Defra Project PE0120 – Project Title: Phosphorus mobilisation with sediment and colloids through drained and undrained grasslands (Final Report) Management and overall support Roland Bol, Kit Macleod, Patricia Butler and Jane Hawkins (North Wyke Research, now Rothamsted Research) and Phil Haygarth (North Wyke Research, now Lancaster University) Objective 1: Richard Brazier (Exeter) and Gary Bilotta (Exeter/North Wyke Research, now Brighton University) Objective 2: Paul Worsfold and Laura Gimbert (Plymouth University) Objective 3: Roland Bol and Steve Granger (North Wyke Research, now Rothamsted Research), Pam Naden and Gareth Old (CEH Wallingford) Objective 4: John Quinton (Lancaster University), Jim Freer (Lancaster University, now Bristol University), Tobias Krueger (Lancaster University), Kit Macleod (North Wyke Research, now Macaulay Institute) General background Phosphorus (P) and sediment/colloids both contribute to problems of water quality, in terms of eutrophication (for P) and silting of river beds and salmon spawning ground (for sediment/colloids). There is particular value in studying them together because P has a strong affinity for attaching to, and being transported with, sediment/colloids. This project focused on P and sediment/colloid losses from intensively managed grasslands. The objective was to provide mechanistic and fundamental knowledge that contributed to the Defra P programme through adopting a multidisciplinary team approach focussed around specific platform sites. The project run from February 2005 to August 2008 and was delivered by a team of soil scientists, geomorphologists, analytical chemists, hydrochemists, hydrologists and mathematical modellers working within 4 linked objectives (see below). Field experimental work was undertaken on the Rowden (box 1) and Denbrook (box 2) platform sites. The project contributed fundamental knowledge to help budget losses, defined new techniques for identification of colloids and sediment, improved mechanistic understanding of sediment, colloid and P transport and contributed to new models for understanding P transport through grassland soils. Scientific objectives Objective 1: To determine the dynamics and mass balances of sheet erosion (with P and C attached), from drained and undrained grasslands. Objective 2: To develop new technologies to separate and fractionate colloidal material in runoff and drainage from grassland and conduct fractogram and P species profiles during event hydrographs. Objective 3: To undertake inductive manipulative tracing studies on small plots and catchments, that contribute new understandings of water, P, sediment/colloids and C transfer linkages/relationships. Objective 4: To develop a mechanistic model of colloidal P transfers and delivery through drained soils that can be used as a research tool. Assess potential for mitigation. The project team published a series of five commentary papers in Hydrological Processes in 2006 and 2007 (Haygarth et al., 2006; Bilotta et al. 2007; Gimbert et al. 2007; Granger et al. 2007; Krueger et al. 2007). These papers again highlighted the scientific relevance of the objectives of this project, i.e. the need to rethink our understanding of sediment and colloid transfers from intensively managed grasslands and readdress critical deficiencies in existing knowledge. The papers focussed on four key areas (see Haygarth et al., 2006. Processes affecting transfer of sediment and colloids, with associated phosphorus, from intensively farmed grasslands: an overview of key issues) as per our set objectives: Erosion; uniqueness and extent of grasslands and their inadequate inventories (Bilotta et al. 2007b) Colloid and sediment characterization; definitions, analytical failings, new challenges and opportunities

(Gimbert et al. 2007b) Tracing issues; organic matter; non-quantified colloid sources (Granger et al. 2007) Modelling; toward greater integration of modelling with field science (Krueger et al. 2007). In addition, other critical issues needed to be addressed to build a holistic understanding of intensively managed grasslands were highlighted: Critical requirement for a wider acceptance of a „continuum‟ of sediment and particles and processes

from molecular to >1 μm, including clarity with operational definitions and an acceptance of the strengths and weaknesses of analytical tools.

Employ new tracer techniques are available for helping assess the role of organic matter in contributing to transfers and how these must be applied.

Field scientists must fully appreciate the variability of observations, and modellers need to understand the uncertainty in model processes and model outputs.

Integrated inter-disciplinary team working not discipline polarization

Overall outcome of the project – What we have learned Grassland areas are generally characterised by high levels of rainfall, steeper slopes and the presence of livestock compared to arable farming systems in the UK. This project has brought new understanding of the baseline losses of SS and TP from intensive grassland systems, advanced our ability to trace the sources, mobilisation and transport of important agricultural pollutants and has made important progress in understanding how farmers may mitigate these losses. We found that intensively managed lowland grassland areas produce significant amounts of SS and TP due to their physical mobilisation associated with high levels of rainfall. The presence of artificial field drains was noted to reduce these baseline losses by up to 50%. The presence of livestock increased mobilisation when the soil was near saturation. In addition, when livestock numbers were above 2 LSU/ha and on steeper slopes (> 15%) increased SS and TP mobilisation. An important component of dairy and beef farms is the production and disposal of slurries and manures. At the field scale drains were shown to be an important conduit of slurry derived material even when a 5 m buffer strip was used. At the farm scale we found that slurry can not only be mobilised from a field, but also transported several hundred meters to nearby surface water bodies during late spring/early summer months. The period when grassland areas may receive large amounts of nutrients to ensure good herbage yield. Another key component of a farmed landscape is the presence of hard standings. In this project we have provided evidence of the importance of these areas for a wide range of potential pollutants and increasing potential for delivery from these areas during high energy summer rainfall events.

Box 1: Rowden site The Rowden Experimental Research Platform (RERP) is situated in Devon, Southwest England (National Grid Reference: SX 650995). It was

established in 1982 on old unimproved pasture on poorly drained sloping land (5–10%) and is typical of much of the permanent grassland in

the south-west of England. The experimental site consists of 14 plot-scale lysimeters which are approximately 1 hectare in size and are

managed as intensive grassland with annual applications of NPK fertiliser in line with the Code of Good Agricultural Practice (Defra, 2009).

They are grazed by beef cattle between June and October, with an average stocking density of four livestock units per hectare. The soil is a

clayey non-calcareous pelostagnogley of the Hallsworth series (Typic Haplaquept (USDA); stagnogley (UK); HOST Class 24), overlying clay

shales of the Crackington Formation (Culm measures). The annual rainfall averages 1055mm, where the majority falls between October and

March. As a consequence of the virtually impermeable clay layer below 30cm the soil remains waterlogged for much of the winter period.

