Long-term Persistence of Stream Nitrate Concentrations (Memory Effect) Inferred from Spectral Analysis and Detrended Fluctuation Analysis (original) (raw)
Related papers
2013
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 (1984-2010) and high-frequency data from 7 hourly samples (2007-2009) 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 only; 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 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.
Using long time series of agricultural-derived nitrates for estimating catchment transit times
The estimation of water and solute transit times in catchments is crucial for predicting the response of hydrosystems to external forcings (climatic or anthropogenic). The hydrogeochemical signatures of tracers (either natural or anthropogenic) in streams have been widely used to estimate transit times in catchments as they integrate the various processes at stake. However, most of these tracers are well suited for catchments with mean transit times lower than about 4-5 years. Since the second half of the 20th century, the intensification of agriculture led to a general increase of the nitrogen load in rivers. As nitrate is mainly transported by groundwater in agricultural catchments, this signal can be used to estimate transit times greater than several years, even if nitrate is not a conservative tracer. Conceptual hydrological models can be used to estimate catchment transit times provided their consistency is demonstrated, based on their ability to simulate the stream chemical signatures at various time scales and catchment internal processes such as N storage in groundwater.
The effects of land use on stream nitrate dynamics
2007
The effects of land use and land use change on stream nitrate are poorly understood. While case studies have been presented, most process work has been done in areas with one land use (minimally disturbed or agricultural) and areas with substantial atmospheric deposition. In this paper we present results from three neighboring headwater catchments in western Oregon with similar (low) atmospheric deposition, size, and geology but with different, spatially consistent land use expressions: forest, agriculture, and residential. The climate in western Oregon has a distinct pattern of a three-month rainless period in the summer, a wetting up with many storms in the fall and winter, and a decrease of storms in the spring. We investigate how human activity alters the export of nitrate, whether the input of nitrate changes throughout the year which may affect storm response (i.e., depletion of soil water nitrate, addition of fertilizer, etc.), and how the changing contribution of source waters throughout the year affects streamflow concentrations. Our results showed marked differences in export rates between the three catchments. The forested catchment showed minimal export for three monitored storms (fall, winter, spring) through the seasonal wetting up of the catchments, and the residential catchment showed high export for all three storms. While the agricultural catchment displayed elevated export in the fall (similar to the residential catchment), exports decreased progressively throughout the rainy period (following late summer manure and green bean application). Overall, our results of storm event nitrate concentrations suggest that varying nitrate inputs have a large affect on nitrate dynamics. While within-storm nitrate concentration response patterns in the residential catchment were the same as the patterns in the reference forested catchment (a ''concentration'' pattern throughout the year), a ''dilution'' pattern was observed in the fall and winter and a ''concentration'' pattern was observed in the spring in the agricultural catchment.
Biogeochemistry, 2010
Seasonal and diurnal variations of instream NO 3 -N concentration oscillations were studied through high-frequency measurements of streamwater's physical, chemical parameters (in-stream NO 3 -N concentration, water temperature, dissolved oxygen, pH) and hydrometeorological variables (stream discharge, solar radiation) under hydrologically stable conditions. The study was carried out in 2006, within the 42 km 2 forested Padež stream watershed in the southwestern part of Slovenia, which is characterized by distinctive hydrogeological settings (flysch) and climate conditions (transitional area between the Mediterranean and continental climate). Fine temporal data resolution (15-min interval) enabled identification of the factors responsible for seasonal variability in the diurnal pattern of the streamwater NO 3 -N concentrations versus seasonal and diurnal behavior of meteorological and other water chemistry constituents. The observed seasonal variability of instream NO 3 -N daily oscillations indicates the important role of primary production uptake, particularly during seasons when deciduous vegetation is dormant and light levels in the stream are high. Highest daily NO 3 -N concentration amplitudes (0.3 mg/l-N) and daily changes in the NO 3 -N flux (0.4-0.5 g/s-N) were observed in spring; the NO 3 -N concentration oscillations in summer showed a considerably smaller effect of the in-stream uptake (maximum NO 3 -N daily concentration amplitude 0.1 mg/l-N; daily change in the NO 3 -N flux 0.02 g/s-N). Seasonal shifts in the timing of daily maximum (up to 6 h) and minimum NO 3 -N concentrations (between 1 and 3 h) provided some additional indications of seasonal changes in the in-stream primary production uptake and its relation to the terrestrial component of the forested watershed.
