Keeping agricultural soil out of rivers: evidence of sediment and nutrient accumulation within field wetlands in the UK

Linear programming the Ramsar convention's criterion IV (case study: Shadegan Wetland, West Asia)

Wetland ecosystems provide critical ecological services to both humans and wildlife. However, many wetlands around the world are facing challenges that threaten their ecological integrity and jeopardize their status as Ramsar Convention sites. The Shadegan Wetland, a Ramsar site since 1975, has been on the Montreux Record since 1993 due to changing conditions in the wetland. This study aims to utilize linear programming (LP) techniques to evaluate the status of criterion IV of the Ramsar Convention within the Shadegan Wetland. Using mathematical language and Excel software, we defined criterion IV and developed a linear model. The Lingo software was employed to verify the model by setting constraints for proxy variables (X variables). We selected constraints based on extreme climatic conditions, such as energy and water limitations, during the study period while considering the trend of each variable. By identifying effective interventions for promoting sustainable use of the wetland while preserving its ecological balance, the LP can support the efforts to re-nominate the Shadegan Wetland as a Ramsar site. Considering the critical conditions, the lowest value of Z in the studied period unravels the critical year as the target. Based on the result, 2015 with the lowest value of the Z index (- 0.36) was identified as the critical year in the entire study period starting from 2001-2019. In the critical year itself, the population of birds equals 50,000 birds, while the average population of birds over the course of the past 20 years was nearly 37,000 birds.

External Phosphorus Loading in New Lakes

The water quality of lakes is highly dependent on external phosphorus (P) loading. The vast external loadings from sewage and other wastewater discharge that European lakes have historically received have been dramatically reduced today by improved wastewater treatment. Gaining knowledge of the catchment characteristics that influence external P-loading should enable predictions of the achievable water quality of lakes. In this study, we tested this proposition. Data from 90 new Danish lakes show no apparent relationship between the mean summer P-concentration and the size or land use of the catchments. The external P-loading and resulting annual P-concentration were further investigated on a representative subset of 12 of the new lakes, using six methods. Three of the methods used empirical estimates of P-transport from catchments, based on the national average P-transport, runoff-dependent P-transport, and crop-dependent P-transport, and the other three methods used different empirical models tested on the lakes. External P-loading was reliably predicted by several of the methods. The predictions of the annual P-concentration were highly dependent on the inclusion of annual runoff. However, the predicted P-concentrations were generally overestimated, most pronounced for the nutrient-poor and most recently established lakes. In these lakes, internal P-loading was found to be the most important factor in predicting achievable water quality.

Enhancing climate resilience of irrigated agriculture: A review

Emerging evidence showing trends in climate change with a strong likelihood those changes will continue elevates the importance of finding affordable adaptations by irrigated agriculture. Successful climate adaptation measures are needed to affordably sustain irrigated agriculture in the face of elevated carbon emissions affecting the reliability of water supplies. Numerous potential adaptation options are available for adjusting irrigated agricultural systems to implement climate risk adaptation. This work focuses on addressing the gap in the literature defined by a scarcity of reviews on measures to elevate the capacity of irrigated agriculture to enhance its climate change resilience. Accordingly, the original contribution of this work is to review the literature describing measures for enhancing climate resilience by irrigated agriculture. In addition, it describes the role of economic analysis to discover affordable measures to enhance resilience by irrigated agriculture. It achieves those aims by posing the question "What principles, practices, and recent developments are available to guide discovery of measures to improve resilience by irrigated agriculture to adapt to ongoing evidence of climate change?" It addresses that question by reviewing several risk reduction measures to control the economic cost of losses to irrigators in the face of growing water supply unreliability. Following this review, a role for optimizing a portfolio of climate adaptation measures is described, followed by a discussion of potential contributions that can be made by the use of hydroeconomic analysis. Results provide a framework for economic analysis to discover economically attractive methods to elevate resilience of irrigated agriculture.

Comparative risk assessment of phosphorus loss from "deep phosphorus stocks" in floodplain subsoils to surface waters

Phosphorus (P) loss from soil may trigger freshwater eutrophication and endanger supply with drinking water regionally. The present paper aims at encouraging discussion and development of sophisticated strategies for risk assessment of P loss from soils of riparian buffer zones (RBZ) as a prerequisite for targeted and effective mitigation of such P losses and their effects on freshwater eutrophication. We use data from a case study on RBZ soils in Germany to compare the performance of different environmental indicators of a risk for P loss from soil. Our data suggest that RBZ soils are temporarily sinks or sources for P. The spatial hotspots of P loss are the topsoils and the deep P stocks (labile P enriched in RBZ subsoils below on average 87.5 cm depth). We discuss four aspects to be considered conceptually and methodologically in the assessment of a risk for P loss from RBZ soils: (1) spatial heterogeneity and spatial bias; (2) temporal heterogeneity and temporal bias; (3) conceptual bias caused by different dynamics of individual P fractions; and (4) adequacy of threshold values. To minimize bias, we propose to assess risk for P loss from RBZ soils using a geospatial, temporally resolved sampling strategy, site-specific or regional threshold values, and a P fractionation approach. For this purpose, we introduce P_dHCl as a risk indicator, which is not susceptible to very short-term dynamics (in contrast to water-soluble P).

