An underground drought
In the arid Shiyang River Basin of Gansu, China, the Wuwei Oasis is sustained by water flowing from the distant Qilian Mountains. This closed basin, meaning no water flows out to the sea, is a mountain-oasis-desert ecosystem. For decades, however, more water has been taken out of the ground than nature puts back in. This overdraft is primarily driven by agricultural expansion; between 1984 and 2007, irrigated farmland expanded at a rate of 13.85 square kilometers per year.
This intense pumping has caused a severe drop in the water table. In some parts of the wider basin, groundwater levels fell by 15 meters in just four decades. From 1984 to 2007, the average decrease in groundwater depth across the Wuwei sub-basin was 24.2 meters. This extraction has consequences, as the over-exploitation of groundwater contributes directly to land desertification and soil salinization. The annual decline rate in some areas reached between 0.5 to 1 meter per year. When large volumes of water are removed from an aquifer, the ground itself can compact and sink, a process known as land subsidence.
Listening to the aquifer
To understand this complex underground system, scientists use specialized tools called piezometers, which measure the pressure, and thus the level, of groundwater. In the Wuwei area, researchers employ "nested" piezometers. This involves installing multiple, separate monitoring tubes within a single borehole, with each tube opening at a different depth. This setup provides a high-resolution profile of the aquifer, showing which specific layers are losing water and which might be more stable.
Data from this monitoring is important. It shows the direct link between human activity and the aquifer's health. Before 2007, for every 1 square kilometer of new farmland, 1.17 square kilometers of natural oasis were lost, and groundwater consumption nearly doubled. These precise measurements also inform conservation efforts. After a comprehensive management plan was implemented in 2007, policies promoting water-saving irrigation and restricting farmland expansion led to a measurable recovery. Between 2008 and 2018, in areas where farmland was reduced, the water level rose by an average of 3.1 meters. This detailed monitoring allows for targeted water management and shows the potential for the potential for recovery.