Dominant mechanisms for the delivery of fine sediment and phosphorus to fluvial networks draining grassland dominated headwater catchments

Terrestrial sources regulate the endogenous phosphorus load in Taihu Lake, China after exogenous controls: Evidence from a representative lake watershed

Identifying phosphorus (P) sources and contributions from terrestrial sources is important for clean water and eutrophication management in lake watersheds. However, this remains challenging owing to the high complexity of P transport processes. The concentrations of different P fractions in the soils and sediments from Taihu Lake, a representative freshwater lake watershed, were obtained using sequential extraction procedure. The dissolved phosphate (PO₄-P) and alkaline phosphatase activity (APA) in the lake's water were also surveyed. The results showed that different P pools in the soil and sediments displayed different ranges. Higher concentrations of P fractions were measured in the solid soils and sediments from the northern and western regions of the lake watershed, indicating a larger input of P from exogenous sources, including agriculture runoff and industrial effluent from the river. Generally, higher Fe-P and Ca-P concentrations of up to 399.5 and 481.4 mg/kg were detected in soils and lake sediments, respectively. Similarly, the lake's water had higher concentrations of PO₄-P and APA in the northern region. A significant positive correlation was found between Fe-P in the soil and PO₄-P concentrations in the water. Statistical analysis indicated that appropriately 68.75% P was retained in the sediment from terrigenous sources, and 31.25% P experienced dissolution and shifted to the solution phase in the water-sediment ecosystems. The dissolution and release in Fe-P in the soils were responsible for the increase of Ca-P in the sediment after the afflux of soils into the lake. These findings suggest that soil runoff predominantly controls P occurrence in lake sediments as an exogenous source. Generally, the strategy of reducing terrestrial inputs from agricultural soil discharge is still an important step in P management at the catchment scale of lakes.

Hydro-chemical responses at different scales in a rural catchment, UK, and implications for managing the unintended consequences of agriculture

Diffuse pollutant transfers from agricultural land often constitute the bulk of annual loads in catchments and storm events dominate these fluxes. There remains a lack of understanding of how pollutants move through catchments at different scales. This is critical if the mismatch between the scales used to implement on-farm management strategies, compared to those used for assessment of environmental quality, is to be addressed. The aim of this study was to understand how the mechanisms of pollutant export may change when assessed at different scales and the corresponding implications for on-farm management strategies. A study was conducted within a 41 km2 catchment containing 3 nested sub-catchments, instrumented to monitor discharge and various water quality parameters. Storm data over a 24-month period were analysed and hysteresis (HI) and flushing (FI) indices calculated for two water quality variables that are typically of environmental significance; NO3-N and suspended sediment (SSC). For SSC, increasing spatial scale had little effect on the mechanistic interpretation of mobilisation and the associated on-farm management strategies. At the three smallest scales NO3-N was chemodynamic with the interpretation of dominant mechanisms changing seasonally. At these scales, the same on-farm management strategies would be recommended. However, at the largest scale, NO3-N appeared unaffected by season and chemostatic. This would lead to a potentially very different interpretation and subsequent on-farm measures. The results presented here underscore the benefits of nested monitoring for extracting mechanistic understanding of agricultural impacts on water quality. The application of HI and FI indicates that monitoring at smaller scales is crucial. At large scales, the complexity of the catchment hydrochemical response means that mechanisms become obscured. Smaller catchments more likely represent critical areas within larger catchments where mechanistic understanding can be extracted from water quality monitoring and used to underpin the selection of on-farm mitigation measures.

