UC Riverside

An artificial capillary barrier (CB), which consists of two layers of gravel and coarse sand, was used to improve the soil water retention capacity of the root zone of sandy soil for the cultivation of Japanese spinach (Brassica rapa var. perviridis). The performance of a CB under specific conditions can be evaluated using numerical simulations. However, there have been relatively few numerical studies analyzing soil water dynamics in CB systems during crop growth. The objectives of this study were (i) to evaluate the performance of a CB during the cultivation of Japanese spinach irrigated at different rates and (ii) to investigate the effect of the irrigation schedule on root water uptake. Numerical analysis was performed using HYDRUS-1D after the soil hydraulic properties of the CB materials were determined. In most cases, the HYDRUS-1D results agreed well with the experimental soil water content data without any calibration when the dual-porosity model describing soil hydraulic properties was used for gravel and coarse sand. We found that the dual-porosity model was able to attenuate the unrealistically steep reduction in the unsaturated hydraulic conductivity predicted by the single-porosity model. The numerical simulations also showed that the CB played an important role in maintaining plant-available water in the root zone while maximizing the water use efficiency. The numerical simulations revealed that the irrigation frequency could be reduced by half during the early growth stage, and the water use efficiency could be greatly improved with the CB layer installed.