The flow of technology

摘要

On a still, warm evening in mid-June, leaves are reflected in the cool water that fills the irrigation channels of a pecan orchard in Clovis, California. This placid scene seems to have barely changed in the past 40 years.Underground, however, the water table tells a different story. Satellite data show that in California's Central Valley, the most productive agricultural region in the United States, water levels in the aquifers have fallen by more than 20 millimetres a year between 2003 and 2012. The total volume of water lost in that period was about 30 billion cubic metres -almost the same as the capacity of the largest reservoir in the United States, Lake Mead in Nevada. The river basins that feed the aquifers are themselves shrinking, owing to a reduction in snowmelt from the surrounding Sierra Nevada mountains.