干湿循环过程中壤质黏土干缩裂缝的开闭规律

为研究干湿循环过程中农田土壤干缩裂缝的开闭规律,在室内试验的基础上,结合数字图像处理技术对壤质黏土干湿循环过程中土壤干缩裂缝网络几何形态特征进行了定量分析。结果表明:干燥过程中土壤含水率随试验时间的变化经历3个阶段。增湿过程中,含水率达到45%时裂缝完全闭合;裂缝面积率、长度密度、面积周长比与连通性指数分别在含水率增加到30%、32%、30%、35%时开始迅速减小。裂缝开裂与闭合是2个不可逆的过程。土壤水分在田间持水率和凋萎系数时裂缝几何参数统计表明,大多数裂缝面积在0~30 mm2之间,长度在0~40 mm之间。从田间持水率干燥到凋萎系数的过程中,裂缝面积与长度的频数分布均显著变化,从凋萎系数增湿到田间持水率的过程中,频数分布几乎没有变化。土壤含水率为凋萎系数时,干燥过程与增湿过程面积、长度的频数分布差异较小,而为田间持水率时差异明显。该成果有助于土壤干缩裂缝开闭机理及裂缝优先流的研究,为基于裂缝网络的精量灌溉制度的制定提供理论基础。

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.