Total and dissolved phosphorus losses from agricultural headwater streams during extreme runoff events

Eutrophication continues to be a concerning global water quality issue. Managing and mitigating harmful algal blooms demands clear information on the conditions promoting large phosphorus losses from contributing watersheds. Of particular concern is the amount and form of phosphorus loading to receiving water bodies during extreme runoff events, which are expected to increase in frequency due to climate change. Five years (2015 to 2020) of water quantity and quality data from 11 agricultural watersheds in the lower Great Lakes basin were analyzed and used to model total and dissolved phosphorus losses. This study aimed to assess temporal dynamics in phosphorus concentrations and losses over runoff events covering a wide range of hydrologic conditions and to quantify their relative importance on annual phosphorus losses. Event concentration-discharge relationships for total and dissolved phosphorus were hysteretic and had contrasting dominant patterns across watersheds. The proportion of annual phosphorus losses during events was highly variable between watersheds, accounting for 47-94 %. Extreme events were particularly impactful: as few as three events per year were found to be responsible for nearly half of total phosphorus (20-50 %) and total dissolved phosphorus (14-44 %) losses. Variability in total and dissolved phosphorus losses and concentrations over a wide range of flow conditions suggests that event magnitude is an important control on the relative mobility of particulate and dissolved phosphorus fractions. This study showed that insights into nutrient dynamics and phosphorus budgets in the lower Great Lakes basin and agriculture dominated environments more broadly can be gained by assessing event nutrient losses with respect to flow conditions and patterns in concentration-discharge relationships.

Buffalo Pound Lake—Modelling Water Resource Management Scenarios of a Large Multi-Purpose Prairie Reservoir

Water quality models are an emerging tool in water management to understand and inform decisions related to eutrophication. This study tested flow scenario effects on the water quality of Buffalo Pound Lake—a eutrophic reservoir supplying water for approximately 25% of Saskatchewan’s population. The model CE-QUAL-W2 was applied to assess the impact of inter-basin water diversion after the impounded lake received high inflows from local runoff. Three water diversion scenarios were tested: continuous flow, immediate release after nutrient loading increased, and a timed release initiated when water levels returned to normal operating range. Each scenario was tested at three different transfer flow rates. The transfers had a dilution effect but did not affect the timing of the nutrient peaks in the upstream portion of the lake. In the lake’s downstream section, nutrients peaked at similar concentrations as the base model, but peaks arrived earlier in the season and attenuated rapidly. Results showed greater variation among scenarios in wet years compared to dry years. Dependent on the timing and quantity of water transferred, some but not all water quality parameters are predicted to improve along with the water diversion flows over the period tested. The results suggest that it is optimal to transfer water while local watershed runoff is minimal.

Degree of Anthropogenic Land Disturbance Controls Fluvial Sediment Hysteresis

Storm events dominate sediment delivery to stream corridors, but the effects of anthropogenic disturbances on altering the sources, pathways, and timing of delivery remain uncertain. To address this knowledge gap, we analyzed 849 events from over a decade of high-frequency turbidity data across five watersheds in an urbanization gradient. Sensing results suggested that hysteresis patterns evolved with land use from clockwise (low-rural) to figure-eight (high-rural) to counter-clockwise (high-urban), indicating a disturbance-driven shift of sediment provenance from proximal to distal. Sediment loading pathways in the lowest-disturbance rural watershed were dominated by a single hysteresis shape (>90% of export by clockwise events), whereas the most-disturbed urban basin had the greatest variability in loading pathways (∼25% of export by clockwise, counter-clockwise, figure-eight, and complex events, respectively). Finally, wastewater treatment facilities modulated the release of "hungry-water" baseflow, causing more-rapid periods of high streamflow variance in catchments with a treatment facility (∼4 h period) than in those without (∼6 h period). Together, our results indicate that anthropogenic disturbances, including tile drainage, impervious surfaces, and roadway density, increase the connectivity of distally located sediment that would-in undisturbed basins-deposit along the sediment cascade. This information is important to watershed managers as they mitigate erosion in developing basins.

