River Lambourn Research Articles

The importance of seasonal macrophyte cover for the behaviour and performance of brown trout (Salmo trutta) in a groundwater‐fed river

Abstract Groundwater‐fed rivers, such as the chalk streams of southern England, exhibit high levels of stability (e.g. flow and temperature) and physical homogeneity (e.g. depth and substrate grain size). However, growth of instream macrophytes is highly variable depending on season, providing an important but ever‐changing source of cover for stream‐dwelling salmonids, such as brown trout (Salmo trutta). In this study, the behavioural ecology of brown trout inhabiting a chalk stream was assessed during periods that included summer and winter. In a reach of the River Lambourn (Berkshire, UK), a combination of physical habitat mapping, electric‐fishing, passive integrated transponder and radio telemetry was used to quantify trout: (1) density relative to physical and thermal characteristics; (2) movement patterns; and (3) performance, in terms of growth. Trout density was positively related to depth during winter (February) and spring (May), but not at the end of summer (September). Despite no statistical relation between trout density and macrophytes, periods of strong and no association between density and depth coincided with sparse and extensive macrophyte cover throughout the study reach, respectively. Despite being greater for some fish in winter compared to summer, the daily distance moved was generally low (<3.5 m/day). While growth was mostly positive, less mass was gained, and performance deviated farther from optimal levels predicted by a growth model, during periods that included winter. A number of factors probably contributed to lower growth in winter, including costs of reproduction, temperatures—which deviated farther from those optimal for growth—and/or an inability to maximise energy intake, e.g. due to time spent holding position in deeper areas as cover provided by macrophytes declined. Despite the lack of extremes in chalk stream environments, the behavioural ecology of brown trout appears to be influenced by seasonal variation in instream cover provided by macrophytes. This emphasises the importance of balancing the management (e.g. cutting and removal) of macrophytes with the ecological benefits they provide.

Projecting impacts of climate change on habitat availability in a macrophyte dominated Chalk River

AbstractClimate change will impact fluvial ecosystems through changes in the flow regime. Physical habitat is an established measure of a river's ecological status when assessing changes to flow. Yet, it requires extensive datasets, is site specific, and does not account for dynamic processes; shortcomings that the use of hydrological and hydraulic models may alleviate. Here, simulated flows along a 600 m reach of the River Lambourn, Boxford, UK, were extracted from the 1D MIKE 11 hydraulic component of an integrated MIKE SHE model of the Centre for Ecology & Hydrology River Lambourn Observatory. In‐channel seasonal macrophyte growth and management through cutting alter water levels, represented in the hydraulic model by manipulating channel bed roughness (Manning's n). Assessment of climate change used outputs from the UK Climate Projections 2009 ensemble of global climate models for the 2080s. River discharge outputs were disaggregated to provide velocity and depth profiles across 41 cross sections along the reach. These were integrated with habitat suitability criteria for brown trout (Salmo trutta) to generate a measure of available physical habitat. The influence of macrophyte growth caused the habitat‐discharge relationship to be unusable in evaluating the sensitivity of brown trout to flow changes. Instead, projected time series were used to show an overall reduction in habitat availability, more for adult than juvenile trout. Results highlighted the impact of weed cutting, and its potential role in mitigating both flood risk and the ecological impacts of climate change. The use of a hydraulic model to assess physical habitat availability has worldwide applicability.

Impacts of phosphorus concentration and light intensity on river periphyton biomass and community structure

Periphyton growth rate has been identified as the key process that leads to river eutrophication. Effort has focused on reducing phosphorus concentrations to control periphyton biomass, but other factors, such as light, are also important. Within-stream flume mesocosms were deployed in the River Lambourn, UK, to investigate how light intensity and phosphorus concentrations affect periphyton biomass and community structure. Soluble reactive phosphorus (SRP) concentrations were tripled in some flumes, and decreased in others by dosing of FeCl3. Increasing SRP concentrations from the ambient concentration of 49 µg l−1 to 155 µg l−1 had no effect on biomass, but community composition (by flow cytometry) shifted from diatom to cyanobacterial dominance. Reducing light levels (equivalent to riparian tree shading) decreased biomass by 40%, showing that the biofilms were light limited at SRP concentration ≥49 µg l−1. Periphyton were phosphorus/light co-limited when SRP concentrations were reduced to 33 µg l−1. Further reductions in SRP concentration (23 µg l−1) resulted in phosphorus limitation of periphyton biomass and increased dominance of diatoms and chlorophytes within the biofilm. Reducing light intensity through providing riparian tree shading could be an important management tool to reduce periphyton biomass and improve ecological status.

