In-stream Nitrate Concentration Research Articles

Flow paths and wetness conditions explain spatiotemporal variation of nitrogen retention for a temperate, humid catchment

Excess use of nitrogen (N) fertilizer has threatened the quality of drinking water and destroyed the structure and functions of aquatic ecosystems. The N retention is defined as a catchment’s capacity to withhold nitrogen from entering surface water bodies during a certain period. Previous studies have focused on N retention in response to catchment characteristics and different climates. However, little has been done to explore the spatiotemporal variation of N retention within a catchment. This study is aimed to explore the spatiotemporal variation of the N retention and the effect of the external N input heterogeneity on the retention capacity and the N export in a small temperate, humid catchment in central Germany. A spatially distributed model was built using the fully coupled surface–subsurface model HydroGeoSphere to simulate the flow and transport of nitrate. Three scenarios considering the external N input heterogeneity, which represent different land-use patterns, were created: (i) the input was uniformly distributed over the catchment, (ii) high input was associated with high retention regions (e.g. near the hillslope top), and (iii) high input was associated with low retention regions (e.g. near the stream). The results showed that N retention exhibits strong spatial heterogeneity, which is controlled by the lengths of flow paths connecting the regions and surface water body and regional wetness conditions within a catchment. N retention showed pronounced seasonality in temperate and humid climates, being higher in wetting, drying and dry periods, but lower in wet periods. The pattern is driven by distinct shifts in the dominance of deeper flow paths and fast shallow flow paths determined by the climate seasonality. Highest denitrification, lowest export fluxes and in-stream nitrate concentration were found in scenario (ii) among the scenarios. Therefore, it was suggested that agricultural activities may avoid regions near surface water bodies to improve the surface water quality. The results of this study provide insights into the nitrogen dynamics and can guide the management of surface water quality.

Effect of topographic slope on the export of nitrate in humid catchments: a 3D model study

Abstract. Excess export of nitrate to streams affects ecosystem structure and functions and has been an environmental issue attracting worldwide attention. The dynamics of catchment-scale solute export from diffuse nitrogen sources can be explained by the changes of dominant flow paths, as solute attenuation (including the degradation of nitrate) is linked to the age composition of outflow. Previous data-driven studies suggested that catchment topographic slope has strong impacts on the age composition of streamflow and consequently on in-stream solute concentrations. However, the impacts have not been systematically assessed in terms of solute mass fluxes and solute concentration levels, particularly in humid catchments with strong seasonality in meteorological forcing. To fill this gap, we modeled the groundwater flow and nitrate transport for a small agricultural catchment in Central Germany. We used the fully coupled surface and subsurface numerical simulator HydroGeoSphere (HGS) to model groundwater and overland flow and nitrate transport. We computed the water ages using numerical tracer experiments. To represent various topographic slopes, we additionally simulated 10 synthetic catchments generated by modifying the topographic slope from the real-world scenario. Results suggest a negative correlation between the young streamflow fraction and the topographic slope. This correlation is more pronounced in flat landscapes with slopes <1:60. Flatter landscapes tend to retain more N mass in the soil (including mass degraded in soil) and export less N mass to the stream, due to reduced leaching and increased degradation. The mean in-stream nitrate concentration shows a decreasing trend in response to a decreasing topographic slope, suggesting that a large young streamflow fraction is not sufficient for high in-stream concentrations. Our results improve the understanding of nitrate export in response to topographic slope in a temperate humid climate, with important implications for the management of stream water quality.

Groundwater‐surface water exchange as key control for instream and groundwater nitrate concentrations along a first‐order agricultural stream

