Upland fields converted from paddy fields often have a hardpan layer that can cause excessive wet and dry conditions. In this study, the water stress affected by this hardpan was evaluated. Data were obtained from a converted maize field that was divided into plots with and without hardpan. A soil-water-atmosphere-plant (SWAP) model was used to estimate the water stress conditions, such as the reduction in the daily transpiration rate and the vertical distribution of reduction in root water uptake for both plots. When hardpan was present, stress due to wet and dry conditions was more intense in the layer above the hardpan, and dry condition persisted after the hardpan dried. Without hardpan, the wet stress decreased more quickly than when the hardpan was present. The vertical distributions of reduction in root water uptake revealed that dry and wet conditions occasionally occurred at the same time in both plots. In this situation, without hardpan, the shallow layer dried and the deeper layer was wet. With hardpan, the same condition occurred, or the hardpan dried and other shallower and deeper layers were wet. It was considered that the latter condition was a unique characteristic of plots with hardpan—converted fields. The vertical distributions of reduction in root water uptake with the SWAP model can be used as an index to examine the relationship between water stress and tillage practices.