Journal of Environmental Quality, 2008
Th e spatial and temporal dynamics in the stream water NO 3-N concentrations in a major European river-system, the Garonne (62,700 km 2), are described and related to variations in climate, land management, and effl uent point-sources using multivariate statistics. Building on this, the Hydrologiska Byråns Vattenbalansavdelning (HBV) rainfall-runoff model and the Integrated Catchment Model of Nitrogen (INCA-N) are applied to simulate the observed fl ow and N dynamics. Th is is done to help us to understand which factors and processes control the fl ow and N dynamics in diff erent climate zones and to assess the relative inputs from diff use and point sources across the catchment. Th is is the fi rst application of the linked HBV and INCA-N models to a major European river system commensurate with the largest basins to be managed under the Water Framework Directive. Th e simulations suggest that in the lowlands, seasonal patterns in the stream water NO 3-N concentrations emerge and are dominated by diff use agricultural inputs, with an estimated 75% of the river load in the lowlands derived from arable farming. Th e results confi rm earlier European catchment studies. Namely, current semi-distributed catchment-scale dynamic models, which integrate variations in land cover, climate, and a simple representation of the terrestrial and in-stream N cycle, are able to simulate seasonal NO 3-N patterns at large spatial (>300 km 2) and temporal (≥ monthly) scales using available national datasets.
Graduation date: 2007 This work furthers the understanding of processes occurring in catchments that affect stream nitrate concentrations using two different approaches: a temporally intensive case study of three headwater catchments with varying land use (through storm event monitoring) and a spatially intensive study on the regional scale (through statistical modeling) of 1st-4th order catchments. At the catchment scale, stream nitrate concentrations during three storm events were monitored in three catchments with different land uses (forested, agricultural, residential) to determine how land use affects nitrate "patterns" during storm events. Overall, results of storm event nitrate concentrations suggest that varying nitrate inputs have a large affect on nitrate dynamics. While within-storm nitrate concentration response patterns in the residential catchment were the same as the patterns in the reference forested catchment (a "concentration" pattern throughou...
Hydrological Processes, 2013
A nitrate sensor has been set up to measure every 10 min the nitrate signal in a stream draining a small agricultural catchment dominated by fertilized crops during a 2-year study period (2006)(2007)(2008) in the south-west of France. An in situ sampling protocol using automatic sampler to monitor flood events have been used to assume a point-to-point calibration of the sensor values. The nitrate concentration exhibits nonsystematic concentration and dilution effects during flood events. We demonstrate that the calibrated nitrate sensor signal gathered from the outlet is considered to be a continuous signal using the Nyquist-Shannon sampling theorem. The objectives of this study are to quantify the errors generated by a typical infrequent sampling protocol and to design appropriate sampling strategy according to the sampling objectives. Nitrate concentration signal and flow data are numerically sampled to simulate common sampling frequencies. The total fluxes calculated from the simulated samples are compared with the reference value computed on the continuous signal. Uncertainties are increasing as sampling intervals increase; the method that is not using continuous discharge to compute nitrate fluxes bring larger uncertainty. The dispersion and bias computed for each sampling interval are used to evaluate the uncertainty during each hydrological period. High underestimation is made during flood periods when high-concentration period is overlooked. On the contrary, high sampling frequencies (from 3 h to 1 day) lead to a systematic overestimation (bias around 3%): highest concentrations are overweighted by the interpolation of the concentration in such case. The in situ sampling protocol generates less than 1% of load estimation error and sample highest concentration peaks. We consider useful such newly emerging field technologies to assess short-term variations of water quality parameters, to minimize the number of samples to be analysed and to assess the quality state of the stream at any time.