Half of the lysimeters at the RERP have been agriculturally drained while the other half has no drainage installed. Lysimeters in which no

drainage has been installed are dependent upon natural drainage via surface-flow and lateral through-flow pathways, and are termed

‘undrained’ (see Fig. 2). Lysimeters which have drainage installed are termed ‘drained’ and have mole drains at a depth of 55cm which are

intercepted by gravel filled trenches with 85cm deep permanent pipe drains (Fig. 2). All the lysimeters have perimeter gravel filled ditches to

a depth of 30cm which collect any surface runoff plus any lateral through-flow (combined referred to as inter-flow). This is then channelled

through V-notch weirs. The drained lysimeters have additional and separate V-notch weirs for measuring water that flows through the

drainage system (termed drain-flow). The stage height (h) of both inter-flow and drain-flow were measured using solar powered Starlevel

flow sensors on a 1 to 5 minute time-step with data recorded by Campbell radio loggers and subsequently transmitted via radio modem to a

central computer. To convert h to discharge (Q) stage-discharge relationships were produced for each weir, including estimates of the errors

involved in the calibration. These were used to produce hydrographs with associated uncertainty bands (Qmax and Qmin). Rainfall at the site

was measured using a tipping-bucket rain gauge (Rainwise, Bar Harbor, ME). The RERP has a very ‘flashy’ hydrological response.

Box 2: Denbrook site The Den Brook catchment (UK grid ref. SX 67712 99685) located in Devon, Southwest England is a first-order headwater catchment 48 ha in size, characterised by a slowly permeable seasonally waterlogged clay soil of the Hallsworth series. The catchment receives high levels of rainfall with an annual 40 year average of 1050 mm, the majority falls in the winter/spring. The catchment has a limited amount of field drainage installed, predominantly draining areas close to the stream. The heavy soil type and wet winter weather conditions cause the hydrological response to rainfall to be flashy, with a large proportion of the response being saturation-excess overland flow. The catchment is predominantly managed as grassland sustaining cattle and sheep and the sward is dominated by perennial ryegrass receiving periodic applications of manure, inorganic fertilizer (N, P and K) and excretal returns during the spring/summer. Within the catchment, there is also a hard-standing area with associated animal housing that is served by a slurry lagoon receiving animal waste and contaminated run-off. This area is connected to the Den Brook by a large drainage pipe which intermittently discharges farmyard run-off into the stream. (Fig xx) The extent of this connection is poorly understood; however, in 2003 the slurry lagoon was rebuilt in an attempt to reduce pollution entering the stream. Waste contained within the lagoon is spread within the catchment area when ground conditions allow, typically during the spring/summer. To the south of the catchment a road drain delivers run-off from the road via a concrete conduit. Discharge from the whole catchment was measured using a trapezoidal flume which has been installed since 2001. Stage height, from which discharge is calculated, is measured on 5 minute time-steps by a pressure transducer in a stilling well, 1 m upstream of the flume and is recorded by a data logger (Campbell Scientific CR10X). Rainfall is recorded at the catchment outlet by a tipping-bucket rain gauge (Rain-wise Bar Harbor, ME), which records the total number of tips per minute (each tip = 0.254 mm rainfall). Within the catchment, a single field has been defined as a sub-catchment and is also monitored for flow. This field at the west of the catchment represents 12% of the catchment area and slopes at about 5° from its SW to NE corner. The field is bounded on its upslope sides to the south and west by earth banks forming hedges (~1 m high) while on its down slope sides only livestock fencing exists. However, on these sides small earth banks (~15 cm high) have developed, possibly as a result of the field’s recent history; it had been ploughed and used for maize production until 2006, when it was returned to grassland. This field, like the majority of the land within the catchment away from the stream channel, has no field drainage installed, and subsurface flow is considered to be negligible due to the low hydraulic conductivity of the subsoil (<10 mm day

-1). Due to the nature of the

slope and the presence of surrounding banks, all surface run-off is contained and directed towards the NE corner of the field. In this corner a ‘V’ notch weir was installed in 2006/07. All surface flow occurring within this field is channelled down to the weir and discharge is calculated from the stage height measured by a stilling-well and ISCO 6712 auto-sampler, integrated bubbler module (Teledyne ISCO Inc, Nebraska, USA). All hydrographs were produced with associated uncertainty bands (Qmax and Qmin).

In the sections below we will report and discuss the main results for each objective of the project. Nearly all of the results reported on in this final report are already published (see references list). Hence, for each individual objective is indicated in the text which scientific papers are referred to. This will allow this final report to focus solely on the research outcomes and provide the reader the opportunity to engage in detail through these published research papers from PEO120 with all aspects of the specific work (background, materials and methods, results and discussion) referred to in the specific sections of this final report. Objective 1: To determine the dynamics and mass balances of sheet erosion (with P and C attached), from drained and undrained grasslands Published papers from objective 1 Bilotta, G.S, Krueger, T., Brazier, R.E., Butler, P., Freer, J. Hawkins, J. M. B., Haygarth P. M., Macleod C. J. A. and

Quinton, J.N. (2010). Assessing catchment-scale erosion and yields of suspended solids from improved temperate grassland. Journal of Environmental Monitoring 12, 731-739.

Bilotta, G.S., Brazier, R.E., Butler, P., Freer, J., Granger, S., Haygarth, P.M., Krueger, T., Macleod, C.J.A, Quinton, J., (2008) Rethinking the contribution of drained and undrained grasslands to sediment related water quality problems. Journal of Environmental Quality 37, 906-914.

Bilotta, G.S., Brazier, R.E (2008). Understanding the influence of suspended solids on water quality and aquatic biota. Water Research 42, 2849-2861.

Bilotta, G.S., Brazier, R.E. and Haygarth, P.M. (2007a). The impacts of grazing animals on the quality of soils, vegetation, and surface waters in intensively managed grasslands. Advances in Agronomy, 94, 237-280.

Bilotta, G.S., Brazier, R.E. and Haygarth, P.M. (2007b) Processes affecting transfer of sediment and colloids, with associated phosphorus, from intensively farmed grasslands: Erosion. Hydrological Processes, 21, 135-139.

Brazier, R.E., Bilotta, G.S., Haygarth, P.M. (2007) A perspective on the role of lowland, agricultural grasslands in contributing to erosion and water quality problems in the UK. Earth Surface Processes and Landforms. 32, 964-967.

To be referred to for detailed information on background, materials and methods, results and discussion, which are not included in the final report. Milestones for objective 1

All milestones for objective1 were met in a timely fashion.

Milestone 1a. Quantify the rates of water erosion under lowland pasture at specific field sites and assess how these are influenced by presence/absence of field drainage. Conduct review on poaching. The influence of subsurface drainage on the hydrological response and the dynamics of SS and P transfer from improved grassland was carried out and published in the Journal of Environmental Quality (Bilotta et al., 2008). High-resolution monitoring was carried out on SS, total phosphorus (TP) and molybdate-reactive phosphorus (MRP) exports in overland flow, throughflow and subsurface drainflow from drained and undrained 1-ha grassland plots ( Figure 1) during hydrological season (2005-2006).