AbstractLower‐order streams (first‐ and second‐order) define the initial, landscape‐related, chemical signature of stream water in catchments. To date, first‐order streams have been perceived as predominantly draining systems, which collect water and solutes from the surrounding groundwater and surface runoff and simply mirror the chemical composition of the inputs. In this study, the impact of stream‐groundwater exchange fluxes on water chemistry of a first order agricultural stream (Schönbrunnen) and its connected groundwater in south‐western Germany was assessed combining222Rn, dissolved ions (chloride, sulphate, nitrate), and salt tracer tests with investigations of stream discharge and groundwater hydraulic gradients. The findings suggest that stream‐water chemistry in lower‐order streams is governed by an intricate interplay between dynamic, bidirectional water and solute exchange between groundwater and the stream leading to a pronounced hydrologic turnover along the studied reaches. High nitrate concentrations (up to 79 mg/L as NO3−) in stream water were attenuated in downstream direction (a mean value of 39 mg/L) without an increase in discharge, suggesting that redox processes occurring during sediment passage in sequential infiltration and exfiltration zones affect stream water chemistry. Nitrate in stream water infiltrating into the aquifer at distinct losing spots was subject to denitrification within the first few decimetres of the streambed, while concurrent exfiltration of low‐nitrate groundwater into the stream at gaining spots compensated for flow losses and in turn diluted instream nitrate concentrations. In summary the findings imply that (1) instream mixing resulting from the bidirectional exchange of water between groundwater and the stream (hydrologic turnover) affects instream nitrate concentrations, (2) denitrification in the streambed of losing reaches and the near‐stream aquifer significantly contributes to reactive nitrate turnover and elimination, and (3) oxidation of ammonium could be a secondary source of nitrate inputs into the stream.

Using nitrate as a tracer to constrain age selection preferences in catchments with strong seasonality

Excess export of nitrate to streams has been an environmental issue attracting attention across the world. Lumped models based on StorAge Selection (SAS) functions have been used widely to investigate this export processes in terms of hydrological and chemical responses in catchment outflux – both in discharge (Q) and in evapotranspiration (ET). The validity of these models depends on the appropriate choice of water age selection preferences, i.e. the parameterization of the SAS functions of Q and ET, which are often calibrated against observed in-stream tracer data. This study attempted to assess the usefulness of in-stream nitrate (N-NO3) concentrations for resolving the SAS functions in terms of plausibility (being physically meaningful) and parameter uncertainty, in a small agricultural catchment in central Germany that experiences strong seasonality. A spatially implicit model was built to simulate the dynamics of nitrate export. The data sets of in-stream nitrate concentrations, nitrogen surplus in the soil, and the median age of Q were used to constrain the model. The best-fit (to all data sets) results indicate high plausibility of the calibrated, temporally dynamic age selection preferences for Q for most time of a year, however, not for ET. The uncertainty analysis using a Monte Carlo approach suggests that lower uncertainty is associated with the SAS functions for Q than that for ET. The uncertainties of the SAS functions can be reduced by 60% combining the three datasets instead of using only in-stream concentration data. This study demonstrated the usefulness of combined data sets for adequately determining dynamic age selection preferences for Q, but not for ET. Better constraining selection preferences will require better in-stream concentration data for the dry period and additional data sets for ET (e.g. age of ET).

Application of Integrated Watershed Management Measures to Minimize the Land Use Change Impacts

Non-point source pollution is a major factor in excessive nutrient pollution that can result in the eutrophication. Land use/land cover (LULC) change, as a result of urbanization and agricultural intensification (e.g., increase in the consumption of fertilizers), can intensify this pollution. An informed LULC planning needs to consider the negative impacts of such anthropogenic activities to minimize the impact on water resources. The objective of this study was to inform future land use planning by considering nutrient reduction goals. We modeled the LULC dynamics and determined the capacity for future agricultural development by considering its impacts on nitrate runoff at a watershed scale in the Tajan River Watershed in northeastern Iran. We used the Soil and Water Assessment Tool (SWAT) to simulate the in-stream nitrate concentration on a monthly timescale in this watershed. Historical LULCs (years 1984, 2001 and 2010) were derived via remote sensing and were applied within the Land Change Modeler to project future LULC in 2040 under a business-as-usual scenario. To reduce nitrate pollution in the watershed and ecological protection, a conservation scenario was developed using a multi-criteria evaluation method. The results indicated that the implementation of the conservation scenario can substantially reduce the nitrate runoff (up to 72%) compared to the business-as-usual scenario. These results can potentially inform regional policy makers in strategic LULC planning and minimizing the impact of nitrate pollution on watersheds. The proposed approach can be used in other watersheds for informed land use planning by considering nutrient reduction goals.

