Developing and improving a simple runoff model of dissolved organic carbon considering soil infiltration and river runoff processes in a forested watershed

Estimation of dissolved organic carbon (DOC) runoff from forested watershed is important for carbon cycle researches and water quality control. Recently, a few process-based models have been proposed to estimate DOC loading to water environments, which assume DOC source in topsoil and transport process to the river. However, the models require large number of input data and are not designed for short time scale calculation of intensive rainfall runoff with in Asian monsoon area. We proposed a new DOC runoff model with both soil and river runoff processes enforced by DOC Source Area (DSA) concept. The data were collected at middle stream (98ha) and downstream (1798ha) in the Mizugaki Experimental Watershed, Yamanashi, Japan from 2009 to 2012. Dry-wet cycle and long-term variation in soil were successfully simulated by advection-diffusion and dissolution formulations of the Soil Submodel. Runoff Submodel included hydrological calculation unit, TOPMODEL, to separately quantify base flow (Qb), subsurface flow (Qsf) and overland flow (Qof). Qsf and Qof were assumed to represent for DSA, and the DOC loading was calculated by Qsf, Qof and the estimated DOC concentration from the Soil Submodel. The results showed that the model performed well for 4 among 11 events at the middle stream while the simulated DOC loadings were overestimated for all 8 events downstream. The reason of the low performance of the model was attributed to the error of Qsf and Qof calculated by TOPMODEL.