Effects of reclaimed water irrigation on the evolution characteristics of desiccation crack of typical subtropical zone soils

The requirement for water resources is ever-increasing with the rapid development of the economy in China. Reclaimed water can be used to greatly alleviate the contradiction between supply and demand for fresh water. The unconventional water sources can also contribute to the structure of water supply, water pollution and ecological security in sustainable agriculture. But, long-term irrigation with reclaimed water may cause the change of soil physical and chemical properties, and then result in the development of desiccation cracks. This study aims to reveal the effects of reclaimed water irrigation on soil desiccation crack and the developmental process in the subtropical region. Four types of subtropical soils were selected as the test materials, including the red, Fluvo-aquic, purple, and paddy soil. Four concentrations of reclaimed water were also used for continuous irrigation, including original reclaimed water (RW) and diluted concentrations of 2(RW-2), 4(RW-4), 6(RW-6) times. An evaporation test was then carried out, where the digital image processing was combined to extract the parameters of soil desiccation cracks. The result showed that the reclaimed water inhibited the formation and development of desiccation cracks in the red soil. The inhibitory effect became more obvious, as the concentration of reclaimed water increased. Specifically, the water content of the initial crack, the area density, and the length density of the desiccation crack dramatically decreased by 57.9%, 54.3%, and 35.4%, respectively. By contrast, the reclaimed water promoted the formation and development of desiccation crack in the Fluvo-aquic, purple, and paddy soil. There were most promoting effects on desiccation crack areas of Fluvo-aquic soil, and desiccation crack length and connectivity of the purple soil, whereas, the least promoting effect was found in the paddy soil. Furthermore, the promoting effect became much stronger, as the concentration of reclaimed water increased, except in the reclaimed water-diluted concentration of 4 times. In addition, there was a balanced process of desiccation crack in the red soil, whereas, that concentrated in the early stage of Fluvo-aquic, purple, and paddy soil, indicating obvious concentration effect of reclaimed water with low concentration. It was also found that there were great influences of soil type on the developmental rate of crack area density in the initial stage, particularly on the developmental rate of connectivity index. Similarly, the concentration of reclaimed water presented a great influence on the length in the deceleration stage, the developmental rate in the accelerated and initial stage, and the maximum developmental rate of crack length density. A three-linear model was utilized to fit the relationship between the developmental degree of soil crack area density, and water content under reclaimed water irrigation. The water content of the initial, accelerated, and decelerated stages were 22.67% to the water content of initial crack, 17.20%-22.67%, and 0-17.20%, respectively, while the developmental rates of each stage were 0.017, 0.037, and 0.020, respectively. A four-parameter log-logistic equation was selected to fit the relationship between the developmental degree of length density and connectivity, as well as the water content. The water content of the accelerated and decelerated stages during the crack length development were 28.06% to the water content of initial crack and 0-28.06%, respectively, where the maximum developmental rate was 4.56, while the water content of the accelerated and decelerated stages during the development of crack connectivity were 26.27% to the water content of initial crack and 0-26.27%, respectively, where the maximum developmental rate was 4.50. The finding can provide a sound reference to develop the quality standards of reclaimed water for irrigation systems in the subtropical zone.