Fluvial response to Late Pleistocene and Holocene environmental change in a Thames chalkland headwater: the Lambourn of southern England

This paper describes the Late Pleistocene to Holocene stratigraphy of the River Lambourn; a minor headwater of the River Thames in the Berkshire Downs. The Quaternary valley-fill comprises around 5–8m of Late Pleistocene gravels overlain by Holocene peats and chalky clays. Quaternary deposits overlie an irregular rockhead erosion surface with deep scouring particularly evident on prominent bends in the valley. The gravels subdivide into a lower unit of chalky gravels overlain by coarse flint gravels. Ground penetrating radar suggests that gravels at depth are relatively structureless, but at the top show well-developed point-bar accretion surfaces which occur in association with peat-filled sinuous channels. These probably date from around the Pleistocene-Holocene boundary and may have formed in response to climate change and increased groundwater outflow as stream hydrology changed from the short-duration, high-magnitude flows of the Lower Dryas to the uniform, low-magnitude flows of the Holocene. Holocene peats initially infilled abandoned floodplain channels at around 10kyr BP but later encroached over much of the Lambourn floodplain. A progressive upward decrease in organic material and an increase in the proportion of chalky clays from around 4kyr BP probably occurred in response to floodplain accretion coupled with increased erosion of the chalk catchment related to agricultural clearance and a wetter climate.

Modelling groundwater/surface water interaction in a managed riparian chalk valley wetland

AbstractUnderstanding hydrological processes in wetlands may be complicated by management practices and complex groundwater/surface water interactions. This is especially true for wetlands underlain by permeable geology, such as chalk. In this study, the physically based, distributed model MIKE SHE is used to simulate hydrological processes at the Centre for Ecology and Hydrology River Lambourn Observatory, Boxford, Berkshire, UK. This comprises a 10‐ha lowland, chalk valley bottom, riparian wetland designated for its conservation value and scientific interest. Channel management and a compound geology exert important, but to date not completely understood, influences upon hydrological conditions. Model calibration and validation were based upon comparisons of observed and simulated groundwater heads and channel stages over an equally split 20‐month period. Model results are generally consistent with field observations and include short‐term responses to events as well as longer‐term seasonal trends. An intrinsic difficulty in representing compressible, anisotropic soils limited otherwise excellent performance in some areas. Hydrological processes in the wetland are dominated by the interaction between groundwater and surface water. Channel stage provides head boundaries for broad water levels across the wetland, whilst areas of groundwater upwelling control discrete head elevations. A relic surface drainage network confines flooding extents and routes seepage to the main channels. In‐channel macrophyte growth and its management have an acute effect on water levels and the proportional contribution of groundwater and surface water. The implications of model results for management of conservation species and their associated habitats are discussed. Copyright © 2015 John Wiley & Sons, Ltd.

Discrete wetland groundwater discharges revealed with a three-dimensional temperature model and botanical indicators (Boxford, UK)