Phosphorus loss index for conservation agriculture systems in Southern Brazil: A new approach to environmental risk assessment

The transport of phosphorus (P) from agricultural fields to watercourses is a worldwide concern. Development of tools to evaluate the environmental P risk is required to assist farmers and field technicians to recommend best management practices. The objective of this work was to develop and test a new approach to assess P loss vulnerability for agricultural conservationist systems in southern Brazil using easily obtainable soil attributes: cover crop residue (CCR), soil organic carbon (OC), visual evaluation of soil structure (VESS), soil test P (STP) and clay content. For that, the principal component analysis was applied, and simulated rainfall was conducted in ten agricultural fields with three replicates under Oxisol, with slopes from 7% to 12%. The database was separated into main and test groups. Significant principal components were correlated with runoff coefficient (RC), final infiltration rate (FIR), total sediment (TS), total P (TP), particulate P (PP), soluble P (SP) and bioavailable P (BAP). Component 1, interpreted as 'increased risk of PP transport', explained 49% of the variation with a strong dependence on CCR, OC, STP and VESS, and correlated with decreased FIR, and increased RC, TS, PP and TP. Component 2, interpreted as 'increased risk of SP transport', explained 21% of the variation with a strong dependence on OC, STP and clay content, and correlated with decreased TS, and increased SP and BAP. The P-index scores from components 1 and 2 were divided into four classes of P loss vulnerability (low, medium, high and very high) considering the first, median and third quartile. The Euclidean distances demonstrated a high similarity of the P-index scores obtained for the main and test groups. Therefore, the developed model can estimate the risk of P loss in agricultural fields with conservative management and moderate slope in conditions similar to the database.

Identifying Flow Pathways for Phosphorus Transport Using Observed Event Forensics and the CRAFT (Catchment Runoff Attenuation Flux Tool)

Identifying key flow pathways is critical in order to understand the transport of Phosphorus (P) from agricultural headwater catchments. High frequency/resolution datasets from two such catchments in Northwest England enabled individual events to be examined to identify the flow (Q) and Total P (TP) and Total Reactive P (TRP) dynamics (forensics). Detailed analysis of multiple flow and water quality parameters is referred to here as the event forensics. Are there more flow pathways than just surface runoff (dominated by overland flow) and baseflow (mainly groundwater) contributing at the outlet of these catchments? If so, hydrograph separation alone will not be sufficient. This forensic analysis gives a classification of four storm event response types. Three classes are based on the balance of old and new water giving enrichment and dilution of TRP pattern in the subsurface flow. A fourth type was observed where a plume of nutrient is lost to the channel when there is no observed flow. Modelling is also essential when used in combination with the event forensics as this additional tool can identify distinct flow pathways in a robust form. A case study will apply the Catchment Runoff Attenuation Flux Tool (CRAFT) to two contrasting small headwater catchments in Northwest England, which formed part of the Demonstration Test Catchments (DTC) Programme. The model will use data collected during a series of events observed in the two catchments between the period 2012 and 2014. It has the ability to simulate fast near surface (that can represent flow in the upper soil horizons and field drains) and event subsurface soil flow, plus slower groundwater discharge. The model can capture P enrichment, dilution and the role that displacement of “old” P rich water has during events by mixing these flows. CRAFT captures the dominant flow and P fluxes as seen in the forensic analysis and can create outputs including smart export coefficients (based on flow pathways) that can be conveyed to policy makers to better underpin decision making.

Identifying critical source areas using multiple methods for effective diffuse pollution mitigation

Diffuse pollution from agriculture constitutes a key pressure on the water quality of freshwaters and is frequently the cause of ecological degradation. The problem of diffuse pollution can be conceptualised with a source-mobilisation-pathway (or delivery)-impact model, whereby the combination of high source risk and strong connected pathways leads to 'critical source areas' (CSAs). These areas are where most diffuse pollution will originate, and hence are the optimal places to implement mitigation measures. However, identifying the locations of these areas is a key problem across different spatial scales within catchments. A number of approaches are frequently used for this assessment, although comparisons of these assessments are rarely carried out. We evaluate the CSAs identified via traditional walkover surveys supported by three different approaches, highlighting their benefits and disadvantages. These include a custom designed smartphone app; a desktop geographic information system (GIS) and terrain analysis-based SCIMAP (Sensitive Catchment Integrated Modelling and Analysis Platform) approach; and the use of a high spatial resolution drone dataset as an improved input data for SCIMAP modelling. Each of these methods captures the locations of the CSAs, revealing similarities and differences in the prioritisation of CSA features. The differences are due to the temporal and spatial resolution of the three methods such as the use of static land cover information, the ability to capture small scale features, such as gateways and the incomplete catchment coverage of the walkover survey. The relative costs and output resolutions of the three methods indicate that they are suitable for application at different catchment scales in conjunction with other methods. Based on the results in this paper, it is recommended that a multi-evidence-based approach to diffuse pollution management is taken across catchment spatial scales, incorporating local knowledge from the walkover with the different data resolutions of the SCIMAP approach.

