COMSOL numerical simulations and experiments of soil water movement under intermittent and continuous sprinkler irrigation

Soil water movement can greatly contribute to the water flow with the soil profile in the engineering of sprinkler irrigation. In this study, a numerical simulation was proposed to explore the soil water movement under intermittent and continuous sprinkler irrigation using COMSOL software. The time variable and non-uniform infiltration boundary conditions were also utilized in the sprinkler irrigation. A series of experiments on soil tanks were conducted to validate the accuracy of the simulation. A comparison was made on the simulated and measured parameters, including the soil water content and downward vertical wetting front. Finally, a systematic investigation was applied to evaluate the effects of sprinkler irrigation and irrigation uniformity on soil wetting and soil water content. Sprinkler irrigation was verified by four patterns (continuous, intermittent 1, 2, and 3) and three uniformity coefficients of sprinkler irrigation (39.77%, 59.87% and 80.15%). The results showed that the COMSOL simulation was used to accurately simulate the soil wetting patterns in sprinkler irrigation. The root mean square error and the coefficient of residual mean square error values were less than 0.014 and 0.017 cm³/cm³, respectively, between the calculated and measured soil water contents at 72 h after the sprinkler irrigation. The coefficient of determination R² value was higher than 0.961. There were higher movement speeds of the soil wetting front and the soil water contents of the wetted zone from 0 to 12 h after the irrigation, indicating the slow-down trending, as time passed. There were no significant effects of sprinkler irrigation patterns on soil water movement and soil water content uniformity. The water content of surface soil decreased, as the intermittent frequency and duration increased, while the water infiltration depth increased after the sprinkler irrigation. Meanwhile, the soil water content decreased with the increasing intermittent frequency and duration at 72 h after the end of irrigation. Christiansen uniformity coefficient was used to evaluate the uniformity of soil water content after simulation. The soil water content uniformity coefficient was higher than the uniformity coefficients of sprinkler irrigation, according to the initial surface-measured distribution. The soil water content uniformity coefficient of shallow-layer soil increased, as time passed, while that of the deeper-layer soil decreased. The soil water content uniformity coefficient at the soil depth of 10 cm was selected to represent the uniformity of soil water content. The soil water content uniformity coefficient increased with the increasing uniformity coefficients of sprinkler irrigation. Furthermore, the soil water content uniformity coefficient values were 88%-90% under the continuous and three intermittent sprinkler irrigations, when the uniformity coefficients of sprinkler irrigation ranged from 40% to 60%. Once the uniformity coefficients of sprinkler irrigation increased to 80%, the soil water content uniformity coefficient values increased higher than 94%. The research space was provided to reasonably reduce the design uniformity coefficients of sprinkler irrigation in sprinkler irrigation. In the low uniformity of sprinkler irrigation, there were high rates of peak water application and a large total irrigation amount using intermittent sprinkler irrigation. The intermittent frequency and duration can be expected to reduce the risk of surface runoff and deep percolation, thus improving the uniformity of soil water content.