Wetlands provide unique goods and services, as habitats of high biodiversity. Hydrology is the principal control on wetland functioning; hence, understanding the water source is fundamental. However, groundwater inflows may be discrete and easily missed. Research techniques are required with low cost and minimal impact in sensitive settings. In this study, the effectiveness of using a three-dimensional (3D) temperature model and botanical indicators to characterise groundwater discharge is explored at the CEH (Centre for Ecology and Hydrology) River Lambourn Observatory, Boxford, UK. This comprises a 10 ha lowland riparian wetland, designated for its scientific interest and conservation value. Temperature data were collected in winter at multiple depths down to 0.9 m over approximately 3.6 ha and transformed into a 3D model via ordinary kriging. Anomalous warm zones indicated distinct areas of groundwater upwelling which were concurrent with relic channel structures. Lateral heat propagation from the channels was minimal and restricted to within 5–10 m. Vertical temperature sections within the channels suggest varying degrees of groundwater discharge along their length. Hydrochemical analysis showed that warmer peat waters were akin to deeper aquifer waters, confirming the temperature anomalies as areas of groundwater discharge. Subsequently, a targeted vegetation survey identified Carex paniculata as an indicator of groundwater discharge. The upwelling groundwater contains high concentrations of nitrate which is considered to support the spatially restricted growth of Carex paniculata against a background of poor fen communities located in reducing higher-phosphate waters.

Connecting large-scale atmospheric circulation, river flow and groundwater levels in a chalk catchment in southern England

Summary Groundwater is an important water resource and globally it represents the largest distributed store of freshwater. In southern England, groundwater is a major source for public water supply, and many aquifers have recently experienced both extreme low and high groundwater levels. In this paper, we use observations of precipitation, river discharge and groundwater levels (1964–2010) and an atmospheric reanalysis to explore the large-scale climate patterns preceding the nine highest and lowest March river discharge and groundwater levels in the chalk catchment of the River Lambourn (Berkshire Downs, southern England). Peak monthly precipitation is shown to occur from October to January, while the highest river discharge and groundwater levels are found from February to April. For high discharge/groundwater levels, composite anomaly patterns of the mean sea level pressure show a stronger than average pressure gradient across the North Atlantic Ocean, with enhanced water vapour transport across southern England. For the lowest discharge/groundwater levels, a blocking high pressure system is found across the British Isles deflecting storms and precipitation to the north. Significantly, the intra-composite variability suggests that different sequences of atmospheric states may lead to high and low discharge/groundwater events.

Evaluation of diffusive gradients in thin-films using a Diphonix® resin for monitoring dissolved uranium in natural waters

Commercially available Diphonix® resin (TrisKem International) was evaluated as a receiving phase for use with the diffusive gradients in thin-films (DGT) passive sampler for measuring uranium. This resin has a high partition coefficient for actinides and is used in the nuclear industry. Other resins used as receiving phases with DGT for measuring uranium have been prone to saturation and significant chemical interferences. The performance of the device was evaluated in the laboratory and in field trials. In laboratory experiments uptake of uranium (all 100% efficiency) by the resin was unaffected by varying pH (4–9), ionic strength (0.01–1.00M, as NaNO3) and varying aqueous concentrations of Ca2+ (100–500mgL−1) and HCO3− (100–500mgL−1). Due to the high partition coefficient of Diphonex®, several elution techniques for uranium were evaluated. The optimal eluent mixture was 1M NaOH/1M H2O2, eluting 90% of the uranium from the resin. Uptake of uranium was linear (R2=0.99) over time (5 days) in laboratory experiments using artificial freshwater showing no saturation effects of the resin. In field deployments (River Lambourn, UK) the devices quantitatively accumulated uranium for up to 7 days. In both studies uptake of uranium matched that theoretically predicted for the DGT. Similar experiments in seawater did not follow the DGT theoretical uptake and the Diphonix® appeared to be capacity limited and also affected by matrix interferences. Isotopes of uranium (U235/U238) were measured in both environments with a precision and accuracy of 1.6–2.2% and 1.2–1.4%, respectively. This initial study shows the potential of using Diphonix®-DGT for monitoring of uranium in the aquatic environment.

