A process-based study for estimating Particulate Organic Carbon load in a forested watershed

River has an important role of carbon cycle globally and locally. It transports significant amount of organic matters as a state of Particulate Organic Carbon (POC). POC sometimes increases two or three times in storm flow than that in base flow so need to propose a method to better estimate POC load through rivers. The load can be estimated by regressing relationship between POC concentrations/loads and discharges, however, the regressed values usually vary in the range of one magnitude and more. We conducted runoff analysis of POC in this study, based on observation of temporal change in quality. The study site located at a headwater stream in a forested catchment. An optical sensor, measuring turbidity of suspended solids in water, was introduced in a river channel and result from on-site sensing was compared with that from load-discharge regression. The result was much better in the case of on-site sensing but still showed two times error when POC load was estimated. We supposed that one of major reason of the error was because carbon content of suspended solids was highly fluctuated specially during storm, POC concentration variations were examined through qualitative analyses. Carbon content of suspended solids was 20-40% during low-flow period and 10-30% during high-flow period. Stable isotope analysis presented that organic matter supposedly derived from leaf litter was discharged first before the peak of water discharge then soil organic matter increased after peak discharge. The result suggest that litter and soil were main sources of POC in the study site, litter with lower density and higher carbon content was transported prior to soil with higher density and lower carbon content. We also examined the effect of low-flow period on the level of POC carbon content. Fine benthic organic matter was accumulated in the river bed during low-flow period and was flashed by subsequent storm flow while no significant increase of carbon content was observed with short low-flow period. Quantifying variable storage size of POC is therefore important for high-precision estimation of POC load in river waters.