[Hydrochemical Evolution in the Yarlung Zangbo River Basin]

In order to explore the hydro-chemical evolution law of the Yarlung Zangbo River Basin from 1973 to 2020, the hydro-chemical characteristics and major ion sources were studied using a Piper diagram, Gibbs diagram, ion ratio, and correlation analysis, and the irrigation applicability of the Yarlung Zangbo River was evaluated using the sodium adsorption ratio (SAR), sodium percentage (Na⁺%), and permeability index (PI). The results showed that the mean value of TDS was (208.30±58.26) mg·L^-1, which increased with time. Ca²⁺ was the dominant cation, accounting for (65.49±7.67)% of the total cations. HCO^-₃ and SO^2-₄ were the dominant anions, accounting for (68.56±9.84)% and (26.85±9.82)% of the main anions, respectively. The annual growth rates of Ca²⁺, HCO^-₃, and SO^2-₄ were respectively 2.07, 3.19, and 4.70 mg·(L·10 a)^-1. The hydro-chemical type of the Yarlung Zangbo River was HCO₃-Ca type, and the main ionic chemistry was controlled by the chemical weathering of carbonate rocks. The weathering of carbonate rocks during the period of 1973 to 1990 was mainly controlled by carbonation, whereas from 2001 to 2020 it was mainly controlled by both carbonation and sulfuric acid. The main ion concentrations in the mainstream of Yarlung Zangbo River were within the range of drinking water standards, with SAR between 0.11-0.93, Na⁺% between 8.00-36.73, and PI values between 0.39-0.87, demonstrating that the waters were suitable for drinking and irrigation. The results were of great significance to the protection and sustainable development of water resources in the Yarlung Zangbo River Basin.