Instream and riparian implications of weed cutting in a chalk river

Macrophyte growth is extensive in the iconic chalk streams that are concentrated in southern and eastern England. Widespread and frequent weed cutting is undertaken to maintain their key functions (e.g. flood water conveyance and maintenance of viable fisheries). In this study, a multidisciplinary approach was adopted to quantify coincident physico-chemical responses (instream and riparian) that result from weed cutting and to discuss their potential implications. Three weed cuts were monitored at a site on the River Lambourn (The CEH River Lambourn Observatory) and major instream and riparian impacts were observed. Measurements clearly demonstrated how weed cutting enhanced flood flow conveyance, reduced water levels (river and wetland), increased river velocities, and mobilised suspended sediment (with associated chemicals) and reduced the capacity for its retention within the river channel. Potential implications in relation to flood risk, water resources, downstream water quality, instream and riparian ecology, amenity value of the river, and wetland greenhouse gas emissions were considered. Provided the major influence of macrophytes on instream and riparian environments is fully understood then the manipulation of macrophytes represents an effective management tool that demonstrates the great potential of working with nature.

Evaluation of DGT as a long-term water quality monitoring tool in natural waters; uranium as a case study

The performance of the diffusive gradient in thin film technique (DGT) was evaluated as a tool for the long-term monitoring of water quality, using uranium as a case study. DGTs with a Metsorb™ (TiO2) sorbent were deployed consecutively at two alkaline freshwater sites, the River Enborne and the River Lambourn, UK for seven-day intervals over a five-month deployment period to obtain time weighted average concentrations. Weekly spot samples were taken to determine physical and chemical properties of the river water. Uranium was measured in these spot samples and after extraction from the DGT devices. The accuracy of the DGT device time weighted average concentrations to averaged spot water samples in both rivers was 86% (27 to 205%). The DGT diffusive boundary layer (DBL) (0.037-0.141 cm - River Enborne and 0.062-0.086 cm - River Lambourn) was affected by both water flow and biofouling of the diffusion surface. DBL thicknesses found at both sites were correlated with flow conditions with an R(2) value of 0.614. Correlations were also observed between the DBL thickness and dissolved organic carbon (R(2) = 0.637) in the River Lambourn, indicating the potential presence of a complex zone of chemical interactions at the surface of the DGT. The range of DBL thicknesses found at the River Lambourn site were also attributed to of the development of macro-flora on the active sampling surface, indicating that the DBL thickness cannot be assumed to be water flow dependant only. Up to a 57% under-estimate of uranium DGT concentration was observed compared to spot sample concentrations if the DBL was neglected. This study has shown that the use of DGT can provide valuable information in environmental monitoring schemes as part of a 'tool-box' approach when used alongside conventional spot sampling methods.

Fingerprinting groundwater pollution in catchments with contrasting contaminant sources using microorganic compounds

Evaluating the occurrence of microorganics helps to understand sources and processes which may be controlling the transport and fate of emerging contaminants (ECs). A study was carried out at the contrasting instrumented environmental observatory sites at Oxford, on the peri-urban floodplain gravel aquifer of the River Thames and Boxford, in the rural valley of the River Lambourn on the chalk aquifer, in Southern England to explore the use of ECs to fingerprint contaminant sources and flow pathways in groundwater. At Oxford compounds were typical of a local waste tip plume (not only plasticisers and solvents but also barbiturates and N,N-diethyl-m-toluamide (DEET)) and of the urban area (plasticisers and mood-enhancing drugs such as carbamazepine). At Boxford the results were different with widespread occurrence of agricultural pesticides, their metabolites and the solvent trichloroethene, as well as plasticisers, caffeine, butylated food additives, DEET, parabens and trace polyaromatic hydrocarbons (PAHs). Groups of compounds used in pharmaceuticals and personal care products of different provenance in the environment could be distinguished, i) historical household and medical waste, ii) long-term household usage persistent in groundwater and iii) current usage and contamination from surface water. Co-contaminant and degradation products can also indicate the likely source of contaminants. A cocktail of contaminants can be used as tracers to provide information on catchment pathways and groundwater/surface water interactions. A prominent feature in this study is the attenuation of many EC compounds in the hyporheic zone.