The study provided the first field-scale evidence of yields of SS and rates of erosion from improved grassland in the UK and demonstrates the important effects of subsurface drainage on hydrology and the

Figure 1. Figure illustrating the location of the Rowden

Experimental Research Platform (North Wyke, Devon), with a

blow-out aerial photograph of the site and further blow-outs

illustrating the design of the 1-ha hydrologically isolated

grassland plots. Adapted from Bilotta et al., (2008).

yields of SS and TP. It showed that, during individual rainfall events, 1-ha grassland plots yield up to ~17 kg of SS, with concentrations in runoff waters of up to ~400 mg L

-1. These concentrations exceed the water

quality standards recommended by the European Freshwater Fisheries Directive (25 mg L-1

) and are beyond those reported to have caused chronic effects on freshwater aquatic organisms. Furthermore, concentrations of TP in runoff waters from these field plots exceeded 800 µg L

1, thus being in excess of TP reported to

cause eutrophication problems in rivers. The data suggested that the presence of subsurface drainage reduces the yields of SS and TP from grassland by as much as 50%. This is a result of drainage acting to lower the zone of saturation within the soil so that when a rainfall event does occur, there is more opportunity for soil moisture storage on drained land compared to undrained land and therefore saturation-excess overland flow is generated less readily. Consequently, there is a lower peak discharge and lower total discharge from drained grassland during rainfall events, reducing the energy available for particle detachment and transport. In addition, the hydrological pathways are modified by drainage, with more water by-passing the pasture surface, where there are readily-available sources of SS and TP. The dataset suggests that we need to rethink the conceptual understanding of grasslands as non-erosive landscapes. In addition, commentaries relating to the state of knowledge in the field of erosion research in grassland environments were written. These commentaries were published in Hydrological Processes and Earth Surface Processes and Landforms (Bilotta et al., 2007b; Brazier.et al., 2007), increasing awareness of the potential for improved grasslands to act as sources of suspended solids (SS), colloids and sorbed contaminants such as phosphorus (P).

Figure 2. Photographs illustrating three forms of physical soil degradation; compaction (left), pugging (middle), and poaching (right), induced by grazing livestock in improved grassland. Photographs from Bilotta et al., (2007a).

The review, published in Advances in Agronomy (Bilotta et al., 2007a), described the processes and factors responsible for the occurrence of soil compaction, pugging (plastic deformation), and poaching (elastic deformation) caused by grazing livestock and associated impacts on the quality of soils, vegetation and surface waters in intensively managed (improved) grasslands (see Figure 2).

1b. Characterise the sediment and associated P eroded under grassland and determine what the likely implications for sediment pollution and nutrient enrichment of adjacent water bodies are.

Bilotta and Brazier [2008] (Water Research) examined the evidence-base in the literature describing the effects of SS on water quality and freshwater biota and questions the appropriateness of current EU water quality guidelines for SS, which currently bear little relation to the scientific evidence-base. This paper highlights the fact that the concentration of SS in the water column is not the only factor that determines the influence on aquatic organisms- the timing and duration of exposure to SS, and the physical and geochemical characteristics of the SS are also important factors determining their influence. This paper explains the need for more advanced, ecosystem-based, water quality guidelines for SS (under the Water Framework Directive), which should be more reflective of the inherent differences in background SS conditions within environments of different typology (e.g. geology, scale, altitude) and be more considerate of the additional factors, such as timing/duration of exposure, that influence the overall effect of SS in aquatic habitats. Development of these guidelines will enable water-quality managers to make informed decisions as to whether the contribution of SS from a certain land-use poses a threat to the aquatic ecosystem associated with that environmental typology.

1c. Determine which management options for drained and undrained land promote accelerated erosion rates and determine what the implications of these rates in terms of both on- and off-site impacts of erosion and nutrient fluxes might be. Field-based, plot-scale experiments were undertaken to investigate the influence of stocking density (perceived to be an important management factor) on the properties of soil, vegetation and the water quality of surface runoff from improved grasslands. This contributed to milestone 1C. Key findings include: 1. Steeper slopes are more susceptible to damage by cattle and yield higher concentrations of SS and P. 2. High stocking densities (>2 LSU ha

-1) are associated with higher yields of SS and P at rates which could

threaten compliance with water quality guidelines. 3. Antecedent moisture conditions play a vital role in controlling the rates of sediment and phosphorus

mobilisation due to cattle activity – when soils are saturated, the rates of mobilisation may be up to one order of magnitude higher than when soils are dry.

Figure 3. Photograph of turbid, saturation-excess overland flow in the Den Brook headwater catchment during one of

the monitored rainfall events of the 2006-2007 hydrological season. Photograph by G.S. Bilotta.

Objective 2: Develop new technologies to separate and fractionate colloidal material in runoff and drainage from grassland and conduct fractogram and P species profiles during event hydrographs Published papers from objective 2 Gimbert, L.J., Worsfold, P.J. (2007). Environmental applications of liquid waveguide capillary cells coupled with

spectroscopic detection, Trends in Analytical Chemistry (TrAC) 26, 914-9930. Gimbert, L.J., Haygarth, P.M., Worsfold, P.J. (2007b) Determination of nanomolar concentrations of phosphate in

natural waters using flow injection with a long path length liquid waveguide capillary cell and solid state spectrophotometric detection, Talanta 71, 1624-1628.

Gimbert, L.J., Haygarth, P.M., Worsfold, P.J. (2007a). Processes affecting transfer of sediment and colloids, with associated phosphorus, from intensively farmed grasslands: colloid and sediment characterisation techniques, Hydrological Processes 21, 275-279.

Gimbert, L.J., Haygarth, P.M., Beckett, R. Worsfold, P.J. (2006). The influence of sample preparation on observed particle size distributions for contrasting soil suspensions using flow field-flow fractionation, Environmental Chemistry 3, 184-191.

To be referred to for specific information not included in the final report.

Milestones for objective 2

All milestones for objective 2 were met in a timely fashion.

2a. Obtain fractograms of particle size distributions in the colloidal size range (focusing on the 0.1 - 1 um fraction) for soil suspensions and runoff waters from different soil types. Obtain high temporal resolution data for size distributions during high energy event hydrographs (e.g. storms).

Materials and methods Flow Field-Flow Fractionation (FlFFF) was used to investigate the particle size distributions (PSDs) in the colloidal size range (0.1-1.0 µm fraction) for soil suspensions. Four lysimeter plots at Rowden were chosen (plots 4, 9, 10 and 14) and soil was sampled from each plot. The sampling strategy involved placing a measuring tape diagonally across each plot and sampling soil at three different depths (0-2, 10-12, and 30-32 cm) at distances of 20, 40, 60, 80 and 100 m along the diagonal line. Each core was then sectioned to give separate samples of the soil at 0-2 and 30-32 cm depth, oven dried at 30 ºC for 5 days and sieved through a 2 mm mesh and then a 63 µm mesh. Soil suspensions of 1% m/v concentration were prepared and shaken for 16 h before settling for 1 h and extracting the <1 µm fraction. Other work included collection of storm runoff samples in February 2006 from a Rowden plot lysimeter for which PSDs were determined by FlFFF.

