Hydroclimatic Regime Shifts and Compound Heat–Dry Events in Rajshahi Division, Bangladesh: Integrating Thermal Gradients, Change-Point Detection, and Heat-Driven Dryness Dynamics (1992–2022)

IQBAL HOSSAIN 

Hydroclimatic variability and compound climate extremes are increasingly affecting monsoon-dominated regions of South Asia, particularly drought-prone northwestern Bangladesh. However, integrated assessments of thermal dynamics, atmospheric moisture, land–atmosphere coupling, and compound heat–dry events remain limited in the regional context. Therefore, this study investigated long-term hydroclimatic changes and emerging heat-driven dryness conditions in Rajshahi Division during 1992–2022. Monthly hydroclimatic data, including air temperature, Earth skin temperature, specific humidity, and precipitation, were obtained from the NASA POWER database for eight districts of the division. Mann–Kendall trend analysis, Sen’s slope estimator, Surface–Air Temperature Gradient (SATG), Heat-Driven Dryness Index (HDDI), compound event analysis, Pettitt change-point detection, and spatial vulnerability assessment were applied to evaluate regional hydroclimatic behavior. The results revealed significant decreases in air temperature and Earth skin temperature, while humidity and precipitation increased across both annual and seasonal scales. SATG exhibited persistent declining trends, indicating weakening land–atmosphere thermal coupling and reduced surface thermal contrast. HDDI values shifted from predominantly positive conditions during the early 1990s to increasingly negative conditions after the late 1990s, reflecting reduced heat-driven dryness and enhanced moisture availability. Compound heat–dry event frequency also declined substantially after the late 1990s, and Pettitt analysis identified significant hydroclimatic regime shifts during this transition period. Spatial assessment further revealed strong district-level variability, with Nawabganj showing the highest vulnerability conditions. Overall, the study demonstrates a gradual hydroclimatic transition from heat-dry dominance toward increasingly moisture-influenced conditions in northwestern Bangladesh. The findings provide an integrated framework for regional drought monitoring, climate adaptation planning, and future hydroclimatic research under changing monsoon conditions.