Modeling Nitrate Export From a Mesoscale Catchment Using StorAge Selection Functions

AbstractStorAge Selection (SAS) functions describe how catchments selectively remove water of different ages in storage via discharge, thus controlling the transit time distribution (TTD) and solute composition of discharge. SAS‐based models have been emerging as promising tools for quantifying catchment‐scale solute export, providing a coherent framework for describing both velocity‐driven and celerity‐driven transport. Due to their application in headwaters only, the spatial heterogeneity of catchment physiographic characteristics, land use management practices, and large‐scale validation have not been adequately addressed with SAS‐based models. Here, we integrated SAS functions into the grid‐based mHM‐Nitrate model (mesoscale Hydrological Model) at both grid scale (distributed model) and catchment scale (lumped model). The proposed model provides a spatially distributed representation of nitrogen dynamics within the soil zone and a unified approach for representing both velocity‐driven and celerity‐driven subsurface transport below the soil zone. The model was tested in a heterogeneous mesoscale catchment. Simulated results show a strong spatial heterogeneity in nitrogen dynamics within the soil zone, highlighting the necessity of a spatially explicit approach for describing near‐surface nitrogen processing. The lumped model could well capture instream nitrate concentration dynamics and the concentration–discharge relationship at the catchment outlet. In addition, the model could provide insights into the relations between subsurface storage, mixing scheme, solute export, and the TTDs of discharge. The distributed model shows results that are comparable to the lumped model. Overall, the results reveal the potential for large‐scale applications of SAS‐based transport models, contributing to the understanding of water quality‐related issues in agricultural landscapes.

Storm event impacts on in-stream nitrate concentration and discharge dynamics: A comparison of high resolution in-situ measured data with model simulations

The relationship between nitrogen and discharge (N-Q) in a stream can be captured with high frequency nitrate nitrogen (NO3−-N) samplers. In Austria, the Raab catchment (998 km2) has high frequency NO3−-N data measured with a spectrometer probe. This study evaluated if the widely-used and typically calibrated eco-hydrological model Soil and Water Assessment Tool (SWAT) can reproduce the hysteresis loop direction and the dilution or accretion effects of NO3−-N dynamics during storm events in this agricultural catchment. The daily aggregated NO3−-N measurements were compared with the daily SWAT simulated discharge and NO3−-N concentrations of 14 storm events by computing hysteresis indices - loop direction and area (h index), loop direction (HInew) and solute gradient (∆C). Overall, the SWAT model was able to replicate the predominant anticlockwise hysteresis and dilution effect of NO3−-N in the Raab catchment. The loop direction was simulated correctly in 9 and 10 events, for the h and HInew indices, respectively. The hysteresis direction inferred from both indices did not always concur due to the differences in the calculation methods. The dilution or accretion effect was simulated correctly in 9 of the events. However, the SWAT model only correctly simulated the N-Q relationships for all three hysteresis criteria in 5 of the 14 events. Due to the aggregation of measured data to the daily time step, information pertaining to the hysteresis shape was sometimes lost, particularly if the storm event was <4 days in duration. Structural limitations of the SWAT as well as specific relevant basin parameters (parameters that have one value for the entire catchment) may restrict simulating N-Q dynamics. An enhanced calibrated and validated model would possibly improve the results, since the events during the better calibrated period more often reproduced the measured hysteresis indices.

Effects of Urban Stormwater Control Measures on Denitrification in Receiving Streams

Urban areas are increasingly adopting the use of ecologically-based technologies for stormwater management to mitigate the effects of impervious surface runoff on receiving water bodies. While stormwater control measures (SCMs) reduce runoff, their ability to influence ecosystem function in receiving streams is not well known. To understand the effect of SCMs on net ecosystem function in stream networks, we measured sediment denitrification in four streams across a gradient of urban and suburban residential development in Charlotte, NC. We evaluated the influence of SCM inputs on actual (DNF) and potential (DEA) denitrification activity in stream sediments at the SCM-stream confluence to quantify microbial processes and the environmental factors that control them. DNF was variable across sites, ranging from 0–6.60 mg-N·m−2·h−1 and highly correlated with in-stream nitrate (NO3-N) concentrations. Sites with a greater impervious area showed a pattern of significantly higher DEA rates upstream of the SCM compared to downstream, while sites with less imperviousness showed the opposite trend. We hypothesize that this is because of elevated concentrations of carbon and nitrogen provided by pond and wetland outflows, and stabilization of the benthic habitat by lower peak discharge. These results suggest that SCMs integrated into the watershed have the potential to create cascading positive effects on in-stream nutrient processing and thereby improve water quality; however, at higher levels of imperviousness, the capacity for SCMs to match the scale of the impacts of urbanization likely diminishes.

