The Qiangtang Basin is a large endorheic basin in the inner part of the Tibetan Plateau, and has been thought to be a dry region in contrast with the surrounding wet outer region that feeds all the major Asian rivers. Combining surface hydrological data with modeling and satellite data from 2002 to 2016 CE, our study reveals that an enormous amount of water, ∼54 ± 4 km3, is unaccounted for annually in the Qiangtang Basin. The amount of missing water is comparable to the total annual discharge of the Yellow River. Data from the Gravity Recovery and Climate Experiment (GRACE) satellite mission show little increase of local terrestrial water storage. Thus, the missing water must have flowed out of the basin through underground passages. Interpreting this result in the context of recent seismic and geological studies of Tibet, we suggest that a significant amount of meteoric water in the Qiangtang Basin leaks out by way of groundwater flow through deep normal faults and tensional fractures along the nearly north-south rift valleys that are oriented subnormal to and cross the surficial hydrological divide on the southern margin of the basin. Cross-basin groundwater outflow of such a magnitude defies the traditional view of a basin-scale water cycle and leads to a very different picture from the previous hydrological view of the Qiangtang Basin. This finding calls for major rethinking of the regional water balance.

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