The Role of Lithology on Concentration‐Discharge Relationships and Carbon Export in Two Adjacent Headwater Catchments

Abstract Headwater catchments have strong impacts on downstream waterways, near‐shore ecosystems, and the quality of water available for growing human populations. Thus, understanding how water and solutes are exported through these upland landscapes is critically important. A growing body of literature highlights the interaction of topography, climate, and the critical zone structure as a key control on streamflow and chemical export. However, more focused work is needed to pinpoint how variability in subsurface structure across lithologically complex regions impacts streamflow and chemical signals at catchment outlets. Here, we aim to better understand how lithology and subsurface critical zones modulate streamflow response and solute export patterns in two central coastal California headwater catchments that are similar in topography, vegetation, and climate but have different lithologies. We monitored streamflow and collected surface water samples at the catchment outlets for dissolved major ions and organic carbon (DOC) for two consecutive water years. The catchment with mélange bedrock displayed much flashier hydrologic behavior with 7.8 times higher peak flow values and 1.9 times higher mean event concentrations of DOC, suggesting shorter and shallower hydrologic flow paths that likely arise from regions of shallower bedrock. Despite distinct hydrologic behavior and DOC export, dissolved major ion concentrations were broadly similar and chemostatic, which may be driven by rapid chemical reactions in the critical zone of both catchments. Our work contributes to building an integrated understanding of how subtle differences in catchment structure can have profound impacts on how water and solutes are routed through headwater catchments.