The Role of Attenuation and Land Management in Small Catchments to Remove Sediment and Phosphorus: A Modelling Study of Mitigation Options and Impacts

It is well known that soil, hillslopes, and watercourses in small catchments possess a degree of natural attenuation that affects both the shape of the outlet hydrograph and the transport of nutrients and sediments. The widespread adoption of Natural Based Solutions (NBS) practices in the headwaters of these catchments is expected to add additional attenuation primarily through increasing the amount of new storage available to accommodate flood flows. The actual type of NBS features used to add storage could include swales, ditches, and small ponds (acting as sediment traps). Here, recent data collected from monitored features (from the Demonstration Test Catchments project in the Newby Beck catchment (Eden) in northwest England) were used to provide first estimates of the percentages of the suspended sediment (SS) and total phosphorus (TP) loads that could be trapped by additional features. The Catchment Runoff Attenuation Flux Tool (CRAFT) was then used to model this catchment (Newby Beck) to investigate whether adding additional attenuation, along with the ability to trap and retain SS (and attached P), will have any effect on the flood peak and associated peak concentrations of SS and TP. The modelling tested the hypothesis that increasing the amount of new storage (thus adding attenuation capacity) in the catchment will have a beneficial effect. The model results implied that a small decrease of the order of 5–10% in the peak concentrations of SS and TP was observable after adding 2000 m3 to 8000 m3 of additional storage to the catchment.

Spatial modeling of sediment transfer and identification of sediment sources during snowmelt in an agricultural watershed in boreal climate

Sediment transfer patterns during snowmelt were studied in a small Finnish agricultural watershed. Erosion rates were high as a consequence of high runoff volumes over saturated soil that partly lacked vegetation cover. Automatic high-frequency monitoring data of sediment and phosphorus concentrations in stream showed a clock-wise hysteresis loop as a dominant pattern. GIS-based modeling of runoff and soil erosion, using LiDAR DTM data, suggested that runoff and erosion mostly came from cropland that had the highest sediment contribution index. Also sediment fingerprinting with Cesium-137 suggested cropland and stream bank were the most important sources of suspended sediments in streams. Because a major part of annual sediment transfer takes place during snowmelt, it is a critical period for annual losses of pollutants. Management practices that minimize springtime sediment and pollutant losses from cropland would be needed to make a marked impact on annual pollution transfer to stream waters.

Evaluating the use of in-situ turbidity measurements to quantify fluvial sediment and phosphorus concentrations and fluxes in agricultural streams

Accurate quantification of suspended sediments (SS) and particulate phosphorus (PP) concentrations and loads is complex due to episodic delivery associated with storms and management activities often missed by infrequent sampling. Surrogate measurements such as turbidity can improve understanding of pollutant behaviour, providing calibrations can be made cost-effectively and with quantified uncertainties. Here, we compared fortnightly and storm intensive water quality sampling with semi-continuous turbidity monitoring calibrated against spot samples as three potential methods for determining SS and PP concentrations and loads in an agricultural catchment over two-years. In the second year of sampling we evaluated the transferability of turbidity calibration relationships to an adjacent catchment with similar soils and land cover. When data from nine storm events were pooled, both SS and PP concentrations (all in log space) were better related to turbidity than they were to discharge. Developing separate calibration relationship for the rising and falling limbs of the hydrograph provided further improvement. However, the ability to transfer calibrations between adjacent catchments was not evident as the relationships of both SS and PP with turbidity differed both in gradient and intercept on the rising limb of the hydrograph between the two catchments. We conclude that the reduced uncertainty in load estimation derived from the use of turbidity as a proxy for specific water quality parameters in long-term regulatory monitoring programmes, must be considered alongside the increased capital and maintenance costs of turbidity equipment, potentially noisy turbidity data and the need for site-specific prolonged storm calibration periods.