The hydrochemistry of a Chalk aquifer during recovery from drought

Chalk groundwater levels typically decline markedly in response to drought, and rebound strongly when the drought breaks. Chalk streams, largely groundwater-fed, are of ecological importance but little research has been conducted on possible water-quality effects accompanying fluctuations in groundwater level. This study monitored springs, boreholes and surface water in the Pang and Lambourn catchments in southern England during a major recovery in 2006–2008. Hydrochemistry, stable isotopes and age indicators were used to characterize the waters. Perennial springs showed little change in water quality over the monitoring period, and even seasonal springs soon became consistent in their hydrochemistry. A similar lack of change was observed in borehole waters and in the River Lambourn. Stable isotopes demonstrated the high degree of damping relative to rainfall inputs, and residence time indicators showed that Chalk groundwater is basically a mixture with an ‘old’ (pre-1950s) component of ≥50%. This being the case, any water quality changes owing to water level fluctuations would inevitably become diluted. Therefore, although future climate predictions for southern Britain include greater extremes in rainfall and temperature, and consequently water level changes of greater amplitude, the buffering effect of the Chalk aquifer should protect the quality of Chalk springs and streams.

Evaluation of DGT techniques for measuring inorganic uranium species in natural waters: Interferences, deployment time and speciation

Three adsorbents (Chelex-100, manganese dioxide [MnO2] and Metsorb), used as binding layers with the diffusive gradient in thin film (DGT) technique, were evaluated for the measurement of inorganic uranium species in synthetic and natural waters. Uranium (U) was found to be quantitatively accumulated in solution (10–100μgL−1) by all three adsorbents (uptake efficiencies of 80–99%) with elution efficiencies of 80% (Chelex-100), 84% (MnO2) and 83% (Metsorb). Consistent uptake occurred over pH (5–9), with only MnO2 affected by pH<5, and ionic strength (0.001–1molL−1 NaNO3) ranges typical of natural waters, including seawater. DGT validation experiments (5 days) gave linear mass uptake over time (R2≥0.97) for all three adsorbents in low ionic strength solution (0.01M NaNO3). Validation experiments in artificial sea water gave linear mass uptake for Metsorb (R2≥0.9954) up to 12h and MnO2 (R2≥0.9259) up to 24h. Chelex-100 demonstrated no linear mass uptake in artificial sea water after 8h. Possible interferences were investigated with SO42− (0.02–200mgL−1) having little affect on any of the three DGT binding layers. PO43− additions (5μgL−1–5mgL−1) interfered by forming anionic uranyl phosphate complexes that Chelex-100 was unable to accumulate, or by directly competing with the uranyl species for binding sites, as with MnO2 and the Metsorb. HCO3− (0.1–500mgL−1) additions formed anionic species which interfered with the performance of the Chelex-100 and the MnO2, and the Ca2+ (0.1–500mgL−1) had the affect of forming labile calcium uranyl species which aided uptake of U by all three resins. DGT field deployments in sea water (Southampton Water, UK) gave a linear mass uptake of U over time with Metsorb and MnO2 (4 days). Field deployments in fresh water (River Lambourn, UK) gave linear uptake for up to 7 and 4 days for Metsorb and MnO2 respectively. Field deployment of the Metsorb-DGT samplers with various diffusive layer thicknesses (0.015–0.175cm) allowed accurate measurements of the diffusive boundary layer (DBL) and allowed DBL corrected concentrations to be determined. This DBL-corrected U concentration was half that determined when the effect of the DBL was not considered. The ability of the DGT devices to measure U isotopic ratios with no isotopic fractionation was shown by all three resins, thereby proving the usefulness of the technique for environmental monitoring purposes.

