Testing nutrient flushing hypotheses at the hillslope scale: A virtual experiment approach

The delivery mechanisms of labile nutrients (e.g. NO3, DON and DOC) to streams are poorly understood. Recent work has quantified the relationship between storm DOC dynamics and the connectedness of catchment units and between pre-storm wetness and transient groundwater NO3 flushing potential. While several studies have shown N and C flushing during storm events as the important mechanism in the export of DOC and DON in small catchments, the actual mechanisms at the hillslope scale have remained equivocal. The difficulty in isolating cause and effect in field studies is made difficult due to the spatial variability of soil properties, the limited ability to detect flow pathways within the soil, and other unknowns. Some hillslopes show preferential flow behavior that may allow transmission of hillslope runoff and labile nutrients with little matrix interaction; others do not. Thus, field studies are only partially useful in equating C and N sources with water flow and transport. This paper presents a new approach to the study of hydrological controls on labile nutrient flushing at the hillslope scale. We present virtual experiments that focus on quantifying the first-order controls on flow pathways and nutrient transport in hillslopes. We define virtual experiments as numerical experiments with a model driven by collective field intelligence. We present a new distributed model that describes the lateral saturated and vertical unsaturated water flow from hypothetical finite nutrient sources in the upper soil horizons. We describe how depth distributions of transmissivity and drainable porosity, soil depth variability, as well as mass exchange between the saturated and unsaturated zone influence the mobilization, flushing and release of labile nutrients at the hillslope scale. We argue that this virtual experiment approach may provide a well-founded basis for defining the first-order controls and linkages between hydrology and biogeochemistry at the hillslope scale and perhaps form a basis for predicting flushing and transport of labile nutrients from upland to riparian zones.