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Modeling of Soil Water Regime and Water Balance in a Transplanted Rice Field Experiment with Reduced Irrigation

Available water resources for agricultural irrigation have decreased worldwide in recent decades, prompting advances in water saving methods. In this study, soil water regime and water balance in a transplanted rice field with reduced irrigation (multiple shallow depth irrigations) in the Taihu Lake Basin of East China were observed and then evaluated using Hydrus-1D model during two consecutive growing seasons. During the 2008 season, irrigation water accounted for 48% of the total water input (TWI, 94.6 cm), but only 20% of TWI (120.4 cm) during the 2009 season. Due to heavy rainfalls during the wet 2009 season, surface runoff accounted for about 6.4% of the TWI, whereas during the much drier 2008 season with higher controlled irrigation inputs, no surface runoff occurred. Simulated evapotranspiration during the 2008 and 2009 seasons accounted for 67.4% and 54.9% of TWIs, respectively. Measured and simulated results indicate that water percolation (approximately 32.5% and 37.6% of TWIs during the 2008 and 2009 seasons, respectively) was the main path of water losses from the transplanted rice fields, suggesting that long and high standing water during the 2009 season increased water percolation. Water productivities evaluated from total irrigations and TWIs were 2.08 and 0.99 kg·m−3 during the 2008 season and 3.85 and 0.77 kg·m−3 during the 2009 season, respectively, and were 1.47 and 1.40 kg·m−3, respectively, when evaluated from modeled evapotranspiration fluxes. The cultivation method with multiple shallow depth irrigation efficiently used rainfall water and reduced the losses due to water percolation and surface runoff by stabilizing rice yields.