Knowledge discovery from high-frequency stream nitrate concentrations: hydrology and biology contributions.

High-frequency, in-situ monitoring provides large environmental datasets. These datasets will likely bring new insights in landscape functioning and process scale understanding. However, tailoring data analysis methods is necessary. Here, we detach our analysis from the usual temporal analysis performed in hydrology to determine if it is possible to infer general rules regarding hydrochemistry from available large datasets. We combined a 2-year in-stream nitrate concentration time series (time resolution of 15 min) with concurrent hydrological, meteorological and soil moisture data. We removed the low-frequency variations through low-pass filtering, which suppressed seasonality. We then analyzed the high-frequency variability component using Pareto Density Estimation, which to our knowledge has not been applied to hydrology. The resulting distribution of nitrate concentrations revealed three normally distributed modes: low, medium and high. Studying the environmental conditions for each mode revealed the main control of nitrate concentration: the saturation state of the riparian zone. We found low nitrate concentrations under conditions of hydrological connectivity and dominant denitrifying biological processes, and we found high nitrate concentrations under hydrological recession conditions and dominant nitrifying biological processes. These results generalize our understanding of hydro-biogeochemical nitrate flux controls and bring useful information to the development of nitrogen process-based models at the landscape scale.

Integrated cost-effectiveness analysis of agri-environmental measures for water quality

This paper presents an application of integrated methodological approach for identifying cost-effective combinations of agri-environmental measures to achieve water quality targets. The methodological approach involves linking hydro-chemical modelling with economic costs of mitigation measures. The utility of the approach was explored for the River Dee catchment in North East Scotland, examining the cost-effectiveness of mitigation measures for nitrogen (N) and phosphorus (P) pollutants. In-stream nitrate concentration was modelled using the STREAM-N and phosphorus using INCA-P model. Both models were first run for baseline conditions and then their effectiveness for changes in land management was simulated. Costs were based on farm income foregone, capital and operational expenditures. The costs and effects data were integrated using ‘Risk Solver Platform’ optimization in excel to produce the most cost-effective combination of measures by which target nutrient reductions could be attained at a minimum economic cost. The analysis identified different combination of measures as most cost-effective for the two pollutants. An important aspect of this paper is integration of model-based effectiveness estimates with economic cost of measures for cost-effectiveness analysis of land and water management options. The methodological approach developed is not limited to the two pollutants and the selected agri-environmental measures considered in the paper; the approach can be adapted to the cost-effectiveness analysis of any catchment-scale environmental management options.

Monsoonal-type climate or land-use management: Understanding their role in the mobilization of nitrate and DOC in a mountainous catchment

Summary The linkage between hydrologic dynamics and the delivery of nitrate and DOC (dissolved organic carbon) to streams was studied in the Haean catchment, a mixed land-use mountainous catchment in South Korea. Three monsoonal precipitation events were analyzed, which varied in total rainfall amount (39–70 mm) and intensities (mean: 1.6–5.6 mm h −1 ), by high-resolution (2–4 h interval) stream water-quality sampling along the topographic elevation gradient of the catchment, from an upland deciduous forest stream, over areas intensively used for agriculture (dryland farming and rice paddies) down to the catchment outlet. The dynamics of river-aquifer exchange were investigated at two piezometer transects at mid and lower elevations. DOC and nitrate sources and their transport pathways to the receiving surface waters differed between the forested and the agricultural stream site. In the forest stream, elevated DOC concentrations (max: 3.5 mgC l −1 ) during precipitation events were due to hydrologic flushing of soluble organic matter in upper soil horizons, with a strong dependency on pre-storm wetness conditions. Nitrate contributions to the forested stream occurred along shallow subsurface transport pathways. At the agricultural sites stream DOC concentrations were considerably higher (max: 23.5 mgC l −1 ) supplied from adjacent rice paddies. The highest in-stream nitrate concentrations (max: 4.1 mgN l −1 ) occurred at river reaches located in the lower agricultural part of the catchment, affected by groundwater inputs. Groundwater nitrate concentrations were high (max: 7.4 mgN l −1 ) owing to chemical fertilizer leaching from dryland fields forced by monsoonal rainfalls. Overall, this study demonstrates that the hydrologic dynamics resulting from the monsoonal climate drive the in-stream DOC dynamics in the forested 1st-order catchment whereas sources and mobilization of DOC in downstream agricultural areas are mainly controlled by the prevailing land-use type and irrigation management. Nitrate dynamics in higher order agricultural streams and their connected aquifers reflect combined effects of land-use type and monsoonal hydrology.

