Hydro-climate and biogeochemical processes control watershed organic carbon inflows: Development of an in-stream organic carbon module coupled with a process-based hydrologic model.

Dissolved organic carbon (DOC) in surface waters directly influences the speciation, transport, and fate of heavy metals, as well as the partitioning of organic contaminants. However, the lack of process-based watershed-scale models for simulating carbon cycling and transport has limited the effective watershed management to control organic carbon fluxes to source waters and throughout the river systems. Here, a process-based in-stream organic carbon (OC) module was developed, coupled with the physically process-based Soil and Water Assessment Tool (SWAT), and linked with its existing soil carbon module to simulate dynamics of both particulate organic carbon (POC) and DOC. The advanced model simulates a large spectrum of OC processes from landscapes to stream networks throughout the watersheds. In-stream organic carbon processes related to POC and DOC as state variables are modeled in the water column, and the transformations between different carbon species and interactions between OC with algae are considered. The module's ability to simulate total organic carbon (TOC) loads was assessed, and the monthly and seasonal variations were captured over 14 years. Simulations for TOC loads suggested that spring snowmelt and summer rainfall runoff events are the main driving forces behind TOC export in the watershed. The parameter sensitivity analysis and dynamic reaction rate simulated in the streams suggested that TOC dynamics in the study area are controlled by both landscape and in-stream processes. The spatiotemporal analysis of the simulated TOC load showed that 55.8% of total terrestrial OC exports into the streams are removed due to in-stream POC settling and DOC mineralization, confirming the necessity of integrating terrestrial and aquatic OC processes for process understanding and for modelling and management of water quality at the watershed scale. The developed OC module is a potentially effective tool for simulating the OC cycle at the watershed scale and can be applied further to water treatment plans and watershed management.