Response mode of hydrochemical types of river water to altitude gradient in alpine regions.

The study investigates the hydrochemical type and characteristics of river water in permafrost regions in the Tibetan Plateau by analyzing 532 samples collected from the source region of the Yangtze River. The hydrochemical type of the river water was Cl--Na+-SO42-, and its hydrochemical characteristics were primarily influenced by the soil sources, though the influence of the sea sources and anthropogenic factors could not be ignored. Significant negative correlations were found between temperature and NO3-, SO42-, Mg2+, Ca2+, and between precipitation, relative humidity, and SO42- and Mg2+ in the river water. River water in the higher altitudes of over 5000 m above sea level was mainly recharged from glacier snowmelt water and by the supra-permafrost water and precipitation at the altitudes between 3500 and 5000 m above sea level. The controlled sources of hydrochemical characteristics of glacier snowmelt water were different for different ablation rates in the area with elevations of over 5000 m above sea level. Different hydrochemical types in different ablation rates implied the hydrochemical type was extremely sensitive to ablation periods in areas with elevation of over 5000 m above sea level. However, hydrochemical type was not sensitive to ablation periods from 3500 to 5000 m above sea level. The ionic concentration of glacier snowmelt water was mainly controlled by pollutants in glaciers and snow. Melting rates of glacier snowmelt water also had a certain effect on ionic concentration. Meanwhile, the stability of the hydrochemical type implied river water mainly controlled the hydrochemical type from 3500 to 5000 m above sea level. Hydrochemical type had no effect on elevation in end ablation.