Figure 4. FlFFF colloidal size distributions for drained (left column) and undrained (right column) lysimeters at different depths (0-2, 10-12 and 30-32 cm) along a 100 m diagonal transect. Results

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Results from only plots 9 and 10 are shown in Fig. 4 for clarity; colloidal material increased with depth in both the undrained and undrained lysimeters. There was also evidence of colloidal material increasing in the 0.4-0.8 µm size range at 30-32 cm depth, resulting in bimodal distributions for drained lysimeter samples. Heterogeneity of the colloidal size distributions also increased significantly with depth over short distances (20 m) across the transect, particularly for the drained lysimeter, as the RSD increased from 9.3 % at 0-2 cm depth to 80.1 % at 30-32 cm depth. No changes in the colloidal profile between samples were observed during the storm event in February 2006 from a Rowden plot (data not shown), which indicated that this event generated limited colloidal transport. Therefore subsequent work, as described in milestone 2e, focused on runoff collected at Den Brook, a system with a higher capacity for sediment/colloid transport. Conclusion Flow field flow fractionation (FlFFF) can be used to determine the particle size distributions (PSDs) in the colloidal size range (0.1-1.0 µm fraction) for soils sampled from managed grasslands. 2b. Quantify colloid sediment transport from drainage and runoff waters by integrating these distributions with flow data. Couple FFF with selective phosphorus detection to quantify phosphorus species associated with colloidal material. Material and methods A liquid waveguide capillary cell (LWCC) of 1 m path length (an emerging technology for spectrophotometric detection) was integrated into a flow injection manifold to improve the analytical figures of merit for phosphorus detection compared with conventional methods. The flow injection manifold was optimised and the new waveguide technology enhanced the sensitivity by a 100-fold compared with a conventional 1 cm flow cell, lowered the limit of detection to 10 nM with a linear range of 10 - 1000 nM and minimised the amount of waste generated (see Gimbert et al., 2007; Gimbert and Worsfold, 2007). Results A typical calibration trace is shown in Fig. 5. Significant challenges were faced when attempting to interface the liquid waveguide directly with the FlFFF, particularly the effect of sample dilution within the separation unit and when merged with the post-column reagent stream. Therefore the recommended strategy for determining the P associated with colloidal material was an off-line approach. The results from this are described under milestones 2c and 2d.

Figure 5. FIA-LWCC calibration using phosphate standards Conclusion A liquid waveguide capillary cell (1µm path length) can be integrated into a flow injection manifold with spectrophotometric detection of molybdenum blue to give a limit of P detection in water samples of 10 nM. 2c. Integrate these studies with digestion techniques to distinguish between inorganic and organic phosphorus species. Conducted ICP analyses of all soils. Material and methods Inorganic and organic P species were determined using an autoclave digestion method followed by flow injection with spectrophotometric detection. The two P fractions were then correlated with different colloidal sized material determined using FlFFF. This was achieved by measuring the PSD of a Rowden soil (0–7 cm top horizon) suspension and integrating the peak areas in the ranges 0-0.2, 0.2-0.45 and 0.45-1 µm

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from the FlFFF profile. Off-line fractionation of the same sample by centrifugation into the same 0-0.2, 0.2-0.45 and 0.45-1 µm size ranges was followed by the determination of inorganic and organic P using a flow injection manifold. This enabled a direct comparison to be made between P species and particle size and the results are shown in Fig. 6. Metal concentrations in Rowden soil samples were characterised using ICP semi-quantitatively after an EDTA extraction and an aqua regia digestion to determine the „bio-available‟ and total concentrations respectively. Results The results show a high percentage of organic P compared with inorganic P in all colloidal size ranges and a higher concentration of both species in the larger colloidal size range (0.45-1 µm). ICP analysis of 41 elements were measured, of which the rare earth elements (REEs) distributions showed interesting features.

Figure 6. Comparison of inorganic and organic P concentrations with the FlFFF peak areas for the size

fractions 0-0.2, 0.2-0.45 and 0.45-1 µm.

Conclusions 1. P was in all colloidal size ranges in Rowden soil predominantly in an organic rather than inorganic form. 2. The „bio-available‟ and total metal concentrations in Rowden soil samples suggested that the REEs could

be used as tracers of soil movement. Furthermore, the feasibility of using REEs incorporated into slurry to study erosion and leaching in grassland did warrant further investigation.

2d. Obtain particle size distributions for soils from Rowden and Denbrook in the size range >1 µm using laser sizing to quantify the relative contributions of the sediment and colloidal phases to the total particulate and phosphorus loads Materials and methods Particle size distributions for soils in the size range >1 µm using laser sizing were determined. Soil was sampled from 4 lysimeter plots at Rowden, 2 drained and 2 undrained. Each core was then sectioned to give separate samples of the soil at 0-2, 10-12 and 30-32 cm depth, oven dried at 30 ºC for 5 days and sieved through a square hole 2 mm stainless steel mesh. The laser data for all the soil samples was separated into the USDA soil classes consisting of fine clay (<0.2 µm), coarse clay (0.2-2 µm), fine and medium silt (2-20 µm), coarse silt (20-50 µm), fine sand (50-250 µm), medium sand (250-500 µm), coarse sand (500-1000 µm) and very coarse sand (1000-2000 µm) fractions. Two soil cores (1 and 2) were sampled from the fodder field in Den Brook. Each core was sectioned to give separate samples of the soil at 0-2 and 30-32 cm depth, oven dried at 30 ºC for 5 days and sieved through a 2 mm mesh and then a 63 µm mesh. Soil suspensions of 1% m/v concentration were prepared and shaken for 16 h before settling for 1 h and extracting the <1 µm fraction. The samples were then injected into the FlFFF to determine the PSDs. Total phosphorus was also determined for soil core 1 at the two different depths of 0-2 and 30-32 cm using an autoclave digestion method followed by segmented flow analysis. This was achieved by using the settled <1 µm fraction obtained in the FlFFF experiment described above and centrifuging to obtain the <0.2 and <0.45 µm fractions. The TP in the two soil samples were then correlated with the FlFFF peak area Results Data plotted as the percentage of particles in each fraction, only plots 9 and 10 are shown in Fig. 7 for clarity. Little variability in the particle size distribution between the soil sampled at 0-2 and 10-12 cm depth for both the undrained and drained lysimeters was observed and the highest percentage of particles (46.2-

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51.5 %) with RSDs ranging from 1.8-3.8 % was in the fine and medium silt fraction (2-20 µm). However, a greater variability (RSD of 12.1 %) was found for drained lysimeter samples at 30-32 cm depth compared to undrained samples (RSD of 5.5 %). Colloidal profiles for the four samples were similar, as more material was present in the 0.45-1.0 µm fraction and a higher proportion of finer colloidal material (0.08-0.2 µm) in the 0-2 cm compared to 30-32 cm depth samples (data not shown). This correlated with a higher concentration of TP in the larger colloidal size range (0.45-1 µm) and a high percentage of TP in the surface (0-2 cm) samples compared with the sub-surface (30-32 cm) in all colloidal size ranges (Figure 8)

Figure 7. Laser sizing particle size distributions for drained (left column) and undrained (right column) lysimeters at different depths (0-2, 10-12 and 30-32 cm) along a 100m diagonal transect.

