Seasonal flood inundation area and repetitive flood exposure shape the fundamental patterns of flood exposure in China from 2000 to 2020

Remote sensing of floods and associated exposure is essential for understanding and managing flood risks, yet exposure differences among floods with varying frequencies remain underexplored. Using MODIS time-series data, this study mapped infrequent flood inundation area (IFIA) and seasonal flood inundation area (SFIA) across China during 2000–2020. Differences in flood extent and built-up land exposure were assessed using the Mann–Kendall test, improved Theil–Sen algorithm, and one-way analysis of variance. Cumulative flood inundation area reached 3.52 × 10⁶ km², with SFIA accounting for 40%. SFIA increased significantly (0.88%·a⁻¹), whereas IFIA declined markedly (−1.32%·a⁻¹). Built-up land exposure to SFIA reached 9.79 × 10⁵ km², representing 62% of total exposure, and increased faster (3.26%·a⁻¹) than exposure to IFIA (2.38%·a⁻¹). Among built-up lands affected by SFIA, 91% experienced repetitive flooding, contributing 99% of cumulative exposure. These findings demonstrate that seasonal and repetitive flooding shape the fundamental patterns of China’s flood exposure over the past two decades, providing insights into flood spatiotemporal dynamics and supporting risk-informed land-use planning.