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

Abstract

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 © 2013 John Wiley & Sons, Ltd.

Mechanisms driving the seasonality of catchment scale nitrate export: Evidence for riparian ecohydrologic controls

“…These diurnal fluctuations in groundwater have been shown to drive diurnal patterns in stream NO 2 3 concentrations [Flewelling et al, 2013]. Further, we argue that summer riparian NO 2 3 production and transport are driven by subdaily variations in soil oxygen caused both by precipitation events and by soil evaporation and transpiration that induce diurnal fluctuations in groundwater tables during the summer [Loheide et al, 2005;Flewelling et al, 2013].…”

“…These diurnal fluctuations in groundwater have been shown to drive diurnal patterns in stream NO 2 3 concentrations [Flewelling et al, 2013]. Further, we argue that summer riparian NO 2 3 production and transport are driven by subdaily variations in soil oxygen caused both by precipitation events and by soil evaporation and transpiration that induce diurnal fluctuations in groundwater tables during the summer [Loheide et al, 2005;Flewelling et al, 2013]. We posit that the marked topographic heterogeneity across the riparian zone along the entire stream reach introduces sufficient variability in transport and transformations that can obscure what would otherwise be strong diel variability in NO [Abit et al, 2008] from the riparian zone to the stream during the growing season.…”

“…We posit that the marked topographic heterogeneity across the riparian zone along the entire stream reach introduces sufficient variability in transport and transformations that can obscure what would otherwise be strong diel variability in NO [Abit et al, 2008] from the riparian zone to the stream during the growing season. Recent work [Flewelling et al, 2013] showed that fluctuations of riparian water levels over the course of the day changed head gradients, which caused the specific discharge through near-stream sediment to vary at the same time scale.…”

“…Pond Branch exhibits diel fluctuations in stream NO from diel patterns at Walker Branch and an agricultural catchment [Flewelling et al, 2013] where concentrations are lowest during the day. In Walker Branch, diel patterns show maximum concentrations at night because instream biota utilize NO 2 3 during the day [Roberts and Mulholland, 2008].…”

Nitrate sinks and sources as controls of spatio-temporal water quality dynamics in an agricultural headwater catchment

“…Basu et al, 2011;Hensley et al, 2015;Thompson et al, 2011) or discharge (e.g. Flewelling et al, 2014). Compared to hydrological export processes (concentration and dilution processes), in-stream removal processes have a smaller impact on total in-stream nitrate concentrations but they can be responsible for nitrate removal (apparent decrease of nitrate concentrations, excluding dilution processes) in the range of 2-10 % at the reach scale (i.e.…”