Estimation model for steady infiltration rate and wetting pattern of aeolian sandy soil under vertical pipe surface drip irrigation

Vertical-pipe drip irrigation is one of the most efficient state-of-the-art techniques for water-saving and temperature conservation, thereby alleviating the combined stress of drought and high temperature in sand-fixing plant seedlings. In terms of long-term engineering, it is necessary to explore the infiltration characteristics of aeolian sandy soil under vertical-pipe drip irrigation. Furthermore, the discharge rate of the drip system is required less than the stable infiltration rate of water in a critical depth, in order to meet the requirement of no flooding seedlings when the emitter is combined with the vertical pipe. In this study, a ponding infiltration test was carried out using a 2 cm constant head in the field to investigate the variation process of cumulative infiltration of aeolian sandy soil under different buried depths and diameters of vertical pipe. A Philip infiltration model was established to fit the data. 9 treatments and 3 control tests were included in the infiltration experiment. Specifically, the diameter of the vertical pipe was designed with the levels of 8.8, 10.6, 12.6, and 14.2 cm, while the buried depth was set at 15, 20, and 25 cm. The results show that the stable infiltration rate of Philip model increased as the diameter of vertical pipe increased, and decreased with the increase of buried depth. A power function relationship (R2> 0.99) was followed between the steady permeability rate and the diameter or buried depth of vertical pipe, where the power function indexes were 2.01 and -0.64, respectively. The well-established estimation formula of stable infiltration rate was also utilized to determine the drip discharge matching with the structural parameters of the vertical pipe, when there was no ponding in the vertical pipe. Subsequently, the infiltration test was conducted for the vertical-pipe drip irrigation. A field test was also carried out to observe the movement of wetting front in aeolian sandy soil under the different dripper discharge, buried depth, and diameter of vertical pipe. A power function was used to process the observed data. 7 treatments and 2 control tests were included in the field test. The dripper discharge was designed with the different levels of 0.9, 1.2, and 1.5 L/h, where the diameters of the vertical pipe were 10.6, 12.6, and 14.2 cm, and the buried depths of the vertical pipe were 15, 20, and 25 cm. The results showed that there was a significant effect of dripper discharge on the migration distance of the wetting front in the vertical downward. The migration distance of the wetting front was much greater in the vertical downward, whereas, slightly increased in the horizontal and vertical upward, as the dripper discharge became larger. Additionally, the migration distance of the wetting front in three directions decreased with the increase in the diameter of the vertical pipe. The migration distance of the wetting front in vertical upward and downward decreased, but the horizontal migration distance changed little, as the depth of the vertical pipe increased. The time of irrigation water was determined to reach the bottom hole of the vertical pipe, according to the water balance equation. A prediction model of the wetting body was established for the vertical-pipe drip irrigation, including the emitter discharge, diameter, and buried depth of the vertical pipe, as well as the irrigation time. The model was also verified by the 8 and the 9 schemes of the irrigation test. The average absolute error of the model was between 0.28 cm and 1.07 cm, while the root mean square error was between 0.19 cm and 1.68 cm, and the Nash efficiency coefficient was greater than 0.91. The finding can offer the accurate prediction of steady infiltration rate and wetting body in the optimal design of vertical-pipe drip irrigation system, thereby creating a relatively suitable environment for sand-fixing plant seedlings. © 2021, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.