Dynamics of dissolved organic carbon in hillslope discharge: Modeling and challenges

Reliable quantitative prediction of water movement and fluxes of dissolved substances - specifically organic carbon - at both the hillslope and the catchment scales remains a challenge due to complex boundary conditions and soil spatial heterogeneity. In addition, microbially mediated transformations of dissolved organic carbon (DOC) are recognized to determine the balance of DOC in soils. So far, only few studies utilized stable water isotope information in modeling and even fewer linked dissolved carbon fluxes to mixing and/or transport models. In this study, stormflow dynamics of O-18/O-16 ratios in the water molecules (expressed as delta O-18) and DOC were analyzed using a physically-based modeling approach. A one-dimensional dual-continuum vertical flow and transport model was used to simulate the subsurface transport processes in a forest hillslope soil over a period of 2.5 years. The model was applied to describe the transformation of input signals of delta O-18 and DOC into output signals observed in the hillslope storm-flow. To quantify uncertainty associated with the model parameterization, Monte Carlo analysis in conjunction with Latin hypercube sampling was applied. delta O-18 variations in hillslope discharge and in soil pore water were predicted reasonably well. Despite the complex nature of microbial transformations that caused uncertainty in model parameters and subsequent prediction of DOC transport, the simulated temporal patterns of DOC concentration in stormflow showed similar behavior to that reflected in the observed DOC fluxes. Due to preferential flow, the contribution of the hillslope DOC export was higher than the amounts that are usually found in the available literature. (c) 2016 Elsevier B.V. All rights reserved.