Test of a groundwater level control scheme for higher soybean yield in multi-purpose paddy fields

In this study, we examined a groundwater control scheme to increase soybean yield in multi-purpose paddy fields. In a multi-purpose paddy field with a box connecting the irrigation canal to underdrain system and a flexible overflow relief well (denoted 'the experimental plot'), the groundwater level was aimed to keep at 10 cm below the soil surface of the field during the blossom and maximum luxuriance stages, i.e. in August when much water is required for soybean growth. On the other hand, in another multi-purpose paddy field with a horizontal relief well (denoted 'the conventional plot'), the outlet of the relief well was kept at 80cm below the soil surface and was always opened. The study was conducted for two years on these plots. The actual groundwater level in the field, soil water content in the topsoil and the soybean yield were measured. In the first year, the actual groundwater level in the experimental plot in August was kept higher (13.4+-5.0 cm below the soil surface) as compared to that in the conventional plot (22.9+-7.5cm). Soybean yield in the former was 25% higher than that in the latter, indicating the effect of the groundwater level control scheme. In the second year, the actual groundwater level in the experimental plot in September was kept higher (from 30 to 40 cm below the soil surface) as compared to that in the conventional plot (from 60 to 70 cm), and soybean yield in the former was 16% higher than that in the latter. Hydraulic conductivity in the plow layer and subsoil was found to become higher through the soybean cultivation. These results showed the possibility of higher yield in the soybean cultivation in multi-purpose paddy fields by adapting the groundwater level control scheme. It was also suggested that the groundwater level should be managed properly throughout the year to maintain low permeability of the subsoil.