Use of spatially distributed time-integrated sediment sampling networks and distributed fine sediment modelling to inform catchment management

Under the EU Water Framework Directive, suspended sediment is omitted from environmental quality standards and compliance targets. This omission is partly explained by difficulties in assessing the complex dose-response of ecological communities. But equally, it is hindered by a lack of spatially distributed estimates of suspended sediment variability across catchments. In this paper, we demonstrate the inability of traditional, discrete sampling campaigns for assessing exposure to fine sediment. Sampling frequencies based on Environmental Quality Standard protocols, whilst reflecting typical manual sampling constraints, are unable to determine the magnitude of sediment exposure with an acceptable level of precision. Deviations from actual concentrations range between -35 and +20% based on the interquartile range of simulations. As an alternative, we assess the value of low-cost, suspended sediment sampling networks for quantifying suspended sediment transfer (SST). In this study of the 362 km² upland Esk catchment we observe that spatial patterns of sediment flux are consistent over the two year monitoring period across a network of 17 monitoring sites. This enables the key contributing sub-catchments of Butter Beck (SST: 1141 t km² yr^-1) and Glaisdale Beck (SST: 841 t km² yr^-1) to be identified. The time-integrated samplers offer a feasible alternative to traditional infrequent and discrete sampling approaches for assessing spatio-temporal changes in contamination. In conjunction with a spatially distributed diffuse pollution model (SCIMAP), time-integrated sediment sampling is an effective means of identifying critical sediment source areas in the catchment, which can better inform sediment management strategies for pollution prevention and control.

Measurement differences between turbidity instruments, and their implications for suspended sediment concentration and load calculations: A sensor inter-comparison study

The use of turbidity for indicating environmentally detrimental levels of suspended and colloidal matter in freshwater systems, and for defining acceptable water quality standards in national and European drinking water regulations, is well established. Turbidity is therefore frequently adopted as a surrogate for suspended sediment concentrations (SSC), or as a relative and objective measure of water clarity in monitoring programmes. Through systematic, controlled experimentation, we tested the response of 12 commercially available turbidity sensors, of various designs, to gauge their measurement consistency when benchmarked against pre-prepared sediment suspensions of known SSC. Results showed that despite calibration to a Formazin standard, sensor responses to identical SSC solutions (in the range of 20-1000 mg L^-1) varied considerably. For a given SSC, up to five-fold differences in recorded turbidity were recorded across the tested instruments. Furthermore, inconsistent measurements were identified across instruments, regardless of whether they operated using backscatter or side-scatter optical principles. While the findings may have implications for compliance with turbidity-based water quality standards, they are less likely to be an issue when turbidity is being used as a surrogate for SSC, provided that instrument use remains constant and that instrument drift is not an issue. In this study, a field comparison of a subset of four study sensors showed that despite very different absolute turbidity readings for a given SSC, well correlated and reliable turbidity - SSC ratings were established (as evidenced by r² coefficients from 0.92 to 0.98). This led to reasonably consistent suspended sediment load estimates of between 64.7 and 70.8 tonnes for a rainfall event analysed. This study highlights the potential for issues to arise when interpreting water turbidity datasets that are often assumed to be comparable, in that measurement inconsistency of the type reported here may remain unknown to water resource decision-makers and practitioners.

