Assessment of planting date adjustments on irrigation water demand under future climate change scenarios in Taiwan

Climate change threatens agricultural productivity in Taiwan, where double-cropping systems are highly sensitive to hydroclimatic variability. This study investigates optimal adjustments of planting dates to evaluate the irrigation water demand (IWD) under climate change using the Food and Agriculture Organization (FAO) AquaCrop model. The model was calibrated with measured canopy cover (R² = 0.99 (season 1) and 0.91 (season 2); RMSE ≤ 13.2%, MAE ≤ 11.1%) and used to estimate IWD for paddy rice under four possible climate scenarios (shared socioeconomic pathway: SSP1–2.6, SSP2–4.5, SSP3–7.0, and SSP5–8.5) from ten CMIP6 model simulation output. A shifting planting-date approach, from 21 days earlier to 21 days later in seven-day increments, was applied to identify the optimal IWD for the future. Historical simulation results showed that the first cropping season requires substantially higher IWD compared to the second cropping season, reaching up to 180 mm and imposing the greatest burden on water resources, whereas the second cropping season mostly requires less supplemental irrigation. Future projections indicated a substantial increase in IWD during the first cropping season, with rises of up to 40 mm along the western coastal plain under the SSP5–8.5 scenarios. Maximum irrigation water savings for a double-cropping system can be achieved by a combination of shifting 14 (21) days earlier for the first (second) season or 21 (14) days later for the first (second) season across different regions in Taiwan. Under a single-cropping system, 80 mm of irrigation water can be saved by shifting the planting date of 50 days later in the central region, whereas in the south this value reaches up to 150 mm by shifting the planting date of 70 days later. These patterns are consistent across all scenarios. These findings indicate that adjusting optimal planting dates could help to mitigate the adverse impacts of climate change on rice IWD under water-stressed conditions in Taiwan.