Dynamics of fluvial hydro-sedimentological, nutrient, particulate organic matter and effective particle size responses during the U.K. extreme wet winter of 2019–2020

The floc size distribution of suspended sediment is a critical driver for in-channel sedimentation and sediment-associated contaminant and nutrient transfer and fate in river catchments. Real-time, in situ, floc size characterisation is possible using available technology but, to date, limited high resolution floc data have been published for fluvial systems draining upland extensive grassland catchments. To that end, suspended sediment floc size distribution and turbidity were characterised at 15-minute intervals using Laser In-Situ Scattering and Transmissometry (LISST) diffraction and a YSI turbidity sonde for six storm events in the upper River Taw (15 km²) catchment in SW England. Maximum event discharges (Q) ranged between 4.3 and 20.0 m³ s⁻¹, with clockwise hysteretic responses (HI = 0.18–0.48) of total suspended solid concentrations (TSS) and Q. The sediment flushing index was highest in the early autumn (0.93) and storm event TSS fluxes varied from 0.04 to 2.9 t km⁻². This suggests a change in sources or composition of sediment during higher Q and highly variable patterns of sediment flux from event-to-event. The proportion of particulate organic matter (POM) to TSS was highly variable (5–89%) and did not increase with Q, indicating POM source limitation. The fine-grained tail (D₁₀ and D₁₆) of the floc size distributions decreased during hydrograph rising limbs, with the finest floc sizes associated with the highest TN and TP concentrations at peak Q. The results suggest that dynamic interactions between wet ground and extreme rainfall events can flush significant amounts of sediment from the relatively undisturbed extensive grassland upland catchment. We strongly encourage a sensors-based approach to reveal the spatio-temporal complexity of floc size and associated pollutant export during high Q generated by extreme rainfall since this can help to elucidate processes and mechanisms and generate high-resolution data for water quality modelling without significant user intervention.

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In situ flocs size and turbidity were studied in sequential high discharge events.

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Hydrographs characterised by clockwise hysteresis with a secondary sediment peak.

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Proportion of POM to TSS in the physical samples was highly variable (5–89%).

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Fine (D₁₀ and D₁₆) floc size diameter was lowest during peak discharges.

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Floc size exerted less influence on TP and TN concentrations.

Sediment and Nutrient Retention in Ponds on an Agricultural Stream: Evaluating Effectiveness for Diffuse Pollution Mitigation

The creation of ponds and wetlands has the potential to alleviate stream water quality impairment in catchments affected by diffuse agricultural pollution. Understanding the hydrological and biogeochemical functioning of these features is important in determining their effectiveness at mitigating pollution. This study investigated sediment and nutrient retention in three connected (on-line) ponds on a lowland headwater stream by sampling inflowing and outflowing concentrations during base and storm flows. Sediment trapping devices were used to quantify sediment and phosphorus accumulations within ponds over approximately monthly periods. The organic matter content and particle size composition of accumulated sediment were also measured. The ponds retained dissolved nitrate, soluble reactive phosphorus and suspended solids during baseflows. During small to moderate storm events, some ponds were able to reduce peak concentrations and loads of suspended solids and phosphorus; however, during large magnitude events, resuspension of deposited sediment resulted in net loss. Ponds filtered out larger particles most effectively. Between August 2019 and March 2020, the ponds accumulated 0.306 t ha−1 sediment from the 30 ha contributing area. During this period, total sediment accumulations in ponds were estimated to equal 7.6% of the suspended flux leaving the 340 ha catchment downstream. This study demonstrates the complexity of pollutant retention dynamics in on-line ponds and highlights how their effectiveness can be influenced by the timing and magnitude of events.

