Candidate Dry-season Refugia and Recent Winter Open-water Decline in Hakaluki Haor, Bangladesh (1995–2026)

Hakaluki Haor is a major freshwater wetland in north-eastern Bangladesh, but the spatial concentration of persistent water and the consistency of recent winter open-water change remain poorly resolved. We combined JRC Global Surface Water v1.4 with Landsat Collection 2 surface reflectance in Google Earth Engine. A reproducible 164.410 km 2 analysis domain was defined from pixels with long-term water occurrence above 15%. At the 80% threshold, persistent permanent-water cores, mapped as candidate dry-season refugia, occupied 20.316 km 2 (12.357% of the domain). JRC monthly recurrence gave a mean of 7.378 recurrence-weighted water-month equivalents over ever-water pixels; 68.569% of that area fell in the 6–12-month-equivalent classes. Lost-water classes exceeded new-water classes in the original domain, two buffered domains and the full bounding box, although the ratio was strongly boundary-dependent (36.7–985.9). Of 32 Landsat winters from 1995 to 2026, 23 passed scene and clear-area requirements. The full Landsat record had no significant monotonic trend (Sen’s slope +0.296 km 2 yr -1 ; p=0.342), whereas 2016–2026 declined significantly (Sen’s slope −1.939 km 2 yr -1 ; p=0.008) at all three tested MNDWI thresholds. Sentinel-1 and Sentinel-2 winter areas covaried strongly during 2018–2026 (Pearson r=0.972), but their absolute extents differed. Published waterbird counts were available for nine matched winters; their moderate association with water area (r=0.413) became weak after adjustment for the number of beels surveyed (r=0.085). Published field studies support the ecological plausibility of retained-water refugia, but the mapped locations remain candidates for field verification.