Reducing uncertainty in the calibration and validation of the INCA-N model by using soft data

Process-based nutrient models are increasingly used to determine the impact of future changes in land use, agriculture production practices and climate on the quantity and timing of nutrients reaching surface waters. Calibration of catchment-scale models to observed conditions can be difficult due to parameter uncertainty and the heterogeneity of catchment processes. Soft data, i.e. knowledge of processes gained through experimentation, have been suggested as one method of reducing uncertainty and producing a more accurate model of the processes that occur in a catchment. In this work, the Integrated Catchment model for Nitrogen was calibrated and validated for the Yläneenjoki catchment in south-western Finland by incorporating soft data. The calibration for 2003–2008 produced an adequate model of the in-stream nitrate concentrations (R2 = 0.45, NS = 0.42). However, model validation using data from 1997–2002 showed that the simulated in-stream nitrate concentrations were above the observed concentrations throughout the entire period (R2 = 0.34, NS &amp;lt; 0). These results show that soft data can be used to constrain model parameters, resulting in a more accurate model of the catchment, but do not guarantee the best validation results as the simulated processes may not occur at the same time and rate as they did in the catchment.

Modelling nitrogen in the Yeşilirmak River catchment in Northern Turkey: Impacts of future climate and environmental change and implications for nutrient management

Recent research in catchments of rapidly developing countries such as Brazil and China suggests that many catchments of the developing world are already showing signs of nitrogen pollution reminiscent of past experiences in developed countries. This paper looks at both the individual and combined effects of future climate change and other likely environmental changes on in-stream nitrate concentrations in a catchment in Northern Turkey. A model chain comprised of simulated future temperature and precipitation from a Regional Circulation Model (RCM), a conceptual hydrological model (HBV) and a widely tested integrated catchment nitrogen model (INCA-N) is used to model future changes in nitrate concentrations. Two future periods (2021–2050 and 2069–2098) are compared to the 1961–1990 baseline period in order to assess the effectiveness of several possible interventions available to catchment authorities. The simulations show that in the urbanised part of the catchment, the effects of climate change and other environmental changes act in the same direction, leading to peak nitrate concentrations of 7.5 mg N/l for the 2069–2098 period, which corresponds to a doubling of the baseline values. Testing different available policy options reveals that the installation of wastewater treatment works (WWTWs) in all major settlements of the catchment could ensure nitrate levels are kept at near their baseline values for the 2021–2050 period. Nevertheless, a combination of measures including WWTWs, meadow creation, international agreements to reduce atmospheric N concentrations and controls on agricultural practises will be required for 2069–2098. The approach presented in this article could be employed in order to anticipate future pollution problems and to test appropriate solutions, some of which will necessitate international co-operation, in other catchments around the world.

Seasonal variability of diurnal in-stream nitrate concentration oscillations under hydrologically stable conditions

Seasonal and diurnal variations of in-stream NO3-N concentration oscillations were studied through high-frequency measurements of streamwater’s physical, chemical parameters (in-stream NO3-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 km2 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 NO3-N concentrations versus seasonal and diurnal behavior of meteorological and other water chemistry constituents. The observed seasonal variability of in-stream NO3-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 NO3-N concentration amplitudes (0.3 mg/l-N) and daily changes in the NO3-N flux (0.4–0.5 g/s-N) were observed in spring; the NO3-N concentration oscillations in summer showed a considerably smaller effect of the in-stream uptake (maximum NO3-N daily concentration amplitude 0.1 mg/l-N; daily change in the NO3-N flux 0.02 g/s-N). Seasonal shifts in the timing of daily maximum (up to 6 h) and minimum NO3-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.