Erratum to: Interaction between groundwater, the hyporheic zone and a Chalk stream: a case study from the River Lambourn, UK

Erratum to: Interaction between groundwater, the hyporheic zone and a Chalk stream: a case study from the River Lambourn, UK

Interaction between groundwater, the hyporheic zone and a Chalk stream: a case study from the River Lambourn, UK

Understanding the processes controlling groundwater/surface-water interaction is essential for effective resource management and for protecting sensitive ecosystems. Through intensive monitoring of Chalk groundwater, shallow gravel groundwater and surface water in the River Lambourn, UK, using a combination of hydrochemical and hydrophysical techniques, a complex pattern of interactions has been elucidated. The river is broadly in hydraulic contact with the streambed sediments and adjacent gravels and sands, but these deposits are mainly hydraulically separate from the underlying Chalk at the site. The hydraulic relationship between the river and underlying alluvium is variable, involving components of groundwater flow both parallel and transverse to the river and with both effluent and influent behaviour seen. While the gravel aquifer is significant in controlling groundwater/surface-water interaction, its importance as a route for flow down the catchment is likely to be modest compared with river discharge. The hydrological complexity revealed in a geological setting typical of lowland UK Chalk streams has implications both for investigation methods and for management such as in the setting of environmental objectives in the European Water Framework Directive.

Potential carbon fixation via methane oxidation in well-oxygenated river bed gravels

Due to a combination of local methanogenesis and high background concentrations in the groundwater, water in the River Lambourn is 51 times supersaturated with methane (162 nmol CH4 L−1). Pore-water concentrations of methane in the gravels of the riverbed were much lower throughout the year (71 nmol CH4 L−1), suggesting significant methane oxidation. To investigate the potential for methane oxidation as a novel chemosynthetic source of carbon to the food web, we made simultaneous measurements, in laboratory chambers, of primary production, respiration, and methane oxidation associated with the gravels. Biomass-specific net primary production was up to 2.7 µmol O2 mg−1 chlorophyll (Chl) h−1 and was similarly high for respiration (2.7 µmmol O2 mg−1 Chl h−1). We also found active methane (CH4) oxidation with the rate increasing in proportion to concentration. At the maximum rate of 0.18 µmol CH4 mg−1 Chl h−1 and a growth efficiency of 0.8, net carbon fixation via methane oxidation was equivalent to 6% of the carbon fixed via net photosynthetic primary production. However, production via methane oxidation could be proportionately much greater under the shade of the profuse instream or riparian vegetation, deep in the gravels, and especially during winter, when light is limiting (< 25 µmol quanta m−2 s−1).

Potential carbon fixation via methane oxidation in well‐oxygenated river bed gravels

Due to a combination of local methanogenesis and high background concentrations in the groundwater, water in the River Lambourn is 51 times supersaturated with methane (162 nmol CH4 L−1). Pore‐water concentrations of methane in the gravels of the riverbed were much lower throughout the year (71 nmol CH4 L−1), suggesting significant methane oxidation. To investigate the potential for methane oxidation as a novel chemosynthetic source of carbon to the food web, we made simultaneous measurements, in laboratory chambers, of primary production, respiration, and methane oxidation associated with the gravels. Biomass‐specific net primary production was up to 2.7 µmol O2 mg−1 chlorophyll (Chl) h−1 and was similarly high for respiration (2.7 µmmol O2 mg−1 Chl h−1). We also found active methane (CH4) oxidation with the rate increasing in proportion to concentration. At the maximum rate of 0.18 µmol CH4 mg−1 Chl h−1 and a growth efficiency of 0.8, net carbon fixation via methane oxidation was equivalent to 6% of the carbon fixed via net photosynthetic primary production. However, production via methane oxidation could be proportionately much greater under the shade of the profuse instream or riparian vegetation, deep in the gravels, and especially during winter, when light is limiting (&lt; 25 µmol quanta m−2 s−1).

Meiofauna versus macrofauna: Secondary production of invertebrates in a lowland chalk stream

The whole metazoan community inhabiting macrophyte stands and gravel beds of the English chalk stream River Lambourn were sampled for 1 yr. Secondary production estimates for specific taxa of macrofauna and meiofauna, usually at species or genus level, were made using the size‐frequency method. Annual standing biomass and production to biomass ratios were also estimated. Total annual secondary production was as high as 64.99 g m−2 yr−1 in the macrophyte stands while within the gravel beds total annual secondary production was 22.55 g m−2 yr−1. Macrofauna contributed &gt;91% to secondary production in both habitats. A corresponding standing biomass of 11.87 g m−2 in the macrophyte stands was also higher than within gravel beds of 5.13 g m−2. Macrofauna contributed &gt;97% to standing biomass in both habitats. A large number of meiofauna and some macrofauna species displayed annual production‐to‐biomass (P : B) values in both habitats significantly higher than 9 or 10. Despite higher densities and turnover rates within the benthos of the River Lambourn, there was a minimal contribution of the meiofauna to total secondary production dominated by the macrofauna.

