Simulating soil water regime in lowland paddy fields under different water managements using HYDRUS-1D

The widely adopted alternate wetting and drying (AWD) irrigation for rice production in lowland paddy fields with shallow groundwater table is increasingly needed to quantify the soil water regime for irrigation schedule design. Field experiments were conducted to compare the soil water flow between paddy fields under AWD and continuously flooded irrigation (CFI), during the rice growing season in 2010–2011. Model simulations using HYDRUS-1D were also conducted based on the measured pressure head distribution of soil profiles. Modeling results show that the pressure head derived from forward simulation using the point estimated soil hydraulic parameters did not agree well with the measured pressure head. However, from inverse modeling of saturated hydraulic conductivities of plow pan (mean of 0.68cmd−1 in AWD plots and 0.54cmd−1 in CFI plots), the HYDRUS-1D model can properly simulate the water flow in multi-layer paddy soil flow, where the plow pan plays an important role in determining the vertical pressure head distribution. The measured pressure head and simulated pressure head derived from inverse modeling agreed well (NSE of 0.93–0.98) during the whole rice growing season. Measurement and simulation results indicated that the practice of AWD decreased the percolation 38.2–40.3% in 2010 and 23.3–27.2% in 2011, compared to that of CFI. It is also found that groundwater capillary rise amounted to 26.1–27.4% in AWD plots, and 10.2–18.1% in CFI plots of respective water input (irrigation and rainfall).