Is satellite-observed surface water expansion a good signal to China’s largest granary?

Water resources are the premise of sustainable agricultural development in Northeast China (NEC), the country’s largest granary. Revealing the long-term evolution of water resources from multiple perspectives, such as surface water area (SWA) and groundwater storage (GWS), is crucial for understanding their current status and predicting future trends to formulate adaptive strategies to promote sustainable water management. Here, firstly, we annually mapped surface water in 1987–2023 using the Google Earth Engine (GEE) cloud service, all these historical 30-m Landsat records, and a mature surface water detection algorithm; then, we calculated annual GWS using Gravity Recovery and Climate Experiment (GRACE) satellite data and compared the trends of SWA and GWS to analyze the current status and evolution direction of water resources. We found continuous expansion of SWA (169.3 km2/yr) since 2002 after a 15-year consecutive decline, while GWS significantly declined (2.37 mm/yr). Quantitative attribution analyses identified the increasing artificial lakes and aquaculture ponds as the major drivers of surface water expansion, with a contribution of 85.0 %, suggesting that the expanding surface water bodies seen from space were just an illusion of increasingly abundant water resources. In addition, we revealed the increasing water evaporation (0.2 km3/yr) and the ratio (0.005–0.2 %/yr) of water evaporation to evapotranspiration (ET) with surface water expansion, indicating that the contribution of water evaporation to ET is more prominent and evaporative water loss has become a new threat to local water security and deserves much more attention. Our research can not only bring new insights into the current status of water security in China’s largest granary but also provide a typical case for reference in analyzing water resource changes in other similar regions.