Evaluation of water stress in soybean based on the difference in canopy temperature between soybean and rice

In order to evaluate water stress in soybean grown in farmer paddy fields in Japan, we developed a water stress index simplified by incorporating canopy temperature of rice into a heat budget equation on the soybean canopy. For the simplification, we assumed that: (1) the aerodynamic resistances in soybean and rice canopies are the same and (2) the canopy resistance of rice does not differ among fields. Based on these assumptions, we expressed the evaporation rate of soybean against that of rice (1-E_S/E_R) as a simple function of the difference in temperature between soybean and rice canopies and gradient of vapor pressure between rice canopy and air. Numerical analysis suggested that estimation error of 1-E_S/E_R is relatively small. Measurement of water stress in soybean in farmer's fields indicated that variation of 1-E_S/E_R among measured occasions was small, and that 1-E_S/E_R was a good index of water stress comparable to the Crop Water Stress Index (CWSI). These results indicated that this method could be used to evaluate water stress in soybean in farmer paddy fields.