Hydrological partitioning of soil water in a hillslope segment using dual continuum model

The existence of a hydraulically interconnected macropore network in the soil profile and the activation of preferential flow through this network during major rainfall events represent a significant difficulty in quantifying the temporal origin of soil water fluxes. The hydrograph separation technique based on the transport of stable water isotopes (or other conservative tracers) in soils in conjunction with a mass balance approach is usually used to partition the runoff into pre-event and event water contributions. For hillslopes located in a temperate climate, the pre-event water is recognized to form a dominant fraction of a stormflow hydrograph. In this study, one- and two-dimensional dual continuum models were used to study the preferential flow of water and the transport of oxygen-18 isotope in a hillslope segment located in a temperate spruce forest. The dual continuum model divides the heterogeneous bulk soil into the soil matrix and the preferential flow domain, with a possible exchange of water and isotope content between the domains. The isotopic composition of the hillslope flow shows distinct signatures of the preferential flow paths and the soil matrix due to the nonequilibrium conditions between the domains. As a result, imperfect mixing of the isotope tracer within the hillslope soil is predicted, leading to isotopically different water pools in the soil matrix and preferential pathways. Despite the dominant role of preferential flow in the generation of hillslope stormflow, the pre-event water formed 52–84% of the total subsurface stormflow, as reflected in the measured isotopic composition of shallow subsurface runoff.