Spatial distribution of sediment phosphorus in a Ramsar wetland

Eutrophication is a significant threat to surface water biodiversity worldwide, with excessive phosphorus concentrations being among the most common causes. Wetland ditches under these conditions shift from primarily submerged aquatic vegetation to algae or duckweed dominance, leading to excessive shading and anoxic conditions. Phosphorus, from both point (e.g. wastewater treatment works) and diffuse (largely agricultural runoff) sources, is currently the central reason for failure in the majority of surface water bodies in England to meet required water quality guidelines. This study assesses phosphorus storage in the ditch systems at West Sedgemoor, a designated site of special scientific interest. Elevated phosphorus concentrations in sediment was observed across the Moor up to 4220 mg Kg^-1, almost 10 times that which may be expected from background levels. The highest concentrations were generally observed at the more intensively farmed sites in the north of the moor, near key inlets and the outlet. Based upon their chemical and physical properties, clear distinction was observed between sites outside and within the Royal Society of the Protection of Birds nature reserve, using principal component analysis.

Identification of Critical Source Areas of Nitrogen Load in the Miyun Reservoir Watershed under Different Hydrological Conditions

The spatiotemporal distribution of critical source areas (CSAs) will change with hydrological conditions. In this study, the CSAs of nitrogen load under different hydrological conditions in the Chaohe River watershed were identified using the cumulative pollution load curve method determined from the nitrogen pollution simulated using the Soil and Water Assessment Tool (SWAT) model. The results showed that: (1) The order of factors impacting nitrogen load intensity is as follows: fertilization intensity, rainfall, runoff, land use type, slope type, and soil type. (2) The primary and secondary CSAs are concentrated in the upper and lower areas of the watershed, where cultivated land (8.36%) and grassland (52.55%) are more abundant. The potential pollution source areas are concentrated in the upper and middle areas of the watershed, where cultivated land (6.99%), grassland (42.37%), and forest land (48.18%) are evenly distributed. The low-risk source areas are concentrated in the middle and left bank of the watershed, where forest land (67.65%) is dominant and the vegetation coverage is highest. The research results have significance for improving the accuracy of the implementation of best management practices, and can provide a reference for the formulation of drinking water protection policies for Beijing.

Functions of traditional ponds in altering sediment budgets in the hilly area of the Three Gorges Reservoir, China

The landscape pattern will affect the sediment transport process. The cluster of ponds is a common landscape, which has traditionally been used for irrigation in the hilly area of the Three Gorges Reservoir (TGR). However, little is known about how the landscape elements temporally changed over the past decades and if the ponds can be applied to function in balancing watershed sediments against soil erosion. The Jinglingxi watershed, covering 20.5 km², was selected as the study area. The changes in pond number, surface area, and drainage catchment were analyzed with aid of high-resolution typographical map and unmanned aerial vehicles imagery. The spatial WaTEM/SEDEM model was developed to simulate watershed soil erosion and sediment deposition under the absence and presence of water bodies scenarios. Results from different simulation scenarios were compared and revealed the trapping effects of the multi-pond system. From 1983 to 2016, the number and total area of ponds roughly doubled. The density reached 30 ponds/km². From 1983 to 2016, the total drainage area of ponds increased from 13.22% to 35.4% of the whole watershed. The sediments deposited at the bottom of ponds can indicate the past specific sediment yield (SSY) in drainage catchments. Our results suggest that the multi-pond system not only reduce watershed sediment export but also alter the sediment deposition in different land uses. The reduced sediments export is expected to prolong the service life of downstream reservoirs at the expectancy of ponds' storage capacities. The ecological compensation from downstream reservoirs' revenues to upstream regions should be established to drive dredging actions for the upstream ponds.

Mitigating river sediment enrichment through the construction of roadside wetlands

Metalled roads have been shown to act as a major pathway for land-to-river sediment transfer, but there currently exists limited research into mitigation solutions to tackle this pollution source. The aim of this study was to assess the effectiveness of three roadside constructed wetlands, installed in September 2016, at reducing sediment enrichment in a tributary of the River Wensum, UK. Two wetland designs were trialled (linear and 'U-shaped'), both of which act as settling ponds to encourage entrained sediment to fall out of suspension and allow cleaner water to discharge into the river. Wetland efficiency was monitored through automated, high-resolution (30 min) turbidity probes installed upstream and downstream of the wetlands, providing a near-continuous record of river turbidity before (October 2011-August 2016) and after (November 2016-February 2018) installation. This was supplemented by lower resolution monitoring of the wetland inflows and outflows, as well as an assessment of sediment and nutrient accumulation rates within the linear wetland. Results revealed median river sediment concentrations decreased up to 14% after wetland construction and sediment load decreased by up to 82%, although this was largely driven by low river discharge post-installation. Median sediment concentrations discharging from the linear wetland (7.2 mg L^-1) were higher than the U-shaped wetland (3.9 mg L^-1), confirming that a longer flow pathway through wetlands can improve sediment retention efficiency. After 12 months of operation, the linear wetland had retained 7253 kg (305 kg ha^-1 y^-1) of sediment, 11.6 kg (0.5 kg ha^-1 y^-1) of total phosphorus, 29.7 kg (1.3 kg ha^-1 y^-1) of total nitrogen and 400 kg (17 kg ha^-1 y^-1) of organic carbon. This translates into mitigated pollutant damage costs of £392 for sediment, £148 for phosphorus and £13 for nitrogen, thus giving a combined total mitigated damage cost of £553 y^-1. With the linear wetland costing £3411 to install and £145-182 y^-1 to maintain, this roadside constructed wetland has an estimated payback time of 8 years, making it a cost-effective pollution mitigation measure for tackling sediment-enriched road runoff that could be widely adopted at the catchment-scale.

