Assessing the dependence of paddy rice yield on field size across the Mekong Delta

The Mekong Delta contributes approximately 7–10% of the global rice trade, produced by about 1.5 million small-scale farmers. Understanding how field size affects rice yield is critical for advancing the sustainability and resilience of smallholder farming systems in the Mekong Delta. However, rice yield variability across field sizes remains poorly understood, due to the complexity of rice cropping systems and the lack of accurate field boundaries for smallholder farms in this region. In this study, we delineated field boundaries across the Mekong Delta using 3-m PlanetScope imagery and analyzed rice yield patterns across field sizes using near-infrared reflectance of vegetation (NIRv) as a yield proxy, derived from 10-m Sentinel-2 observations spanning 2019–2025. To delineate smallholder farms, we fine-tuned the Segment Anything Model (SAM), which generated field boundaries with an accuracy of 78%, an F1-score of 0.54, and a Matthew’s correlation coefficient (MCC) of 0.40. Using these boundaries, we assessed rice yield variability across field sizes and found that yield increased with field size (r = 0.30, p < 0.001). This relationship remained stable across years, indicating that smaller farms consistently experienced lower yields. This study contributes to understanding rice yield patterns within smallholder farming systems for better management practices in the Mekong Delta.