Figure 8. Comparison of total phosphorus concentrations with the FlFFF peak areas for the size fractions 0-0.2, 0.2-0.45 and 0.45-1 µm at two different depths (0-2 and 30-32 cm) in the Den Brook fodder field. Conclusion FlFFF could be used to determine the PSDs of the <1 µm fraction of soil sampled from the fodder field in Den Brook, with conventional particle sizing using laser scattering providing complementary data to FlFFF for the larger particles (1 - 2000 µm).

2e. Assess the impact of colloidal material from agricultural runoff on catchment water quality by characterizing the <1 µm fraction in storm runoff samples from Denbrook using Fl-FFF Material and methods

Lysimeter 9 (drained) Lysimeter 10 (undrained)

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Particle size distributions of the <1 µm fraction of the runoff sampled from the fodder field in Den Brook during storm events were determined using FlFFF and compared with total phosphorus and suspended solids. The runoff water was settled for 1 h after 10 min shaking and the top 20 mL pipetted out as this layer contained the <1 µm particles. The samples were then injected (20 µL) into the FlFFF. Results Three storm profiles were obtained but only the results from the 24

th November 2006 storm event are shown

in Fig. 9 for clarity. The colloidal profile changed significantly during the storm. The peak area of each of the particle size distributions between 0.08-0.2 µm, 0.2-0.45 µm and 0.45-1.0 µm were therefore calculated and these data plotted against the storm hydrograph data. This showed that the amount of colloidal material changed with discharge and that more material was present in the 0.2–0.45 µm size range. When the data in Fig. 9 are compared with total phosphorus and suspended solids a similar pattern is observed with the changing hydrograph i.e. there is a correlation of colloidal material, TP and SS with stage height (Table 1).

Figure 9. Discharge for storm event 24

th November 2006, with peak areas for different size fractions (0.08-

0.2 µm, 0.2-0.45 µm and 0.45-1.0 µm) calculated from the particle size distributions determined by FlFFF. Table 1. Comparison of TP, SS and peak area calculated from FlFFF particle size distributions with stage height for storm event 24

th November 2006.

Conclusions 1. FlFFF can be used to determine the PSDs of the <1 µm fraction of the runoff samples from the

grasslands during storm events. 2. Colloidal profiles change significantly during the storms with more material present in the 0.2–0.45 µm

size range, with a good correlation of colloidal material, TP and SS with stage height.

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18:00 0.052 2820 750 0.0408

19:00 0.118 2050 450 0.0527

20:00 0.152 1990 424 0.0566

21:00 0.261 6090 1929 0.0820

22:30 0.188 2180 446 0.0572

00:30 0.098 1500 323 0.0411

02:00 0.074 950 107 0.0336

03:30 0.076 1015 147 0.0297

05:30 0.064 885 98 0.0277

Objective 3: To undertake inductive manipulative tracing studies on small plots and catchments, that contribute new understandings of water, P, sediment/colloids and C transfer linkages/relationships. Published papers from objective 3 Granger, S.J., Bol, R., Dixon,

L., Naden, P., Old, G., Marsh, J.K., Bilotta, G., Brazier, R., White, S.M. and Haygarth,

P.M. (2010a). Assessing multiple novel tracers to improve the understanding of the contribution of agricultural farm waste to diffuse water pollution. Journal of Environmental Monitoring 12, 1159-1169.

Granger, S.J. Hawkins, J.M.B., Bol, R., White, S.M., Naden, P., Old, G., Bilotta, G.S., Brazier, R.E., Macleod, C.J.A. and Haygarth, P.M.

. (2010b). High temporal resolution monitoring of multiple pollutant responses in drainage from

an intensively managed grassland catchment caused by a summer storm. Water, Air and Soil Pollution. 205, 377-393.

Granger, S.J., Heaton, T. H.E.., Bol, R., Bilotta, G.S., Butler, P., Haygarth, P.M. and Owens, N. (2008) Using 15

N and

18O to evaluate the sources and pathways of NO3

- in rainfall event discharge from drained agricultural

grassland lysimeters at high temporal resolutions. Rapid Communications in Mass Spectrometry 22, 1681-1689. Granger, S. J., Bol, R. , Butler, P. J., Haygarth, P. M. , Naden, P. , Old, G., Owens, N.P., Smith, B.P.G. (2007).

Processes affecting transfer of sediment and colloids, with associated phosphorus, from intensively farmed grasslands: tracing sediment and organic matter. Hydrological Processes 21, 417-422.

Naden, P.S., Old, G.H., Eliot-Laize, C., Granger, S.J., Hawkins, J.M.B., Bol, R., Haygarth, P. (in press) Assessment of natural fluorescence as a tracer of diffuse agricultural pollution from slurry spreading on intensely-farmed grasslands. Water Research 44, 1701-1712.

To be referred to for specific information not included in the final report.

Milestones for objective 3 The focus in objective 3 was on the role of agricultural amendments (specifically slurry spreading) on pollutant losses from intensively farmed grasslands. Milestones 3a, 3b were met on time; 3c was modified and partly incorporated into an extended milestone 3a and new milestone 3d. Milestone 3d was subsequently delayed due to late delivery of the results for artificial fluorescent particles. Materials and methods (general) A range of novel tracing techniques was employed in three separate experiments (Table 2) to investigate the role of different pathways (surface/near-surface versus drains) and sources in the transfer of slurry-derived pollutants. Used together, the tracers help to corroborate experimental findings. Three main tracing techniques were used to index or mimic the various phases of slurry: 1) Natural fluorescence: dissolved/colloidal phase, 2) Carbon isotopes: dissolved/colloidal and particulate phases, and 3) Fluorescent labelled particles: particulate phase, being matched to the organic particles in slurry (Granger et al., 2007). The experimental research platforms at Rowden (plot scale) and Denbrook (small catchment scale) were used. To complement these and to assess the generic applicability of the results, the variability in the phosphorus content (by particle size fraction) as well as the natural fluorescence signal of animal slurries from nine farms around Devon was analysed and spatial and storm sampling of the Denbrook catchment undertaken in January 2007. Table 2 Summary of experimental work undertaken

Expt Sampling points

Slurry application Storms sampled

Tracers used

Rowden spring 2006

3 drained and 3 undrained plots

21m3ha

-1 applied with 10m buffer by

spreader: 2 drained plots 18

April

2 undrained plots 25 April

2 control plots monitored

19, 21 and 24-25 May

Carbon isotopes Natural fluorescence Artificial fluorescent particles (DNA)

Rowden autumn 2007

6 drained and 6 undrained plots

49m3ha

-1 applied with 5m buffer by

trailing shoe to half the plots 11-13 October

16-17 Oct 18-22

Nov

Natural fluorescence

Denbrook spring 2008

fodder field and catchment outlet

49m3ha

-1 applied to 5ha fodder field

with 5m buffer by spreader 13-14 May 2008

29-30 Mar 28-29 May 2-3 June 9-12 July

Natural fluorescence Artificial fluorescent particles

3a. Determine the timing and contribution of slurry-derived vs. soil-derived C-P in dissolved (<0.45µm, colloidal) and particulate (>0.45µm) pathways.