The application of high temporal resolution data in river catchment modelling and management strategies

Modelling changes in river water quality, and by extension developing river management strategies, has historically been reliant on empirical data collected at relatively low temporal resolutions. With access to data collected at higher temporal resolutions, this study investigated how these new dataset types could be employed to assess the precision and accuracy of two phosphorus (P) load apportionment models (LAMs) developed on lower resolution empirical data. Predictions were made of point and diffuse sources of P across ten different sampling scenarios. Sampling resolution ranged from hourly to monthly through the use of 2000 newly created datasets from high frequency P and discharge data collected from a eutrophic river draining a 9.48 km² catchment. Outputs from the two LAMs were found to differ significantly in the P load apportionment (51.4% versus 4.6% from point sources) with reducing precision and increasing bias as sampling frequency decreased. Residual analysis identified a large deviation from observed data at high flows. This deviation affected the apportionment of P from diffuse sources in particular. The study demonstrated the potential problems in developing empirical models such as LAMs based on temporally relatively poorly-resolved data (the level of resolution that is available for the majority of catchments). When these models are applied ad hoc and outside an expert modelling framework using extant datasets of lower resolution, interpretations of their outputs could potentially reduce the effectiveness of management decisions aimed at improving water quality.

Unravelling organic matter and nutrient biogeochemistry in groundwater-fed rivers under baseflow conditions: Uncertainty in in situ high-frequency analysis

In agricultural catchments, diffuse nutrient fluxes (mainly nitrogen N and phosphorus P), are observed to pollute receiving waters and cause eutrophication. Organic matter (OM) is important in mediating biogeochemical processes in freshwaters. Time series of the variation in nutrient and OM loads give insights into flux processes and their impact on biogeochemistry but are costly to maintain and challenging to analyse for elements that are highly reactive in the environment. We evaluated the capacity of the automated monitoring to capture typically low baseflow concentrations of the reactive forms of nutrients and OM: total reactive phosphorus (TRP), nitrate nitrogen (NO₃-N) and tryptophan-like fluorescence (TLF). We compared the performance of in situ monitoring (wet chemistry analyser, UV-vis and fluorescence sensors) and automated grab sampling without instantaneous analysis using autosamplers. We found that automatic grab sampling shows storage transformations for TRP and TLF and do not reproduce the diurnal concentration pattern captured by the in situ analysers. The in situ TRP and fluorescence analysers respond to temperature variation and the relationship is concentration-dependent. Accurate detection of low P concentrations is particularly challenging due to large errors associated with both the in situ and autosampler measurements. Aquatic systems can be very sensitive to even low concentrations of P typical of baseflow conditions. Understanding transformations and measurement variability in reactive forms of nutrients and OM associated with in situ analysis is of great importance for understanding in-stream biogeochemical functioning and establishing robust monitoring protocols.

Simulating high frequency water quality monitoring data using a catchment runoff attenuation flux tool (CRAFT)

High resolution water quality data has recently become widely available from numerous catchment based monitoring schemes. However, the models that can reproduce time series of concentrations or fluxes have not kept pace with the advances in monitoring data. Model performance at predicting phosphorus (P) and sediment concentrations has frequently been poor with models not fit for purpose except for predicting annual losses. Here, the data from the Eden Demonstration Test Catchments (DTC) project have been used to calibrate the Catchment Runoff Attenuation Flux Tool (CRAFT), a new, parsimonious model developed with the aim of modelling both the generation and attenuation of nutrients and sediments in small to medium sized catchments. The CRAFT has the ability to run on an hourly timestep and can calculate the mass of sediments and nutrients transported by three flow pathways representing rapid surface runoff, fast subsurface drainage and slow groundwater flow (baseflow). The attenuation feature of the model is introduced here; this enables surface runoff and contaminants transported via this pathway to be delayed in reaching the catchment outlet. It was used to investigate some hypotheses of nutrient and sediment transport in the Newby Beck Catchment (NBC) Model performance was assessed using a suite of metrics including visual best fit and the Nash-Sutcliffe efficiency. It was found that this approach for water quality models may be the best assessment method as opposed to using a single metric. Furthermore, it was found that, when the aim of the simulations was to reproduce the time series of total P (TP) or total reactive P (TRP) to get the best visual fit, that attenuation was required. The model will be used in the future to explore the impacts on water quality of different mitigation options in the catchment; these will include attenuation of surface runoff.

