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

Estimation of dissolved organic carbon (DOC) runoff load in forested watershed is important for the assessment of the global carbon cycle as well as for the control of regional water environments. A few process-based models have been proposed to estimate the DOC loads to water environments, which assume DOC source in topsoil and transport processes to the river, however, these models exhibited difficulties with the availability of input data and applicability to short time-scale rainfall-runoff processes in the Asian monsoon area. With the six years' data from middle stream (98ha) and downstream (1798ha) in the Mizugaki Experimental Watershed, Yamanashi, Japan, this study developed a new process-based model that consists of two separate systems for determining DOC loads enforced by DOC Source Area (DSA) concept. For the runoff system, a semi-distributed hydrological modelling unit (`modified-TOPMODEL') was installed, by which surface and subsurface water flows, representing for DSA, were sequentially simulated. For the soil system, a wet-dry cycle was successfully simulated by an advection-diffusion and dissolution formulation as well as seasonal temperature effect, integrated with root-zone water fluxes calculated by modified-TOPMODEL. The results of storm event calculation showed that the model performed well for the middle stream while the simulated DOC loadings were overestimated at downstream, implying the need of improvement for DSA calculation with a large gap in topographic characteristics.