… Assembly 2010, held …, 2010
1] Nitrate pollution of surface waters is widespread in lowland catchments with intensive agriculture. For identification of effective nitrate concentration reducing measures the nitrate fluxes within catchments need to be quantified. In this paper we applied a mass transfer function approach to simulate catchment-scale nitrate transport. This approach was extended with time-varying travel time distributions and removal of nitrate along flow paths by denitrification to be applicable for lowland catchments. Numerical particle tracking simulations revealed that transient travel time distributions are highly irregular and rapidly changing, reflecting the dynamics of rainfall and evapotranspiration. The solute transport model was able to describe 26 years of frequently measured chloride and nitrate concentrations in the Hupsel Brook catchment (6.6 km 2 lowland catchment in the Netherlands) with an R 2 value of 0.86. Most of the seasonal and daily variations in concentrations could be attributed to temporal changes of the travel time distributions. A full sensitivity analysis revealed that measurements other than just surface water nitrate and chloride concentrations are needed to constrain the uncertainty in denitrification, plant uptake, and mineralization of organic matter. Despite this large uncertainty, our results revealed that denitrification removes more nitrate from the Hupsel Brook catchment than stream discharge. This study demonstrates that a catchment-scale lumped approach to model chloride and nitrate transport processes suffices to accurately capture the dynamics of catchment-scale surface water concentration as long as the model includes detailed transient travel time distributions. Citation: van der Velde, Y., G. H. de Rooij, J. C. Rozemeijer, F. C. van Geer, and H. P. Broers (2010), Nitrate response of a lowland catchment: On the relation between stream concentration and travel time distribution dynamics, Water Resour. Res., 46, W11534,
Water Resources Research, 2010
1] Nitrate pollution of surface waters is widespread in lowland catchments with intensive agriculture. For identification of effective nitrate concentration reducing measures the nitrate fluxes within catchments need to be quantified. In this paper we applied a mass transfer function approach to simulate catchment-scale nitrate transport. This approach was extended with time-varying travel time distributions and removal of nitrate along flow paths by denitrification to be applicable for lowland catchments. Numerical particle tracking simulations revealed that transient travel time distributions are highly irregular and rapidly changing, reflecting the dynamics of rainfall and evapotranspiration. The solute transport model was able to describe 26 years of frequently measured chloride and nitrate concentrations in the Hupsel Brook catchment (6.6 km 2 lowland catchment in the Netherlands) with an R 2 value of 0.86. Most of the seasonal and daily variations in concentrations could be attributed to temporal changes of the travel time distributions. A full sensitivity analysis revealed that measurements other than just surface water nitrate and chloride concentrations are needed to constrain the uncertainty in denitrification, plant uptake, and mineralization of organic matter. Despite this large uncertainty, our results revealed that denitrification removes more nitrate from the Hupsel Brook catchment than stream discharge. This study demonstrates that a catchment-scale lumped approach to model chloride and nitrate transport processes suffices to accurately capture the dynamics of catchment-scale surface water concentration as long as the model includes detailed transient travel time distributions. Citation: van der Velde, Y., G. H. de Rooij, J. C. Rozemeijer, F. C. van Geer, and H. P. Broers (2010), Nitrate response of a lowland catchment: On the relation between stream concentration and travel time distribution dynamics, Water Resour. Res., 46, W11534,
Although nitrate export in agricultural catchments has been simulated using various types of models, the role of groundwater in nitrate dynamics has rarely been fully taken into account. We used groundwater dating methods (CFC analyses) to reconstruct the original nitrate concentrations in the groundwater recharge in Brittany (Western France) from 1950 to 2009. This revealed a sharp increase in nitrate concentrations from 1977 to 1990 followed by a slight decrease. The recharge concentration curve was then compared with past chronicles of groundwater concentration. Groundwater can be interpreted as resulting from the annual dilution of recharge water in an uncontaminated aquifer. Two aquifers were considered: the weathered aquifer and the deeper fractured aquifer. The nitrate concentrations observed in the upper part of the weathered aquifer implied an annual renewal rate of 27 to 33% of the reservoir volume while those in the lower part indicated an annual renewal rate of 2-3%. The concentrations in the deep fractured aquifer showed an annual renewal rate of 0.1%. The river concentration can be simulated by combining these various groundwater reservoirs with the recharge. Winter and summer waters contain i) recharge water, or water from the variably saturated zone with rapid transfer and high nitrate concentrations, and ii) a large contribution (from 35 to 80% in winter and summer, respectively) from the lower part of the aquifer (lower weathered aquifer and deep fractured aquifer). This induces not only a relatively rapid response of the catchment to variations in agricultural pressure, but also a potential inertia which has to be taken into account.