Evaluating Maize Drought and Wet Stress in a Converted Japanese Paddy Field Using a SWAP Model

Japanese government recommend farmers to cultivate upland crops in paddy fields ("converted fields") to suppress the overproduction of rice. Converted fields are subject to excessively wet and dry conditions that reduce the yield of non-rice crops. Drought and wet stresses are critical to crop growth within specific growth periods. To provide data for use in mitigating crop yield reduction, we evaluated drought and wet stresses in maize (Zea mays L.). A SWAP (soil-water-atmosphere-plant) model was applied to a converted maize field. Observations were carried out in 2019 and 2018 for model calibration and validation. Thereafter, we evaluated the water stress of maize in 2019 (actual conditions) and at a tillage depth 11 cm deeper (scenario conditions). We found that (1) drought and wet stresses occurred within the relevant critical growth periods under actual conditions; (2) in the critical periods, the drought and wet stresses under scenario conditions were 33%-75% and 10%-82%, respectively, of those under actual conditions; (3) water stress at depths of 10 and 20 cm was lower under the scenario conditions than under the actual conditions. These results indicate that deeper tillage may mitigate both drought and wet stresses and can be used to reduce water stress damage in converted fields.