Abstract:
The effect of controlled irrigation and drainage on water transformation for paddy field was investigated by controlling root zone soil water content and water table depth using a lysimeter equipped with an automatic water table control system. Four treatments that combined irrigation and drainage managements were implemented: controlled irrigation (CI) + controlled water table depth 1 (CWT1), CI + controlled water table depth 2 (CWT2), CI + controlled water table depth 3 (CWT3) and flooding irrigation (FI) + CWT1. Experiments were conducted in 12 drainage type plots with a mobile shelter and gallery. Each plot had a size of 2.5 m × 2 m and a depth of 1.3 m. Each plot was individually irrigated and drained using a pipe installed with a water meter and a tube installed at 1.2 m below the soil surface, respectively. Subsurface drainage was conducted based on the water table control by using an automatic water table control system, which was installed on each drain tube in the gallery. The soil moisture was measured daily by a Trease system (6050X3) when no pond water remained in the paddy field. The pond water depths in the paddy field were measured daily using a vertical ruler. A water table observation well was installed in the field outside the plot. The actual field water table depths were measured daily after the re-greening stage. One water table observation tube was mounted on the drain tube in each plot. The water table depths of each plot were measured daily after the re-greening stage using a vertical ruler. A water meter installed on the pipe of each plot recorded the irrigation volumes. The water leakage volumes were measured using a tipping bucket gauge. The results showed that irrigation water, subsurface drainage water, and evapotranspiration in paddy field reduced significantly under controlled irrigation and drainage, 41.7%, 49.9% and 24.9% lower than those under conventional irrigation and drainage management, respectively, and meanwhile, rice yield reduced slightly. Therefore, water production efficiency increased significantly under controlled irrigation and drainage. Rice yield and water production efficiency under controlled irrigation and drainage were 1.9% lower and 35.5% higher than those under conventional irrigation and drainage management, respectively. Increases in water table control levels resulted in less irrigation water, subsurface drainage water and evapotranspiration and steady rice yield for paddy field under controlled irrigation. Therefore, water production efficiency under controlled irrigation increased with the increase in water table control levels. The increases in water table control levels reduced irrigation water by lengthening the duration of soil moisture depleted to the lower threshold for irrigation. The proportion of water table levels lower than the control levels increased as the increase in water table control levels, which shortened the duration of subsurface drainage and reduced subsurface drainage water. The effect of irrigation water on evapotranspiration was higher than that of subsurface drainage water. Therefore, the reduction in irrigation water accompanied by the increase in water table control levels reduced evapotranspiration in paddy field. The application of increasing water table control level to a suitable level in paddy field under controlled irrigation can effectively realize high water use efficiency.