Exploring the First Order Controls of Hillslope Scale N and DOC Flushing: A Virtual Experiment Approach

The delivery mechanisms of N and DOC to streams are poorly understood. Recent work has quantified the relationship between storm DOC dynamics and the connectedness of catchment units (McGlynn and McDonnell, 2002 WRR in press) and between pre-storm wetness and transient groundwater N flushing potential (McHale et al., 2002 WRR 38: 10.1029). While several studies have shown N and DOC flushing during storm events as the important mechanism in the export of DOC and N 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 reduced 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 (Buttle et al., 2001, HP 15: 1065-1070); others do not. Thus, field studies are only partially useful in equating DOC and N sources with water flow and transport. This paper presents a new approach to the study of hydrological controls on DOC and N flushing at the hillslope scale. We present virtual experiments that focus on quantifying the first order controls on flow pathways, transport and residence distributions in hillslopes. We define virtual experiments are 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 finite and infinite N and DOC 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 N and DOC at the hillslope scale. We argue that this new 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.