Seasonal and interannual variations of hydrochemistry in the middle Yellow River

The Chinese Loess Plateau is a huge carbon reservoir that is strongly influenced by climate change. However, the seasonal and interannual variations in chemical weathering processes and their responses to extreme weather events remain poorly understood. Weekly water samples were collected from 2014 to 2016 at the Tongguan hydrological station in the middle Yellow River (MYR). Multiple methods, including ion ratios, forward model, chemical weathering rate estimation, net carbon budget calculation, and correlation analysis, were coupled to investigate contemporary weathering processes. The river water of the MYR was weakly alkaline, with its riverine solutes primarily dominated by evaporite dissolution (55 ± 0.5%) and carbonate weathering (33 ± 0.5%), while precipitation and anthropogenic inputs were limited. Evaporite dissolution contributed more to riverine solutes during the monsoon season, whereas carbonate weathering showed the opposite trend, with minor seasonal variation of silicate weathering. For interannual variations, evaporite dissolution contribution increased from 2014 (52%) to 2016 (57%), while carbonate weathering decreased from 2014 (35%) to 2016 (31%), with less silicate weathering contribution (6%). In the MYR, the physical erosion rate (1–7180 t/km²/yr) varied greatly, with high values in the monsoon season. The net CO₂ consumption budget by chemical weathering gradually decreased from 4.9 × 10¹⁰ mol/yr in 2014 to 3.9 × 10¹⁰ mol/yr in 2015, and further declined to 3.3 × 10¹⁰ mol/yr in 2016. Storm events triggered rapid sediment mobilization, with two events in 2016 accounting for 60% of annual flux. Storm events also substantially enhanced the net carbon consumption budget, contributing 6–9% of the total annual amount. Since 2001, increased runoff has led to a sustained rise in chemical weathering rates. These findings indicate that chemical weathering exhibits strong interannual variability and that storm events have a significant impact on chemical weathering flux.