Newly-established free water-surface constructed wetland to treat agricultural waters in the low-lying Venetian plain: Performance on nitrogen and phosphorus removal

Constructed wetlands offer promising solutions for controlling nutrient pollution in agricultural systems with relatively low costs and energy inputs. In mainly central and northern Italy, semi-natural and reconstructed Free-Water Surface Constructed Wetlands (FWS CWs) are designed to treat nonpoint-source pollution from agricultural catchments. However, their performance depends on system design and time of establishment. This paper evaluates the efficiency of a recently established FWS CW to remove nonpoint-source nutrient pollution due to agricultural drainage in the low-lying Venetian plain (NE Italy). The system was established in 2014 by creating five consecutive sub-basins vegetated with macrophytes to restore a semi-natural wetland, and later monitored in terms of water quality parameters and nutrients removal over three consecutive agricultural seasons (2014-2016). Total (TN) and nitrate (N-NO₃) nitrogen concentrations showed peaks (16.37 and 15.31 mg l^-1 for TN and N-NO₃, respectively) in the various sub-basins during spring 2015, associated with fertilisation of surrounding croplands and intense rain events. Performance improved over the three years, with increasing median removals of TN (33.3-49.0%) and N-NO₃ (32.2-80.5%), corresponding to average mass of 1355 kg y^-1 and 1011 kg y^-1 for TN and N-NO₃. Concentrations of ammonium (N-NH₄) and orthophosphate (P-PO₄) were generally low (<1 and <0.3 mg l^-1 for N-NH₄ and P-PO₄, respectively), with average yearly mass removals of 50 kg for N-NH₄ and 9 kg for P-PO₄. According to the overall treatment performance, the FWS CW could treat a total area of about 30 ha with a wetland/catchment ratio of 7%. However, we expect that treatment efficiency will increase as a result of bank stabilisation and improvement of the aquatic environment, together with increases in surface vegetation.

Sediment and nutrient storage in a beaver engineered wetland

Beavers, primarily through the building of dams, can deliver significant geomorphic modifications and result in changes to nutrient and sediment fluxes. Research is required to understand the implications and possible benefits of widespread beaver reintroduction across Europe. This study surveyed sediment depth, extent and carbon/nitrogen content in a sequence of beaver pond and dam structures in South West England, where a pair of Eurasian beavers (Castor fiber) were introduced to a controlled 1.8 ha site in 2011. Results showed that the 13 beaver ponds subsequently created hold a total of 101.53 ± 16.24 t of sediment, equating to a normalised average of 71.40 ± 39.65 kg m2. The ponds also hold 15.90 ± 2.50 t of carbon and 0.91 ± 0.15 t of nitrogen within the accumulated pond sediment. The size of beaver pond appeared to be the main control over sediment storage, with larger ponds holding a greater mass of sediment per unit area. Furthermore, position within the site appeared to play a role with the upper-middle ponds, nearest to the intensively-farmed headwaters of the catchment, holding a greater amount of sediment. Carbon and nitrogen concentrations in ponds showed no clear trends, but were significantly higher than in stream bed sediment upstream of the site. We estimate that >70% of sediment in the ponds is sourced from the intensively managed grassland catchment upstream, with the remainder from in situ redistribution by beaver activity. While further research is required into the long-term storage and nutrient cycling within beaver ponds, results indicate that beaver ponds may help to mitigate the negative off-site impacts of accelerated soil erosion and diffuse pollution from agriculturally dominated landscapes such as the intensively managed grassland in this study. © 2018 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.

