Yield, Water Balance and Nitrate Leaching under Different Irrigation Regimes in Rice Cultivation under Sub-tropical Environment: Field Observation and Simulation Modeling using HYDRUS-1D

Abstract

For irrigated rice, increasing water productivity is essential as water supplies become more limited. Water consumption and risk management can benefit from monitoring the long-term impacts of fertilization and irrigation on the components of the water balance. A field-Lysimeter investigation was carried out and the Hydrus-1D model was calibrated to simulate the nitrate leaching and water balance components. Three distinct irrigation treatments were applied in the lysimetric study: traditional irrigation (continuous ponding) (T1), irrigation three days after the ponded water disappeared from the soil surface (referred to as three days AWD) (T2), and irrigation five days AWD (T3). The results of field experiments indicated that, the conventional irrigation treatment (T1) yielded the highest grain production (6.77–6.94 t ha⁻¹), but demanded the most water (50.6–70.9 cm). Severe irrigation restriction (T3) achieved significant water savings up to 39.3%, yet resulted in substantial yield reductions of up to 26.4%. The relative crop responses remained consistent across both years. A moderate irrigation reduction (T2) provided a balanced trade-off, saving 16.6–22.8% of water while limiting yield penalties to roughly 13%. Consequently, T2 emerged as the optimal management strategy to improve water productivity and sustain rice production in water-limited environments without causing severe yield losses. The Hydrus-1D model was well fitted with the field data (calibration efficiency was about 94%). The HYDRUS-1D simulations demonstrated that AWD irrigation reduced soil water storage and increased root-zone drying compared with continuous ponding. However, reductions in cumulative root water uptake and nitrogen uptake were relatively small, particularly under the three-day AWD treatment. Surface, root-zone, and bottom-boundary nitrogen dynamics showed similar temporal patterns among treatments, indicating that moderate AWD can maintain crop water and nitrogen acquisition while reducing water use. Among the AWD treatments, three-day AWD (T2) appeared to provide the best balance between water conservation and maintaining favorable root-zone conditions. Overall, integrating field observations with model simulations identified three-day AWD (T2) as the most suitable irrigation strategy for improving water productivity while sustaining rice yield and efficient water and nitrogen use.