3c. Evaluate the effects of successive slurry applications through time. Rowden experiment in spring 2006 This experiment investigated the different pathways by which dissolved/colloidal and particulate phases of slurry and associated pollutants are lost from both drained and undrained grassland plots using multiple, simultaneous tracers (Table 2). Material and methods Slurry (grass and maize derived) were applied to the 1 ha Rowden experimental plots (Devon) in April 2006. Water samples were taken from surface flows (drained and undrained) and drains on drained plots during 3 storm events following slurry application. Results Relationships were found during the storm events, for both the drained and undrained plots, between discharge (Q), measured pollutants and natural fluorescence, counts of artificial fluorescent particles and variations in

13C in the drainage waters (Granger et al. 2010a). For example, the artificial fluorescent

particles (shown as bead counts) behaved in a similar fashion to the way sediment behaved, i.e. concentrations increased and decreased with increasing/decreasing Q. This was most pronounced in the drain flow during the third (i.e. largest) storm event. Movement of slurry-derived material was also reflected by an enriched

13C signal observed in the particulate fractions of the maize-slurry amended plots, when

compared to the grass-slurry amended plots during events 2 and 3 which corroborates the slurry source. Specifically, for events 2 and 3 mean differences of 0.5‰ (± 0.3 SED) and 0.8‰ (± 0.4 SED) in the carbon isotope signal were measured. This equates to 2-8% and 5-13%, respectively, of the applied slurry C present in the drain flow samples (Granger et al. 2010a). Conclusion The study also suggested that dissolved phases of slurry may be successfully traced using natural fluorescence, while particulate phases may be traced using artificially labelled fluorescent particles and naturally occurring carbon isotopes. Despite the low rate of slurry application and the following four-week dry period, the experiment demonstrated that multiple tracers could be used to follow the mobilisation and transport of slurry at the 1ha scale and provide evidence of the link between hydrological events and slurry losses from soil to water. This clearly confirms, in line with findings of objective 4 (next section), the need to represent both these hydrological pathways and agricultural amendment in models to improve our ability to predict the movement of diffuse pollutants from drained agricultural fields. Rowden experiment in autumn 2007 The second Rowden experiment, carried out in conjunction with the Defra-funded cracking clays project (WQ0118), was designed to investigate dissolved slurry losses from drained and undrained plots. Materials and methods Slurry was applied at the higher rate of 49m

3ha

-1, leaving a 5m buffer, using a trailing-shoe method (Table

2). The two events sampled were an initial small event immediately after spreading when the plots were in different states of wetness, and a more substantial double event some five weeks later; the intervening period being relatively dry. Results Figure 9 shows a summary of the natural fluorescence results expressed as the ratio of the tryptophan index to the fluvic/humic index (TI:FI) for each of the plots and drainage paths for all sample data. High values of this index are indicative of slurry losses. Most of the samples have TI:FI ratios less than 1.0. The exceptions to this are samples taken, during the storm immediately after slurry spreading, from the drain pathways of the plots receiving slurry. These high values of TI:FI tend towards the values found in the raw slurry, are associated with high values of TI and ammonium, and are thought to indicate the occurrence of incidental slurry losses through the drain pathways. An ANOVA test revealed no significant difference (p-value>0.05) between the two treatments for the surface/interflow pathway of either the undrained or drained plots for either of the storms. However, for the drain pathway there was a significant enrichment of the TI:FI ratio for those plots in receipt of slurry for both the October and November storms. (Naden et al., 2010) Conclusions 1. The importance of field drains for enrichment of the TI:FI ratio following slurry application. 2. The estimate of the incidental slurry losses from plot 9 during the first small storm event, taking into account uncertainty in the flow measurements, was 2-8 kg slurry. While this represents only 0.004-0.016%

Catchment flume

Field weir

0 300m

Map of Denbrook catchment

of what was applied to the 1 hectare plot, given the high SRP and BOD content of slurry, it may well be significant in terms of impact on stream ecology. 3. The lower level of enrichment seen in the second storm may be due to transport of decomposed slurry through the drain system or may be associated with enhanced microbial activity. The high TI:FI ratios in this case are not associated with high values of the TI index thus agreeing with our experiments on temporal changes in the natural fluorescence of slurry. 4. Further work is needed to test the mechanisms responsible for the reported results and the use of natural fluorescence to indicate soil carbon processes in relation to slurry application.

Figure 10. Summary results from autumn 2007 Rowden plot experiment (given as boxplots). Data arranged in pairs by drainage path: red denotes slurry applied; green denotes no slurry; pathways are surface/ interflow – undrained (U), surface/interflow - drained (S), drains (D). Numbers beneath the graph indicate the number of samples measured for fluorescence. 3b. Assess contribution from different farm areas (e.g. grassland vs. fodder field vs. hard standings)

3d. Assess the influence of slurry components from the fodder field to the drainage waters at the outlets of the fodder field and catchment using tracer techniques at the field and catchment scale Denbrook experiment in spring 2008 This experiment was focused at the small catchment scale (see map insert) and was designed to investigate (i) the relative importance of the hard standing in diffuse pollutant losses; (ii) the transfer of diffuse pollution from the fodder field to the headwater stream. Materials and methods Both natural fluorescence, as a tracer of animal waste, and a suite of diffuse pollutants were measured in the stream every 30 minutes during an intense summer storm. Artificial fluorescent particles were employed to help understand the transfer of particulate matter across the agricultural landscape from soil to headwater streams. Two different colours of fluorescent particles were used to represent one to represent the more organic (yellow; specific gravity 1.5) and a second for the more mineral fractions (magenta; specific gravity 2.7) of slurry applied to the fodder field.

Results Nitrite (92 µg l

-1), particulate phosphorus (107 µg l

-1) and

soluble phosphorus (74 µg l-1

), exceeded environmental limits during base flow. Concentrations of nitrate and nitrite were decreased during the storm event, whereas all other pollutants generally increased and frequently exceeded environmental limits, especially when associated with a small subsidiary peak on the rising limb of the main hydrograph (Fig. 11). This subsidiary hydrograph is thought to be a result of runoff from the farm hard-standing within the catchment. The associated incidental transfers of pollutants dominated the overall pollutant response and were only detected due to the high temporal resolution of the measurements (Granger et al. 2010b). The results were corroborated natural fluorescence (Fig 12).

