Chemical weathering and CO2 consumption rates of the Koshi River Basin: modelling and quantifying

The Koshi River is a typical river originating from the Tibetan Plateau that drains into the Indian Plain. To quantify the chemical weathering and CO2 consumption rates of the upstream alpine meltwater-supplied mountainous and downstream rainfall-impacted temperate plain regions, 46 river water samples were collected at 23 sites during monsoon and post-monsoon seasons, and major ions concentrations were measured in the laboratory. The forward model was applied to calculate carbonate weathering rate (CWR), silicate weathering rate (SWR), and their CO2 consumption rates ([CO2]car and [CO2]sil). For investigating the hydrological impacts, correlation analysis was applied between the chemical weathering rates and different runoff components based on the Spatial Process in Hydrology model at each sampling site. Results show that carbonate and silicate rock weathering control the river water chemistry. The average silicate weathering rate is 7.37 tons/km2/ yr, consuming 3.40 x 105 mol/km2/yr CO2 while considering sulfide-involved carbonate weathering, the carbonate weathering rate is 26.35 tons/km2/yr, consuming 3.82 x 105 mol/km2/yr CO2. Overall, chemical weathering and CO2 consumption rates are enhanced in monsoon season in both the upstream and downstream regions owing to the intense hydrological cycle. These rates are relatively higher in the alpine upstream region during the post-monsoon season due to intense physical erosion, and higher in downstream during the monsoon season due to the greater volume of water. The significant positive correlations between snowmelt runoff with CWR and [CO2]carindicated that the rising precipitation and temperature leading to enhanced snowmelt would increase the carbonate weathering rate. Hence, this study provides a valuable understanding of the spatiotemporal variations in river hydrochemistry and the role of hydrology in shaping chemical budget within the snow and glacier-fed Himalaya River Basins.