Spatiotemporal Evolution and Migration of Compound Heat and Drought Events during the Summer Maize Growing Season in the Huang-Huai-Hai Plain

Compound heat and drought events pose severe challenges to crop growth and development through antagonistic and additive effects. In the Huang-Huai-Hai Plain, summer maize specifically encounters "dual water-heat stress" as its growing season overlaps with peak hazard periods. However, while the spatial patterns of compound events are evolving, existing studies predominantly adopt a static perspective and rely on meteorological indices, thereby overlooking direct root-zone water constraints and lacking the analysis of dynamic migration trajectories over long-term sequences. To address this, our study focuses on the core summer maize production region of China—the Huang-Huai-Hai Plain. Based on daily root-zone soil moisture and air temperature data during the growing seasons from 1980 to 2020, we constructed a Compound Heat and Drought Event (CHDE) index by substituting traditional meteorological indices with the Soil Moisture Deficit Index (SMDI)—which better reflects root-zone water stress—combined with the Temperature Condition Index (TCI).  This study targets daily-scale compound events and analyzes their spatiotemporal characteristics. Building upon static analysis, we introduced a Barycenter Migration to establish a dynamic spatiotemporal analysis framework, tracking evolutionary trajectories across three dimensions: Frequency, Duration, and Severity. Results indicate that the negative correlation between root-zone soil moisture and high temperature follows a "weak-strong-weak" evolution throughout the growing season; the jointing-tasseling (V6-VT) stage exhibits the strongest negative correlation, highest hazard severity, and most frequent occurrence, thus being identified as the critical phenological stage. Spatially, hazard hotspots demonstrate a distinct "central-to-south" migration during crop development, shifting from the central plains during the vegetative growth stage to the southern regions during the reproductive growth stage, with the timing of occurrence expanding toward earlier growth stages. The exposure to compound events experienced a trough in the 1990s, reversed from a decreasing to an increasing trend around 2000, and underwent a abrupt change in 2011–2012. Notably, approximately 60% of the region showed an increase in frequency over the last two decades, exhibiting a distinct spatial asymmetry: increases were primarily concentrated in the southern plains (e.g., Henan, northern Anhui), whereas the northern regions (e.g., Hebei, northern Shandong) were characterized mainly by decreases or stability .Through the spatiotemporal analysis of compound events, this study reveals the evolutionary patterns and regional heterogeneity of compound stress during the summer maize growing stage in the Huang-Huai-Hai Plain, providing a scientific basis for formulating maize irrigation strategies.