The role of environmental land use conflicts in soil fertility: A study on the Uberaba River basin, Brazil

In the Uberaba River basin (state of Minas Gerais, Brazil), pastures for livestock production have invaded areas of native vegetation (Cerrado biome), while already existing pastures were invaded by crop agriculture, with an expansion of sugar cane plantations in the most recent years. In some areas of the basin, these land use changes were classified as environmental land use conflicts because the new uses were not conforming to land capability, i.e. the soil's natural use. Where the areas in conflict became dense, some soil properties have changed significantly, namely the organic matter content and the exchangeable potassium concentration, which have decreased drastically (5kg/m(3) per 10% increase in the conflict area) threatening the fertility of soil. Besides, these changes may have triggered a cascade of other environmental damages, specifically the increase of soil erosion and the degradation of water quality with negative impacts on aquatic biodiversity, related to a disruption of soil organic matter structural functions. Because half the Uberaba catchment has been considered is a state of accentuated environmental degradation, not only caused by environmental land use conflicts, conservation measures have been proposed and requested for immediate implementation across the watershed.

Changing climate and nutrient transfers: Evidence from high temporal resolution concentration-flow dynamics in headwater catchments

We hypothesise that climate change, together with intensive agricultural systems, will increase the transfer of pollutants from land to water and impact on stream health. This study builds, for the first time, an integrated assessment of nutrient transfers, bringing together a) high-frequency data from the outlets of two surface water-dominated, headwater (~10km(2)) agricultural catchments, b) event-by-event analysis of nutrient transfers, c) concentration duration curves for comparison with EU Water Framework Directive water quality targets, d) event analysis of location-specific, sub-daily rainfall projections (UKCP, 2009), and e) a linear model relating storm rainfall to phosphorus load. These components, in combination, bring innovation and new insight into the estimation of future phosphorus transfers, which was not available from individual components. The data demonstrated two features of particular concern for climate change impacts. Firstly, the bulk of the suspended sediment and total phosphorus (TP) load (greater than 90% and 80% respectively) was transferred during the highest discharge events. The linear model of rainfall-driven TP transfers estimated that, with the projected increase in winter rainfall (+8% to +17% in the catchments by 2050s), annual event loads might increase by around 9% on average, if agricultural practices remain unchanged. Secondly, events following dry periods of several weeks, particularly in summer, were responsible for high concentrations of phosphorus, but relatively low loads. The high concentrations, associated with low flow, could become more frequent or last longer in the future, with a corresponding increase in the length of time that threshold concentrations (e.g. for water quality status) are exceeded. The results suggest that in order to build resilience in stream health and help mitigate potential increases in diffuse agricultural water pollution due to climate change, land management practices should target controllable risk factors, such as soil nutrient status, soil condition and crop cover.

Storm Event Suspended Sediment-Discharge Hysteresis and Controls in Agricultural Watersheds: Implications for Watershed Scale Sediment Management

Within agricultural watersheds suspended sediment-discharge hysteresis during storm events is commonly used to indicate dominant sediment sources and pathways. However, availability of high-resolution data, qualitative metrics, longevity of records, and simultaneous multiwatershed analyses has limited the efficacy of hysteresis as a sediment management tool. This two year study utilizes a quantitative hysteresis index from high-resolution suspended sediment and discharge data to assess fluctuations in sediment source location, delivery mechanisms and export efficiency in three intensively farmed watersheds during events over time. Flow-weighted event sediment export was further considered using multivariate techniques to delineate rainfall, stream hydrology, and antecedent moisture controls on sediment origins. Watersheds with low permeability (moderately- or poorly drained soils) with good surface hydrological connectivity, therefore, had contrasting hysteresis due to source location (hillslope versus channel bank). The well-drained watershed with reduced connectivity exported less sediment but, when watershed connectivity was established, the largest event sediment load of all watersheds occurred. Event sediment export was elevated in arable watersheds when low groundcover was coupled with high connectivity, whereas in the grassland watershed, export was attributed to wetter weather only. Hysteresis analysis successfully indicated contrasting seasonality, connectivity and source availability and is a useful tool to identify watershed specific sediment management practices.