Groundwater dynamics and channel activity as major controls of in-stream nitrate concentrations in a lowland catchment system?

Summary In-stream nitrogen concentrations of a small agricultural lowland study catchment in Saxony-Anhalt, Germany, show high seasonal dynamics. Field data lead to the hypothesis that these dynamics are controlled by groundwater level fluctuations and the associated falling dry and rewetting of intermittent channels and not by seasonal variations of soil nitrogen leaching processes. To test this hypothesis a hypothetical model experiment was carried out using MODFLOW and MT3DMS. We simulated steady state and transient flow and nitrate transport in a two-dimensional model transect, driven by average and monthly lysimeter data of recharge and nitrate leaching. Falling dry and rewetting of open channels was accounted for. The transient groundwater flow model can capture the supposed mechanisms quite well. For conservative transport, the resulting concentration changes are in the order of 25 mg/l NO 3 -N, for the reactive transport simulation, the concentration ranges was still in the order of 12 mg/l NO 3 -N. It can therefore be concluded, that the dynamics of groundwater flow considerably control the dynamics of in-stream nitrate concentrations. The results of the steady state simulation based on averaged inflows are not equivalent to the averaged outflows given by the transient simulation, reflected in different loads. In such a dynamic system the steady state flow approach is not an acceptable representation of the system. Observed data and simulations showed similar relations of in-stream nitrate concentrations to groundwater level and discharge. Minor differences were explained by the simplified two-dimensional modelling concept in contrast to a spatially distributed, three-dimensional real-world catchment system. In a real-world system further processes superpose the assumed hydrological controls, such as in-stream or hyporheic zone denitrification, contribution of tile drains and observed fertilization practice.

Modeling nitrate fluxes at the catchment scale using the integrated tool CAWAQS

Nitrates fluxes in the Grand Morin basin (1200 km(2)), that is subjected to intense agricultural pressure, are considered using in-stream observations (around 250 sampling days over 5 years) and physically based simulations using the CAWAQS model (CAtchment WAter Quality Simulator). In-stream nitrate concentration averaged 6 mg N L(-1), increasing by approximately 0.2 mg N L(-1) yr(-1) around this value (period 1991-1996). Our results show that, over the period of 1991-1996, the differences between in-stream observed nitrate concentrations and simulated nitrate concentrations result from nitrate losses at the basin scale. These losses are due to denitrification by transfer through wetlands, alluvial plains, the hyporheic zone, and by benthic processes in rivers. A mean annual mass balance at the basin scale indicates that 40% of the infiltration flux (3360 kg N km(-2) yr(-1)) is removed from the system via the river network, 40% is stored in aquifers and 20% is lost by denitrification (period 1991-1996).

Integrated Nitrogen CAtchment model (INCA) applied to a tropical catchment in the Atlantic Forest, São Paulo, Brazil

Abstract. Stream-water flows and in-stream nitrate and ammonium concentrations in a small (36.7 ha) Atlantic Forest catchment were simulated using the Integrated Nitrogen in CAtchments (INCA) model version 1.9.4. The catchment, at Cunha, is in the Serra do Mar State Park, SE Brazil and is nearly pristine because the nearest major conurbations, São Paulo and Rio, are some 450 km distant. However, intensive farming may increase nitrogen (N) deposition and there are growing pressures for urbanisation. The mean-monthly discharges and NO3-N concentration dynamics were simulated adequately for the calibration and validation periods with (simulated) loss rates of 6.55 kg.ha−1 yr−1 for NO3-N and 3.85 kg.ha−1 yr−1 for NH4-N. To investigate the effects of elevated levels of N deposition in the future, various scenarios for atmospheric deposition were simulated; the highest value corresponded to that in a highly polluted area of Atlantic Forest in Sao Paulo City. It was found that doubling the atmospheric deposition generated a 25% increase in the N leaching rate, while at levels approaching the highly polluted São Paulo deposition rate, five times higher than the current rate, leaching increased by 240%, which would create highly eutrophic conditions, detrimental to downstream water quality. The results indicate that the INCA model can be useful for estimating N concentration and fluxes for different atmospheric deposition rates and hydrological conditions.

