When Snowpacks Shrink: Declining Weathering and Export Rates but Rising Stream Solute Concentrations in Warming Mountains

ABSTRACT Silicate weathering produces solutes, consumes CO 2 , and regulates Earth's long‐term climate. Rates of silicate weathering are changing, particularly in rapidly warming mountains with dwindling snowpacks. Although hydrologic shifts in warming mountains have been extensively studied, their impacts on water chemistry and weathering have been sparsely examined. Here we ask the question: How does changing mountain hydrology, in particular shrinking snowpacks and early snowmelt, alter silicate weathering, solute export, and stream chemistry? We answer this question using a catchment‐scale reactive transport model (BioRT‐HBV) calibrated with streamflow and chemistry data (2010–2020) from the Sagehen Creek Experimental Forest Catchment, a snow‐dominated headwater catchment in Sierra Nevada Mountains, CA, USA. We show that snow‐dependent water storage controls silicate weathering and solute export, especially in the shallow subsurface, contrasting the general perception that silicate weathering is slow and kinetically controlled with limited dependence on water storage. In the driest year with the least snow water equivalent ( SWE , 10% of the wettest year), rates of silicate weathering and export rates of weathering products (such as calcium) were about half and a third of those in the wettest year with the highest SWE . In fact, annual rates of weathering and solute export scale linearly with annual total precipitation and maximum SWE . The timing of weathering and solute export closely followed that of snowmelt, with rates peaking about 3 months earlier in the driest years compared to those in the wettest years. Mean stream calcium concentrations however were about 1.6 times higher in the driest year, with highest concentrations in the dry summer. These results suggest that as mountains shift to a low‐to‐no snow future, catchments transition from exporters to reactors with a lower weathering capacity and even lower export capacity, leading to higher solute concentration in streams.