Evaluation of the effects of transplanting date shifts and drainage outlet raising management practices in paddy farming regions under future climates using coupled APEX-Paddy and SWAT models

Predicting adaptation and the response of agricultural water resources to climate change is a challenging. As changes in future climate will change agricultural environments, agricultural conservation practice (ACP) assessments are needed as a response strategy to reduce water pollution and preserve crop yield in agricultural watersheds. This study used coupled SWAT and APEX-Paddy models (SWAPX) to evaluate conservation practices for hydrology, water quality, and crop yield in paddy-dominated watersheds for three future periods (2020s, 2030s, and 2040s). We estimated future hydrology and water quality using the well-validated SWAPX model and RCP 8.5 scenario for 10 general circulation models (GCMs) and evaluated the ability of transplanting date shift (TDS) and drainage outlet raising (DOR) management strategies to reduce runoff and total nitrogen (T-N) load and preserve rice yield. Runoff in future climate scenarios was higher than the baseline (2008–2019), and the T-N load was predicted to increase rapidly in the near future and then gradually decrease until the 2040s (2041–2049). The R40 scenario (combined TDS and DOR), which was the most effective, reduced runoff by 9.2%, T-N load by 13.6%, and water productivity by 47.7% compared with the CB (baseline) scenario. When the ACPs were applied to the entire paddy area in the watershed, the reduction rates of runoff and T-N load were 4.1% and 7.4%, respectively. This result demonstrates the effectiveness of ACPs on a spatial–temporal scale and implies that ACP efficiency, applied area, and placement in target regions are important to consider when developing plans to improve water quality and crop yields at the watershed scale.