An agricultural heartland is sinking
The Hetao Plain is a vast, fertile agricultural area in Inner Mongolia, approximately 250 kilometers long and 50 kilometers wide. For over 2,200 years, since the Qin Dynasty, farmers have diverted water from the Yellow River to irrigate this arid region. The area receives only about 150-200 mm of rain annually but contends with evaporation rates as high as 2,200 mm per year. This climate makes irrigation essential. The modern Hetao Irrigation District covers more than 730,000 hectares and produces much of the grain and oil for China.
To meet the intense water demands of modern agriculture, extensive groundwater pumping supplements the water drawn from the Yellow River. This over-extraction is causing the ground itself to fall. When water is pumped from underground aquifers, the pressure that supports the overlying sediment drops. The fine-grained clays and silts in the aquifer system compact under the weight, leading to a gradual lowering of the land surface, a process called subsidence. The loss of underground water storage is permanent; once the clay layers have compressed, they cannot be refilled. This widespread subsidence now threatens infrastructure and the long-term viability of farming in the region.
Millimeter-precise satellite monitoring
Scientists track the sinking of the Hetao Plain using a remote sensing technique called Interferometric Synthetic Aperture Radar (InSAR). Earth-orbiting satellites, like those in the Sentinel-1 mission, repeatedly bounce radar signals off the ground and measure the returning waves. By comparing the phase of the radar waves from two different satellite passes over the same location, researchers can detect tiny changes in the ground's elevation. This method has a precision of millimeters and can cover vast areas, making it useful for monitoring large-scale subsidence.
The InSAR data creates detailed deformation maps that show where the ground is sinking fastest, often in bowl-shaped depressions corresponding to areas of heavy groundwater extraction. These maps reveal subsidence rates that can exceed 100 mm per year in some parts of the world. By tracking these changes over time, scientists can model the stress on the Earth's crust and predict where ground fissures and sinkholes are likely to appear. This information is used for land planning and managing the risk to buildings, roads, and the complex canal systems that support the region's agriculture.