Upland streamwater nitrate dynamics across decadal to sub-daily timescales: a case study of Plynlimon, Wales

Abstract

Abstract. Streamwater nitrate dynamics in the River Hafren, Plynlimon, mid-Wales were investigated over decadal to sub-daily timescales using a range of statistical techniques. Long-term data were derived from weekly grab samples and high-frequency data from 7-hourly samples (2007)(2008)(2009)) both measured at two sites: a headwater stream draining moorland and a downstream site below plantation forest. This study is one of the first to analyse upland streamwater nitrate dynamics across such a wide range of timescales and report on the principal mechanisms identified. The data analysis provided no clear evidence that the long-term decline in streamwater nitrate concentrations was related to a decline in atmospheric deposition alone, because nitrogen deposition first increased and then decreased during the study period. Increased streamwater temperature and denitrification may also have contributed to the decline in stream nitrate concentrations, the former through increased N uptake rates and the latter resultant from increased dissolved organic carbon concentrations. Strong seasonal cycles, with concentration minimums in the summer, were driven by seasonal flow minimums and seasonal biological activity enhancing nitrate uptake. Complex diurnal dynamics were observed, with seasonal changes in phase and amplitude of the cycling, and the diurnal dynamics were variable along the river. At the moorland site, a regular daily cycle, with minimum concentrations in the early afternoon, corresponding with peak air temperatures, indicated the importance of instream biological processing. At the downstream site, the diurnal dynamics were a composite signal, resultant from advection, dispersion and nitrate processing in the soils of the lower catchment. The diurnal streamwater nitrate dynamics were also affected by drought conditions. Enhanced diurnal cycling in Spring 2007 was attributed to increased nitrate availability in the post-drought period as well as low flow rates and high temperatures over this period. The combination of high-frequency short-term measurements and long-term monitoring provides a powerful tool for increasing understanding of the controls of element fluxes and concentrations in surface waters.

Integrated management of ash from industrial and domestic combustion: a new sustainable approach for reducing greenhouse gas emissions from energy conversion

“…In the case of nitrogen compounds, their concentrations are clearly affected by the biological activity of the catchment. This is because during the growing period, nitrogen is assimilated by plants and the concentrations of its compounds are significantly lower, which is confirmed by other studies (Gardner and McGlynn 2009;Halliday et al 2013). In turn, higher concentrations of nitrogen compounds are observed in autumn and winter, when the biological activity is definitely lower.…”

Green light: gross primary production influences seasonal stream N export by controlling fine‐scale N dynamics

“…For instance, we estimated that spring diel NO − 3 variation may reduce catchment NO − 3 exports by ~70 g N/ha (~16%) at the Ichetucknee River (770 km 2 , 8900 L/s), based on mean daily minima and maxima NO − 3 concentrations (0.38 and 0.46 mg N/L) reported by Heffernan and Cohen ( 2010 ). The contribution of fi nescale N dynamics to reduce catchment N export was even larger at the Upper Hafren River in UK (122 ha, 60 L/s), an open stream where spring diel NO − 3 variations (from 0.14 to 0.18 mg N/L) reduced stream NO − 3 loads by 154 g N/ha (22%) (Halliday et al 2013 ). These back-of-the-envelope calculations highlight that fi ne-scale N dynamics can not only indicate the preferential mechanisms of in-stream N uptake, but also provide a relevant evaluation of their contribution on regulating NO − 3 downstream fl uxes at the catchment scale.…”

“…The infl uence of fi ne-scale N patterns on N fl uxes could be even higher in open-canopy and lowland streams for which reported diel NO − 3 variations are larger than for headwater forested streams (Grimm 1987, Heffernan et al 2010, Halliday et al 2013. For instance, we estimated that spring diel NO − 3 variation may reduce catchment NO − 3 exports by ~70 g N/ha (~16%) at the Ichetucknee River (770 km 2 , 8900 L/s), based on mean daily minima and maxima NO − 3 concentrations (0.38 and 0.46 mg N/L) reported by Heffernan and Cohen ( 2010 ).…”

Water Flux Tracking With a Distributed Hydrological Model to Quantify Controls on the Spatio‐temporal Variability of Transit Time Distributions

“…Despite being more time-consuming and characterized by some degree of subjectivity, manual calibration helps reduce the risk of considering parameter sets that fit the calibration data but lead to poor internal consistency in model response (Boyle et al, 2000;Fatichi et al, 2015;Moradkhani & Sorooshian, 2008). The model was run more than 200 times with different sets of parameters, which were varied over ranges suggested by literature (Table 2) (e.g., Bell, 2005;Benettin et al, 2015b;Brandt et al, 2004;Halliday et al, 2013;Kirby et al, 1997;Shand et al, 2005). The different parameter combinations were tested in each model run and the parameter sets which resulted in higher performance measures and physically plausible simulations of the simulated variables (e.g., amount of groundwater seepage or mean basin saturation) were then (Criss & Winston, 2008) for discharge.…”