This appears to be downstream, so not a source of water for China, but shrinking due to China's increased water use. This article doesn't mention what kind of agriculture (that is, cotton is not discussed here), but it does imply that changes in land use in the last 15 years is having strong effects on the region's water, which suggests my concerns about cotton have ground to stand on.

This research is even more specific, saying that observed data proves that Chinese are cultivating significantly more and grazing less than they used to ("Changes in LULC were most evident on the Chinese side of the border, where crop production areas increased and grazing areas decreased between 1995 and 2015") https://iopscience.iop.org/article/10.1088/2515-7620/ab5e1f/pdf