With the onset of rainfall the impervious farmyard areas that were contaminated with animal wastes immediately generate runoff that is quickly routed to the catchment outlet. Initial peaks in TI, FI, TI:FI and flow are pronounced during the June 2008 storm event when the wider catchment is relatively slow to

respond to rainfall (Figure 12) During winter events (e.g. January 2007) when the catchment is wetter and more responsive to rainfall, discharge from the hard standing (pipe 2) is more likely to coincide with higher flows in the Denbrook from the fodder field and grassland (pipe 1) and therefore experience greater dilution. Our two component mixing model develop to quantify the relative contributions suggested that even in winter, measurements of natural fluorescence from a January storm suggested that the pipe draining the area which includes the hard-standing contributed 26-55% of the catchment outflow and 61-81% of slurry-derived pollutants. The relatively number of magenta and yellow fluorescent particles lost in the field drainage waters for the three storms accounted for 0.015 and 0.009%, respectively, of the total number of applied particles. Particle numbers at the catchment outlet showed that of those found in the field drainage waters 52% magenta and 60% yellow fluorescent particles subsequently left the catchment. All particles in drainage water leaving the field were in the 2-16 µm range, with >95% being in the 2-6 µm size range.The patterns in the counts of yellow and magenta fluorescent particles in drainage waters during the storm event follow that of the (discharge) hydrographs for the fodder field and catchment, but are more evident in the simpler hydrograph of storm event 1 compared to the multi-peaked storm 3. (Fig. 13). A lag in response at the catchment outlet is clearly evident in storm 1 and relates to the time it takes for the water and particles to travel from the field to the weir at the catchment outlet (Granger et. al. (submitted). Tracing the movement of agricultural slurry material within a small headwater catchment using artificial fluorescent particles) Conclusions 1. Field drains are important for the loss of slurry-derived dissolved material even in small storms immediately after slurry-spreading. 2. Farm hard-standing can be the dominant areas for pollutant losses in grassland catchments. 3. There is a requirement for high temporal resolution monitoring to quantify losses 4. The occurrence of surface transfers of particulate material in summer storms from catchment areas not directly connected to the channel (ca. 50-60% tracer particulates lost from the fodder field appearing at the catchment outlet).

Figure 11. The hydrological response of the catchment to rainfall and chemiograms of some of the

pollutants monitored (NO2-, NH4

+, PP, SP, SS)

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Figure 12. Response of Denbrook flow and tryptophan index to fulvic/humic index (TI:FI) ratio at the catchment outlet to a high rainfall event (2 June 2008). Uncertainty bars based on measurement of 10 consecutive samples on 11

th January 2007

Figure 13 Patterns in discharge and counts of artificial magenta and yellow fluorescent particles from the

fodder field and catchment during two storm events (storm 1 and 3).

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Objective 4. To develop a mechanistic model of colloidal P transfers and delivery through drained soils that can be used as a research tool. Assess potential for mitigation. Published papers from objective 4 Krueger, T., J. Freer, J. N. Quinton, C. J. A. Macleod, G. S. Bilotta, R. E. Brazier, P. Butler and P. M. Haygarth

(2010). Ensemble evaluation of hydrological model hypotheses. Water Resources Research. 46 Article Number: W07516.

Krueger, T., Quinton, J.N., Freer, J., Macleod, C.J.A., Bilotta, G.S., Brazier, R.E., Butler, P., Haygarth, P.M. (2009).Uncertainties in data and models to describe event dynamics of agricultural sediment and phosphorus transfer. Journal of Environmental Quality 38, 1137-1148

Krueger T, Freer J, Quinton J.N., Macleod C.J.A. (2007). Processes affecting transfer of sediment and colloids, with associated phosphorus, from intensively farmed grasslands: a critical note on modelling of phosphorus transfers. Hydrological Processes 21, 557-562.

To be referred to for specific information not included in the final report. Milestones All milestones were met. Final mitigation suggestions are provided in the final section of the report and represent the suggestions made by the whole research team, not only objective 4.

4a. Develop a conceptually realistic, yet parameter efficient, model which can identify the different sources of particulate P delivered to a catchment outlet. Model development was approached in a top-down way, i.e. aiming at the effective behaviour at the scale where observations were available (the field scale of Rowden and the small catchment scale of Den Brook, including nested sub-catchments). This approach was framed in a Generalised Likelihood Uncertainty Estimation (GLUE; Beven and Binley, 1992; Beven, 2006) which took explicit account of the uncertainties in model structures, parameters and data where possible. Figure 13 outlines the model process compartments of phosphorus transfer suggested by analyses of the available data, exemplified here for Rowden. Important is the prominent position of the sediment behaviour in shaping the phosphorus dynamics in these intensively managed grassland environments.

Figure 13. The model process compartments of phosphorus transfer that were suggested by analyses of the available data. The schematic is an example from the Rowden study (Krueger et al., 2009) and notes the available data and uncertainty information as well as the proposed models and uncertainty analyses (UA). At the Rowden field scale, the hydrology compartment was conceptualised using simple soil moisture accounting stores (Figure 14), taking account of model structural uncertainty by formulating an ensemble of multiple structures. Analysis of sediment dynamics as a function of hydrology (i.e. discharge) pointed to hysteretic behaviour in the sediment-discharge relationship, hence this compartment was conceptualised using a hysteresis model:

2

1 3SS

dQC Q

dt

(1)

with suspended solids concentration CSS, discharge Q, local slope of the hydrograph dQ/dt and parameters θ1, θ2 and θ3. Phosphorus dynamics were modelled as

6

4 5TP SSC C

(2)

with total phosphorus concentration CTP and parameters θ4, θ5 and θ6.

Figure 14. Ensemble of 72 soil moisture accounting stores evaluated to describe the hydrology of the un-drained fields at Rowden (Krueger et al., in revision). P is precipitation; ET is evapotranspiration with indices denoting different formulas to translate estimated potential ET into actual rates; S1 is the active soil moisture store; S0 is a hypothetical in-active store only accessible by evapotranspiration; QOF is saturation excess overland flow; QIF is sub-surface interflow. At the Den Brook catchment scale, the hydrology compartment was set up using Dynamic Topmodel (Beven and Freer, 2001), with local slope and contributing area as the principal drivers of the spatial variability in hydrological response plus variations in the conceptualisation of Hydrological Similarity Units (HSUs) to account for sealed areas (i.e. farmyard) and variable drainage density (Figure 15). The topographic analysis was carried out on a high resolution Digital Elevation Model (DEM) that was based on a detailed Differential GPS survey. The sediment dynamics model was developed further from the field scale study. Three competing model hypotheses were tested: Equation (1),

(3)

and

(4) (4)

The description of phosphorus dynamics was carried over from the field scale study in Equation (2).

Figure 15. The model setup of Den Brook

catchment. (Krueger et al., in preparation,

Learning about catchment hydrology

through model rejection using imprecise

sub-catchment flux measurements.

4b. To use spatial measurements of sediment transport and deposition in overland and subsurface flow as a means of constraining the uncertainty of a distributed model of particulate P mobilisation in a small catchment Available data were used to assess the uncertainties in model predictions. The overall uncertainties were separated into their components relating to input data, model structure, model parameters and evaluation data were information to characterise these explicitly were available (Figures 16 and 17). The uncertainties in the stage-discharge rating curves of the gauges at Rowden and Den Brook were quantified using fuzzy rating curves derived from detailed rating experiments (Krueger et al., in revision). Figure 17 illustrates how

quantity and quality of measurements (i.e. number and size of data boxes and their distribution over the observed stage range) constrain the relative uncertainty of the derived rating curve (i.e. width of envelope relative to flow range).

