Scaling hydrological and biogeochemical processes remains a fundamental challenge in watershed science. Historically, research has emphasized small area catchments where inputs and outputs can be precisely measured. Nested long-term monitoring data sets can be used to investigate how processes upscale and propagate from headwaters to downstream areas before reaching the ocean. Here, we use 15 years of data from the Lamprey River Hydrological Observatory, a temperate watershed, to understand the mechanisms that control both flow and dissolved organic carbon (DOC) production, transport, export, and across nested catchments varying sizes from small (7 km²), medium (170 km²), to large (480 km²). We developed a parsimonious model that optimizes discharge and DOC simultaneously to investigate multiple interacting factors that ultimately determine the variability in concentrations of DOC. Factors include temperature, water storage and mixing, flow path depth, snowpack, antecedent precipitation, and the availability of organic matter. We calibrate the model using daily discharge data and monthly and weekly DOC measurements across nested catchments. Our model simulations successfully represented the Lamprey River's discharge (KGE ~ 0.60) and DOC (KGE ~ 0.55) concentration dynamics across all scales. Model results show that rapid surface and overland flow contribute more to the total annual flow, with decreasing catchment area reaching up to 40% of the annual flow in the smallest catchment, compared to ~30% in the medium-sized catchment and less than 20% in the largest catchment. Overall, DOC production did not show a relation with scale; however, increased loss was observed with increasing scale (0.57, 0.65, and 0.67, respectively, from small to large scale). In addition, antecedent accumulated precipitation (over 45 days) significantly influenced DOC dynamics across catchment scales, emphasizing the importance of connectivity in catchment response. Upscaling hydrological and biogeochemical processes using parsimonious models could play a fundamental role in understanding the effects of climate change and other anthropogenic factors in complex ecosystems.