Spatial Relationships Between Water Quality and Pesticide Application Rates in Agricultural Watersheds

Pesticide applications to agricultural lands in California, USA, are reported to a central data base, while data on water and sediment quality are collected by a number of monitoring programs. Data from both sources are geo-referenced, allowing spatial analysis of relationships between pesticide application rates and the chemical and biological condition of water bodies. This study collected data from 12 watersheds, selected to represent a range of pesticide usage. Water quality parameters were measured during six surveys of stream sites receiving runoff from the selected watershed areas. This study had three objectives: to evaluate the usefulness of pesticide application data in selecting regional monitoring sites, to provide information for generating and testing hypotheses about pesticide fate and effects, and to determine whether in-stream nitrate concentration was a useful surrogate indicator for regional monitoring of toxic substances. Significant correlations were observed between pesticide application rates and in-stream pesticide concentrations (p < 0.05) and toxicity (p < 0.10). In-stream nitrate concentrations were not significantly correlated with either the amount of pesticides applied, in-stream pesticide concentrations, or in-stream toxicity (all p > 0.30). Neither total watershed area nor the area in which pesticide usage was reported correlated significantly with the amount of pesticides applied, in-stream pesticide concentrations, or in-stream toxicity (all p > 0.14). In-stream pesticide concentrations and effects were more closely related to the intensity of pesticide use than to the area under cultivation.

Towards an improved understanding of the nitrate dynamics in lowland, permeable river-systems: Applications of INCA-N

The Integrated Catchment Model of Nitrogen (INCA-N) was applied to the Lambourn and Pang river-systems to integrate current process-knowledge and available-data to test two hypotheses and thereby determine the key factors and processes controlling the movement of nitrate at the catchment-scale in lowland, permeable river-systems: (i) that the in-stream nitrate concentrations were controlled by two end-members only: groundwater and soil-water, and (ii) that the groundwater was the key store of nitrate in these river-systems. Neither hypothesis was proved true or false. Due to equifinality in the model structure and parameters at least two alternative models provided viable explanations for the observed in-stream nitrate concentrations. One model demonstrated that the seasonal-pattern in the stream-water nitrate concentrations was controlled mainly by the mixing of ground- and soil-water inputs. An alternative model demonstrated that in-stream processes were important. It is hoped further measurements of nitrate concentrations made in the catchment soil- and ground-water and in-stream may constrain the model and help determine the correct structure, though other recent studies suggest that these data may serve only to highlight the heterogeneity of the system. Thus when making model-based assessments and forecasts it is recommend that all possible models are used, and the range of forecasts compared. In this study both models suggest that cereal production contributed approximately 50% the simulated in-stream nitrate load in the two catchments, and the point-source contribution to the in-stream load was minimal.

Impacts of climate change on in-stream nitrogen in a lowland chalk stream: An appraisal of adaptation strategies

The impacts of climate change on nitrogen (N) in a lowland chalk stream are investigated using a dynamic modelling approach. The INCA-N model is used to simulate transient daily hydrology and water quality in the River Kennet using temperature and precipitation scenarios downscaled from the General Circulation Model (GCM) output for the period 1961–2100. The three GCMs (CGCM2, CSIRO and HadCM3) yield very different river flow regimes with the latter projecting significant periods of drought in the second half of the 21st century. Stream-water N concentrations increase over time as higher temperatures enhance N release from the soil, and lower river flows reduce the dilution capacity of the river. Particular problems are shown to occur following severe droughts when N mineralization is high and the subsequent breaking of the drought releases high nitrate loads into the river system. Possible strategies for reducing climate-driven N loads are explored using INCA-N. The measures include land use change or fertiliser reduction, reduction in atmospheric nitrate and ammonium deposition, and the introduction of water meadows or connected wetlands adjacent to the river. The most effective strategy is to change land use or reduce fertiliser use, followed by water meadow creation, and atmospheric pollution controls. Finally, a combined approach involving all three strategies is investigated and shown to reduce in-stream nitrate concentrations to those pre-1950s even under climate change.