Measurement of wetting process of soil under dripper with automatic system for point source infiltration

Abstract: Drip irrigation provides greater efficiency in terms of water usage. It requires adequate design and conscientious management. Soil water movement process is an important content of drip irrigation system design and operation management. Soil infiltration has important impacts on soil water movement and distribution process. Therefore, in this study, the automatic measurement system for point source infiltration of soil was used to measure the wetted areas at soil surface and the wetted depths of soil profile through image recognition technology and computer-controlled digital camera in real time. The digital camera was used to take images of the run-on water advancing in real time before they were processed and then by analysis the wetted areas were determined. According to the wetted soil surface areas, the system automatically computed the soil infiltration rate with the mathematic models outlined in the point source run-on method before the infiltration curve could be displayed on the real-time basis. The soil infiltration measurement system was based on the advancing process of the wetted radius at the soil surface under point source of water supply and based upon the assumption of the ellipsoid-shaped wetted soil body and the principle of water balance. An equilateral triangular-prism soil box of 50 cm long and 50 cm high, which was made of plexiglass, was used in this study for indoor drip irrigation experimental tests, to measure the processes of infiltration and water movement processes. Transparent plastic sheets were pasted on 2 side walls of the soil boxes. The soil materials, with the texture of silty loam consisting of 15% clay, 50.2% silt and 34.8% sand particles, were air-dried before passing through a 2 mm sieve, and then packed into the soil box in 5 cm depth layer at a bulk density of 1.3 g/cm3. The experiments involved 3 dripper discharge rates of 2, 4 and 8 L/h, with 3 replicates. Digital camera that was connected to and controlled by a computer was used to take picture of wetted areas at soil surface in 4 min time interval to get the changing process of the wetted surface areas. Meanwhile, at the time moments of 1, 2, 4, 6, 8, 10, 12, 20, 30, 40, 50, 60, 70, 80, 90, 100, 120, 140, 160 and 180 min, the surface wetting radius and vertical wetting front process were also manually recorded by using the transparent plastic sheets. AUTOCAD software was used to highlight the wetted surface areas and the wetting fronts before measuring the wetted radius at the surface and the vertical wetted areas by infiltration water. Based on the data, the soil infiltration process under point source was calculated by the automatic measuring system. The wetted surface areas and the vertical wetted areas by infiltration water were computed through the recorded images. The calculated infiltration results were compared with the measured data to verify the accuracy of the measurement. The results showed that the manually measured wetted surface areas were slightly larger than those measured by the automatic measurement system for soil infiltration under point source. The relative measurement errors in wetted soil surface areas were 2%-15%. The predicted vertical infiltration depths calculated by the computed infiltration rate were slightly higher than the manually measured soil infiltration depths. The errors in infiltration depth between the estimated and the manually measured were 1%-8%. Philip and Kostiakov infiltration models/equations were used to fit the measured infiltration processes. Kostiakov equation provided better predictions of the infiltration processes and Philip model produced dramatically different and irrational stable infiltration rates (negative values). The research results indicate that the automatic measurement system can accurately describe the point source surface wetting process of drip irrigation and soil water infiltration process, and can be used to predict the vertical infiltration depth under different dripper discharge. Soil infiltration automatic measurement system has realized the automation in the soil infiltration measurement under point source. The system and the method are verified by laboratory test data, which can provide basic tools to get the parameters of drip irrigation system, and the relevant reference for the design of drip irrigation system.