Assessment of drought-heatwave and waterlogging-heatwave compound events during summer soybean growth stages in the Sichuan Basin under climate change

Under intensifying climate change, compound drought‑heatwave events (CDHWs) and waterlogging‑heatwave events (CWHWs) severely affect agricultural production. However, systematic analyses of their spatiotemporal changes and dominant drivers during specific crop growth stages remain limited. Based on historical (1990–2019) and future (2021–2080, shared social economic paths scenarios SSP2–4.5 and SSP5–8.5 in the Coupled Model Intercomparison Project Phase 6) climate data, this study employed the standardized precipitation requirement index and temperature condition index combined with run theory to quantify the duration, intensity, and frequency of CDHWs and CWHWs across summer soybean growth stages in the Sichuan Basin. Copula functions were further applied to estimate return periods at multiple quantiles and to identify shifts in dominant driving factors. The results showed that in 1990–2019, the mean annual cumulative days of CDHWs and CWHWs were 22 days and 10 days, respectively. Under future scenarios in 2021–2050 and 2051–2080, CDHWs were projected to remain at 17–20 days, whereas CWHWs were expected to increase substantially, reaching 20–50 days. Both CDHWs and CWHWs were characterized by extremely short‑duration events (1–2 days), whose proportion was expected to increase by 4%–26% in the Pod-setting to Seed-filling (R3–R5) and Seed-filling to Maturity (R5–R8) stages in the future. The intensity of long-duration (>10 days) CDHWs in the future showed a downward trend (decrease by 0.2), while that of long-duration CWHWs increased (up to 0.76). Compared with CDHWs, CWHWs were projected to have shorter return periods in the future. Under SSP5–8.5, 62% of stations in 2021–2050 and 46% in 2051–2080 exhibited return periods of less than 1 year at the 70th percentile for CWHWs. The dominant factor of both CDHWs and CWHWs was projected to shift from moisture‑related conditions to heatwave‑dominated conditions. Under SSP5–8.5 in 2051–2080, heatwave‑driven areas of CDHWs were expected to cover about 60% of the region, with 69%–77% of stations being heatwave-dominated in the Sowing to Branching Stage (VE–Vn), Branching to Flowering (Vn–R1) and Seed-filling to Maturity (R5–R8) stages. This study provided theoretical support for agricultural water management in summer soybean production and offered scientific insights for climate‑change adaptation in the Sichuan Basin.