Survey of Wet and Dry Stress in Soybean in Japanese Farmers' Fields using Dry and Wet Indices Based on FAO56 Model

Abstract

Major factors that decrease soybean yields in Japan include excessive soil moisture and drought stress. In the present study, a WI (Wet Index) was developed using soil moisture estimates based on the FAO56 model to evaluate both risks of soil dryness and wetness. In addition, to assess the risk of wet losses, the days when ratio of WI was equal to one (RWI), which implied that soil moisture content reached a maximum, was adopted for every growth stage. When the relationship between WI or RWI and yield was evaluated using a partial correlation coefficient, WI had a positive correlation with emergence period approximately 30–40 days after emergence, seed filling phase, and maturity phase, and RWI had a negative correlation with emergence period approximately 20–30 days after emergence and approximately one week before flowering period. We evaluated moisture and drought stress risks, which present challenges in farmers’ fields, and revealed the growing stages at which moisture and drought pose the highest risks. The results could offer critical insights for the estimation of dryness and wetness losses in the field.