Figure 16. (a) Variability of rainfall at Rowden, expressed as cumulative rain measured at four gauges over the 2006 Water Year which let to six rain scenarios (Krueger et al., in revision). (b-c) Relative uncertainty range of hourly flow-weighted mean concentrations as a function of number of samples per hour (Krueger et al., in preparation b).

Figure 17. Fuzzy stage-discharge rating curves for (a) one Rowden weir, (b) the fodder field weir at Den Brook, and (c) the Den Brook outlet flume (note the differences in scale). Shown are the raw measurement repeats as dots, the estimated data uncertainty boxes, and the derived uncertainty envelopes over the entire range of observed stages (Krueger et al., in revision; Krueger et al., in preparation a). Model structural and parameter uncertainties contributed, in parts, more to the overall prediction uncertainty than data uncertainties. This was attributed to flaws in the models and data limitations in estimating parameters. Nevertheless, some competing model hypotheses (i.e. structures and parameter sets) could be rejected which helped to narrow down formally the possible effective behaviour of the study sites. So did the model ensemble experiment at Rowden lead to a revision of the perception of the fields as being hydrologically isolated (Krueger et al., in revision). This was indicated by relatively high performances of models that simulated in-active soil moisture stores and evapotranspiration rates that both were too large to be realistic for this site, thereby compensating for water seepage (most likely along the field perimeter drains) that was not accounted for in any of the models. In evaluating simple relationships between total phosphorus and suspended solids (Figure 18a), it was found that the phosphorus signature of the top-soil seemed to dominate surface and drain pathways and that dissolved phosphorus was of little importance (Krueger et al., 2009). This could be indicative of macro-pore facilitated transport of phosphorus-rich top-soil into the drains in these grassland systems. With respect to the crucial role that sediment dynamics played in the transfer of phosphorus, the empirical hysteresis model that was developed offers a coherent formal description of the characteristic hysteretic behaviour (Figure 18b-c). To tackle the observed temporal and spatial variability of this behaviour, part of the parameter changes between events could be predicted by antecedent event characteristics (Krueger et al., in preparation b). If the empirical model is understood as the „top‟ level, it can then be investigated further „down‟ into the small scale processes that might generate hysteretic behaviour. Hypotheses such as erosion of bare areas, sub-surface mobilisation and transport, re-suspension in drains and pipes and locally varying hydraulic conditions were generated. Yet, the need for more and different measurements to test them (e.g. sediment tracing using techniques developed by objective 3 and spatially nested hydraulic and sediment flux measurements) was identified.

Figure 18. (a) Total phosphorus-suspended solids relationship observed at Rowden (for stage > 40mm; the low flow relationship was more uncertain) and model uncertainty bounds simulated using Equation (2). (b) A high model performance and (c) a low model performance example of suspended-solids-discharge hysteresis observed at Rowden in black (including estimated discharge uncertainty) and model uncertainty bounds simulated using Equation (1) in grey (including estimated discharge uncertainty and overall prediction uncertainty). 4c. To validate model developed in 4a for scales ranging from controlled plot (<1 ha) to small catchment (<1 km) and how does uncertainty in our predictions change as we move between scales. The catchment scale hydrological model yielded acceptable simulations at the catchment outlet but failed to reproduce the timing of internal contributing areas. This demonstrated the need for spatial evaluation of distributed models of diffuse pollution while predictions made on the grounds of models calibrated to single locations were misleading. Conclusions 1. A parameter efficient sediment and phosphorus transfer model was developed and evaluated using data

for which uncertainties had been established. Modelling sediment dynamics was crucial in predicting phosphorus transfer, yet the dominant processes are complex and variable in space and time.

2. Capturing the large variability in environmental conditions and measured data by frequent repeated sampling is essential for robust evaluation of hydrology and water quality models.

3. Spatially distributed models of diffuse pollution need to be evaluated using spatial data, calibration at a single location results easily in falsely acceptable predictions (“right looking for the wrong reasons”).

4. Only in an iterative learning process of field experimentation, data uncertainty estimation, model development and model evaluation can fundamental understanding of the environment be gained and models be developed which are robust under uncertainty and therefore credible in decision making.

References Beven, K. J. and A. Binley (1992). Hydrological Processes 6(3): 279-298. Beven, K. J. and J. E. Freer (2001). A dynamic TOPMODEL. Hydrological Processes 15(10): 1993-2011. Beven, K. (2006). A manifesto for the equifinality thesis. Journal of Hydrology 320(1-2): 18-36. 4d. Suggest mitigation measures. The plot, field and farm scale evidence from modelling, tracing and erosion studies within the current project leads us to suggest the following mitigation methods (senso stricto DPWA User manual 2007): Soil management: reduce the amount of highly erodible crops and bare soil area, reduce the use of fodder and forage fields esp. on steep slopes and proximal or with strong connectivity to water courses (Implement method 1 and 2). Livestock management: reduce bare areas and compaction through animal management (less animals, different animals, don‟t let them out when wet etc.). Specifically, reduce overall stocking rates on livestock farms, i.e. stocking densities no higher than 2 LSU ha

-1 on steep (>15º slopes) (method 13) and reduce field

stocking rates when soils are wet, i.e. consideration of soil moisture contents when grazing (at all times of year) (method 15). Fertiliser management: reduce fertiliser/slurry use and apply when crops will use nutrients (method 23, 24). Farm infrastructure: reduce connectivity and potential impact from agricultural point sources (e.g. manage farmyards and hard standings, re-site gateways) (41) and introduce landscape buffer features to reduce delivery of sediment and nutrients to stream channels, i.e. disconnect key sources of diffuse substances from receptors (e.g. manipulate the pipe network, put in buffers, or ponds or wetlands) (43, 44).

Issue of field drains: Mobilisation and transport of soil and livestock-associated materials is complex involving a number of physical and biogeochemical processes. The results from intensive monitoring of several rainfall-runoff events, have shown that the presence of field drains significantly reduces the total flux of sediment and total P compared to fields with no artificial drains. This implies that field drains have a positive role in reducing fluxes of SS and TP. However, this project has also demonstrated that fractions of recently applied slurry can be preferentially transported through agricultural field drains compared to interflow/surface runoff pathways or undrained fields. This implies that management strategies should be focused on reducing losses via field drains (i.e. implement method 12 and allow field drainage systems to deteriorate), particularly at those times of year when riverine biota may be more at risk from high biochemical oxygen demand (BOD) or high dissolved nutrient levels. Future studies: may want to examine this (unresolved) field drains issue further, particularly in relation to antecedent conditions prior to rainfall events and variability between wet and dry years.