Diel patterns in coastal-stream nitrate concentrations linked to evapotranspiration in the riparian zone of a low-relief, agricultural catchment

Evapotranspiration (ET) can cause diel fluctuations in the elevation of the water table and the stage in adjacent streams. The diel fluctuations of water levels change head gradients throughout the day, causing specific discharge through near-stream sediment to fluctuate at the same time scale. In a previous study, we showed that specific discharge controls the residence time of groundwater in streambed sediment that, in turn, exerted the primary control on NO3- removal from groundwater passing through the streambed. In this study, we examine the magnitude of diel specific discharge patterns through the streambed driven by ET in the riparian zone with a transient numerical saturated-unsaturated groundwater flow model. On the basis of a first-order kinetic model for NO3- removal, we predicted diel fluctuations in stream NO3- concentrations. Model results indicated that ET drove a diel pattern in specific discharge through the streambed and riparian zone (the NO3- removal zones). Because specific discharge is inversely proportional to groundwater travel time through the NO3- removal zones and travel time determines the extent of NO3- removal, diel changes in ET can result in a diel pattern in NO3- concentration in the stream. The model predictions generally matched observations made during summertime base-flow conditions in a small coastal plain stream in Virginia. A more complicated pattern was observed following a seasonal drawdown period, where source components to the stream changed during the receding limb of the hydrograph and resulted in diel fluctuations being superimposed over a multi-day trend in NO3- concentrations. Copyright (c) 2013 John Wiley & Sons, Ltd.