Experiments and numerical simulations of soil water movement under mobile drip irrigation system

Abstract: The low-pressure sprinklers have to be modified to replace the high flow rate pressure-compensating driplines at a certain spacing, in order to reduce the wind drift and evaporation loss of large-sized sprinkler irrigation machines. This mobile drip irrigation (MDI) system can be used to realize drip irrigation when moving, due to the self-propelled characteristics of the irrigation machines. This system can be utilized to combine the center pivot and drip irrigation, leading to a large coverage area of irrigation, high automation, low evaporation and drift losses. It is necessary to optimize the design parameters for the soil water infiltration under mobile drip irrigation. Therefore, laboratory experiments were carried out with the MDI test system. The mobile dripline was dragged and moved on the soil surface of a lysimeter. EC-5 soil moisture sensors were installed in the lysimeter to detect the dynamic change of the soil water contents at observation points. The irrigation depth was controlled to adjust the speed of the dripline. Three irrigation depths were set as 30, 40, and 50 mm. A numerical model was established to improve the soil water infiltration and redistribution under MDI using HYDRUS-2D software. The comparison between the simulated and the measured data showed that the water movement in the soil profile under MDI was followed by the non-point source infiltration model, indicating the better agreement of the model with the measured. The NRMSE value of simulated water distribution in the soil profile was less than 20%, while the simulated water content change in the wetting body was generally lower than 25%, indicating the high accuracy of the model. HYDRUS-2D model was used to clarify the influences of three soil textures (sandy loam, loam, and silty loam), three irrigation depths (20, 30, and 40 mm), and the five initial soil water contents (0.050, 0.075, 0.100, 0.125, and 0.150 cm3/cm3) on the soil water movement under MDI. The HYDRUS-2D performed better to simulate the soil water distribution after irrigation under the MDI system. The simulation results show that the soil texture posed a great impact on the shape and size of the wetting body. Specifically, the stronger the soil sandiness was, the larger the wetting front transport distances were, suitable for the larger installation spacing of driplines. However, much attention should be paid to avoiding the deep percolation of the soil with a coarser texture. In addition, the root distribution of crops should be considered, when designing an irrigation system, or a smaller dripline spacing should be used for the finer soil texture. Therefore, the high irrigation depth and the initial soil water content can be expected to increase the transport distance of the wetting front in the tested sandy loam. As such, irrigation uniformity can be improved to overcome the greater risk of deep percolation. These findings can offer practical significance for the decision-making on the mobile drip irrigation system.