Abstract: Mineral weathering by sulfuric acid plays an important role in chemical weathering in the catchment, with consequences on global carbon cycle. Highfrequency sampling and analysis can accurately assess the sources and biogeochemical processes of dissolved solutes in the river. However, few studies have focused on the temporal variation of sulfate in rivers and rock weathering by sulfuric acids. Here, a monthly sampling campaign was conducted in the Nanpan, Beipan and Hongshui rivers along the upper reaches of Xijang River during a whole hydrological year (October 2013 to September 2014). The seasonal variation of riverine sulfate was affected by the monsoon climate in the upper reaches of the Xijang River basin. Heavy isotopes were generally enriched in sulphate during the dry season compared to that in the wet season. The isotopicevidence showed that the riverine sulfate was mainly derived from sulfide oxidation, whereas evaporite dissolution could be an important source during the dry season. Mine drainage would be an important source of sulfate to those rivers during the wet season, mainly due to the responses of the fast surface flow to frequent heavy rain in monsoonal climate regions. A relatively highproportion of HCO₃^- was derived from rock weathering by sulfuric acid during the wet season compared to that during the dry season. Approximately 1/4 of the HCO₃^- in the Hongshui River originated from carbonate weathering by sulfuric acid, which derived from the oxidation of sulfide. In all, information from the specific dual isotopic characteristics of riverine sulfate throughout a hydrological year can provide unique evidence for understanding the temporal dynamics of sulfate concentrations and weathering processes in the rivers.

Key words: GIMMS NDVI, SPOT NDVI, consistency, correlation, trend, MODIS NDVI, spatial distribution