length density and connectivity index varied with the increasing of the moisture content in two typical stages, i.e., remaining stable in the initial stage of wetting process and decreasing quickly after the moisture content reached 32% and 35%, respectively. Results suggested that the relationships of crack area ratio with the moisture content in drying and wetting process could be fitted to Logistic function (R2=0.9981) and BiDoseResp function (R2=0.9972), respectively. The changes of crack length density with the moisture content in drying and wetting process were in line with Logistic function, and their determination coefficients were 0.9962 and 0.9978, respectively. The fitting functions showed that during cyclic drying-wetting process, the change of crack length was partly reversible, but on the whole, the propagation and closure of cracks were two processes that were completely irreversible. The statistical analysis of geometric parameters of cracks at different soil moisture contents during cyclic drying-wetting process indicated that crack area mainly ranged from 0 to 30 mm2, and most of the cracks were shorter than 40 mm at different moisture contents. During the drying process from field capacity to wilting coefficient, the frequency distributions of both crack area and crack length significantly changed, and the number of the cracks increased greatly, for example, the number of crack whose area was less than 60 mm2 increased by 367% and that whose length was less than 40 mm increased by 470%. However, there was no significant change between the frequency distributions of crack area during the wetting process from wilting coefficient to field capacity, neither did the frequency distributions of crack length. The comparison of crack geometric parameters at the same moisture content during the drying and the wetting process showed that the frequency distributions of crack area and length were almost the same when the moisture content was at wilting coefficient. On the contrary, the frequency distributions of crack area and length were quite different at the moisture content of field capacity, and there were a difference of 341% between the numbers of the cracks whose area was less than 60 mm2 and a difference of 430% between the numbers of the cracks whose length was less than 40 mm during the drying and the wetting process. The research results will contribute to the study on the propagation and closure mechanism of soil desiccation cracks and the crack preferential flow, and provide the theoretical basis for determining precise irrigation system based on the crack network.