The study looked at water vapor sources in Northwest China by analyzing deuterium excess in precipitation. They found that deuterium excess values were higher in the dry season and lower in the wet season, with a negative correlation to air temperature and vapor pressure. This suggests that monsoon moisture comes from far away during the wet season, while continental air masses contribute to higher deuterium excess in the dry season. The findings were consistent with reanalysis data, highlighting the importance of monsoon moisture in summer precipitation at these sites.