Effects of depressions and drainage systems on soybean ( <i>Glycine max</i> (L.) Merill) growth during the early growth stages under excess water stress conditions

Soybean (Glycine max (L.) Merill) productivity is severely compromised under conditions of excess water, particularly during the early growth stages. Depressions in converted paddy fields are believed to significantly contribute to excess water stress. Farmers typically use drainage systems, such as field ditches and pipe drains to improve field drainage. In this study, we aimed to evaluate the impacts of depressions and drainage systems on soybean growth during the early growth stages in farming fields. Using uncrewed aerial vehicles and imaging processing software, we observed the soybean coverage ratio at two and four weeks after sowing. We analyzed the relationship between the coverage ratio and depression depth, distance from field ditches, and distance from pipe drains. Depressions deeper than 2 cm were present in all study fields, and the coverage ratio increase from 2 to 4 weeks after sowing was negatively correlated with depression depth. Although the distance from pipe drains was negatively related to the coverage ratio increase, the distance from field ditches had a limited effect. The construction of pipe drains significantly improved the coverage ratio increase, particularly in areas with deep depressions and near pipe drains, by up to 23.0 times in this study. Therefore, constructing pipe drains near depressions is an effective way to mitigate the impact of depressions and excess water stress on soybean growth during the early stages. Our results will be useful for developing strategies to improve drainage and soybean growth during early growth stages.