Background <p>The volumetric water content, which remains temporarily constant (i.e., quasi-saturated) during the downward infiltration of rainwater into the soil, can serve as a useful indicator for evaluating slope stability under rainfall. To apply a volumetric-water-content-based failure prediction method to real slopes, it is necessary to establish a quantitative evaluation approach that accounts for variations in rainfall intensity, soil permeability, and water retention characteristics. We performed a series of vertical rainfall infiltration experiments on a uniform, horizontal sand deposit using a finely tuned rainfall simulator capable of reproducing specified rainfall intensities in a centrifuge.</p> Results <p>Two-fluid type spray nozzles with adjustable pneumatic pressure, arranged in series to overlap spray distributions, enabled reproduction of three constant rainfall intensities in the centrifuge with small droplet sizes and low injection velocities. Sprayed rainwater infiltrated the soil, increasing its volumetric water content, and the process from the initial state through the quasi-saturated state to the final saturated state was captured. Higher rainfall intensities or higher viscosity of the sprayed liquid led to greater quasi-saturated volumetric water contents.</p> Conclusions <p>The relationship between the volumetric water content at a quasi-saturated state and relative hydraulic conductivity was generally consistent with that obtained from the Mualem-van Genuchten model. As the model is derived from the soil water characteristic curve and the saturated hydraulic conductivity, it has been demonstrated that the volumetric water content in the quasi-saturated state can be estimated from the soil water characteristic curve, the saturated hydraulic conductivity, and the rainfall intensity.</p>