An investigation into nearly 1,700 aquifers across more than 40 countries found that groundwater levels in almost half have fallen since 2000. Only about 7 percent of the aquifers surveyed had groundwater levels that rose over that same time period.

The new study is one of the first to compile data from monitoring wells around the world to try and construct a global picture of groundwater levels in fine detail.

The declines were most apparent in regions with dry climates and a lot of land cultivated for agriculture, including California’s Central Valley and the High Plains region in the United States. The researchers also found large areas of sharply falling groundwater in Iran.