Effects of pressure head and vertical tube diameter on infiltration characteristics of soil moisture in subsurface irrigation system with vertical tube emitter

Abstract: Subsurface irrigation is a type of water-saving technology. However, poor anti-clogging abilities and large energy consuming of the existing subsurface irrigation emitters, presents a significant obstacle to the development of subsurface irrigation. Aimed to this problem, a new type of subsurface irrigation emitter, vertical tube emitter, was proposed. This study investigated the effects of soil texture, pressure head and vertical tube diameter on infiltration characteristics of soil moisture under irrigation system with vertical tube emitter. Laboratory experiments were carried out in State Key Laboratory Base of Eco-hydraulic Engineering in arid area in silt loam and sandy loam. In the experiment on the effect of pressure head on infiltration, the pressure head was designed with different levels of 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0 m, and the vertical tube diameter was fixed at 16 mm. In the experiment on the effect of vertical tube diameter on infiltration, the vertical tube diameter was 4, 8, 12, and 16 mm and the pressure head was fixed at 2.0 m. During the experiment, the infiltration was measured by difference of water level in Markov bottle. The water level was recorded every 1, 2, 3, and 5 min in the first 50 min, every 10 min from 50 to 100 min, and every 30 min from 100 to 180 min. The experiment lasted for 5 h. Each treatment was replicated 3 times. The infiltration process was described by Philip formula and the infiltration rate was calculated. The results showed that: 1) Soil texture had great influence on the cumulative infiltration. Heavy texture tended to low infiltration. The cumulative infiltration in sandy loam was higher than that in the silt loam under the same pressure head and vertical tube diameter; 2) The effect of pressure head on cumulative infiltration was largest and then gradually became weak; 3) The infiltration process could be well described by Philip formula with R2 higher than 0.99 for different soil under different pressure head and vertical tube diameter; 4) The cumulative infiltration increased with increasing vertical tube diameter under the pressure head of 2 m, but its change decreased with experimental time; 5) Based on the curves of diameter and cumulative infiltration, a vertical tube diameter-based model was established to estimate cumulative infiltration. The model was validated with measured values under pressure head of 1.0 and 2.5 m and diameter of 8 and 12 mm. The validation revealed R2 higher than 0.99 and root of mean square error from 0.0552 to 0.1723 L, indicating that the established model was good at cumulative infiltration estimation under irrigation system with vertical tube emitter; and 6) The infiltration rate decreased greatly during the first 1 h of experiment then stabilized after 3-4 h in both soils. The stable infiltration rate was 0.53-1.25 L/h when the pressure head was 0.5-3.0 m and the vertical tube diameter was 4-16 mm. Compared with micro-infiltration irrigation, the stable infiltration rate had wide range and high value, indicating that the vertical tube emitter could meet the water requirement of crop. In the future, more factors such as initial soil moisture, soil bulk density should be considered in studying the infiltration characteristics of soil moisture in the irrigation system with vertical tube emitter.