Over-parameterised, uncertain ‘mathematical marionettes’ — How can we best use catchment water quality models? An example of an 80-year catchment-scale nutrient balance

The Integrated Catchment Model of Nitrogen (INCA-N) was applied to the River Lambourn, a Chalk river-system in southern England. The model's abilities to simulate the long-term trend and seasonal patterns in observed stream water nitrate concentrations from 1920 to 2003 were tested. This is the first time a semi-distributed, daily time-step model has been applied to simulate such a long time period and then used to calculate detailed catchment nutrient budgets which span the conversion of pasture to arable during the late 1930s and 1940s. Thus, this work goes beyond source apportionment and looks to demonstrate how such simulations can be used to assess the state of the catchment and develop an understanding of system behaviour. The mass-balance results from 1921, 1922, 1991, 2001 and 2002 are presented and those for 1991 are compared to other modelled and literature values of loads associated with nitrogen soil processes and export. The variations highlighted the problem of comparing modelled fluxes with point measurements but proved useful for identifying the most poorly understood inputs and processes thereby providing an assessment of input data and model structural uncertainty. The modelled terrestrial and instream mass-balances also highlight the importance of the hydrological conditions in pollutant transport. Between 1922 and 2002, increased inputs of nitrogen from fertiliser, livestock and deposition have altered the nitrogen balance with a shift from possible reduction in soil fertility but little environmental impact in 1922, to a situation of nitrogen accumulation in the soil, groundwater and instream biota in 2002. In 1922 and 2002 it was estimated that approximately 2 and 18 kg N ha − 1 yr − 1 respectively were exported from the land to the stream. The utility of the approach and further considerations for the best use of models are discussed.

Fine sediment delivery and transfer in lowland catchments: modelling suspended sediment concentrations in response to hydrological forcing

AbstractFine sediment delivery to and storage in stream channel reaches can disrupt aquatic habitats, impact river hydromorphology, and transfer adsorbed nutrients and pollutants from catchment slopes to the fluvial system. This paper presents a modelling tool for simulating the time‐dependent response of the fine sediment system in catchments, using an integrated approach that incorporates both land phase and in‐stream processes of sediment generation, storage and transfer. The performance of the model is demonstrated by applying it to simulate in‐stream suspended sediment concentrations in two lowland catchments in southern England, the Enborne and the Lambourn, which exhibit contrasting hydrological and sediment responses due to differences in substrate permeability. The sediment model performs well in the Enborne catchment, where direct runoff events are frequent and peak suspended sediment concentrations can exceed 600 mg l−1. The general trends in the in‐stream concentrations in the Lambourn catchment are also reproduced by the model, although the observed concentrations are low (rarely exceeding 50 mg l−1) and the background variability in the concentrations is not fully characterized by the model. Direct runoff events are rare in this highly permeable catchment, resulting in a weak coupling between the sediment delivery system and the catchment hydrology. The generic performance of the model is also assessed using a generalized sensitivity analysis based on the parameter bounds identified in the catchment applications. Results indicate that the hydrological parameters contributing to the sediment response include those controlling (1) the partitioning of runoff between surface and soil zone flows and (2) the fractional loss of direct runoff volume prior to channel delivery. The principal sediment processes controlling model behaviour in the simulations are the transport capacity of direct runoff and the in‐stream generation, storage and release of the fine sediment fraction. The in‐stream processes appear to be important in maintaining the suspended sediment concentrations during low flows in the River Enborne and throughout much of the year in the River Lambourn. Copyright © 2007 John Wiley &amp; Sons, Ltd.