Assessing the potential impacts of a revised set of on-farm nutrient and sediment 'basic' control measures for reducing agricultural diffuse pollution across England

The need for improved abatement of agricultural diffuse water pollution represents cause for concern throughout the world. A critical aspect in the design of on-farm intervention programmes concerns the potential technical cost-effectiveness of packages of control measures. The European Union (EU) Water Framework Directive (WFD) calls for Programmes of Measures (PoMs) to protect freshwater environments and these comprise 'basic' (mandatory) and 'supplementary' (incentivised) options. Recent work has used measure review, elicitation of stakeholder attitudes and a process-based modelling framework to identify a new alternative set of 'basic' agricultural sector control measures for nutrient and sediment abatement across England. Following an initial scientific review of 708 measures, 90 were identified for further consideration at an industry workshop and 63 had industry support. Optimisation modelling was undertaken to identify a shortlist of measures using the Demonstration Test Catchments as sentinel agricultural landscapes. Optimisation selected 12 measures relevant to livestock or arable systems. Model simulations of 95% implementation of these 12 candidate 'basic' measures, in addition to business-as-usual, suggested reductions in the national agricultural nitrate load of 2.5%, whilst corresponding reductions in phosphorus and sediment were 11.9% and 5.6%, respectively. The total cost of applying the candidate 'basic' measures across the whole of England was estimated to be £450 million per annum, which is equivalent to £52 per hectare of agricultural land. This work contributed to a public consultation in 2016.

Population stress: A spatiotemporal analysis of population change and land development at the county level in the contiguous United States, 2001-2011

The past century has witnessed rapidly increasing population-land conflicts due to exponential population growth and its many consequences. Although the measures of population-land conflicts are many, there lacks a model that appropriately considers both the social and physical contexts of population-land conflicts. In this study we introduce the concept of population stress, which identifies areas with populations growing faster than the lands available for sustainable development. Specifically, population stress areas are identified by comparing population growth and land development as measured by land developability in the contiguous United States from 2001 to 2011. Our approach is based on a combination of spatial multicriteria analysis, zonal statistics, and spatiotemporal modeling. We found that the population growth of a county is associated with the decrease of land developability, along with the spatial influences of surrounding counties. The Midwest and the traditional "Deep South" counties would have less population stress with future land development, whereas the Southeast Coast, Washington State, Northern Texas, and the Southwest would face more stress due to population growth that is faster than the loss of suitable lands for development. The factors contributing to population stress may differ from place to place. Our population stress concept is useful and innovative for understanding population stress due to land development and can be applied to other regions as well as global research. It can act as a basis towards developing coherent sustainable land use policies. Coordination among local governments and across different levels of governments in the twenty-first century is a must for effective land use planning.

A tool for cost-effectiveness analysis of field scale sediment-bound phosphorus mitigation measures and application to analysis of spatial and temporal targeting in the Lunan Water catchment, Scotland

The cost-effectiveness of six edge-of-field measures for mitigating diffuse pollution from sediment bound phosphorus (P) runoff from temperate arable farmland is analysed at catchment/field scales. These measures were: buffer strips, permanent grassland in the lowest 7% of arable fields, dry detention bunds, wetlands, and temporary barriers such as sediment fences. Baseline field P export was estimated using export coefficients (low risk crops) or a modified Universal Soil Loss Equation (high risk crops). The impact of measures was estimated using simple equations. Costs were estimated from gross margin losses or local data on grants. We used a net cost:benefit (NCB) factor to normalise the costs and impacts of each measure over time. Costs minimisation for target impact was done using PuLP, a linear programming module for Python, across 1634 riparian and non-riparian fields in the Lunan Water, a mixed arable catchment in Eastern Scotland. With all measures in place, average cost-effectiveness increases from £9 to £48/kg P as target P mitigation increases from 500 to 2500kg P across the catchment. Costs increase significantly when the measures available are restricted only to those currently eligible for government grants (buffers, bunds and wetlands). The assumed orientation of the average field slope makes a strong difference to the potential for storage of water by bunds and overall cost-effectiveness, but the non-funded measures can substitute for the extra expense incurred by bunds, where the slope orientation is not suitable. Economic discounting over time of impacts and costs of measures favours those measures, such as sediment fences, which are strongly targeted both spatially and temporally. This tool could be a useful guide for dialogue with land users about the potential fields to target for mitigation to achieve catchment targets.

Effects of land disturbance on runoff and sediment yield after natural rainfall events in southwestern China

Severe soil erosion occurs in southwestern China owing to the large expanses of urbanization and sloping land. This field monitoring study was conducted to record the rainfall events, runoff, and sediment yield in 20-, 40-, and 60-m plots under conditions of artificial disturbance or natural restoration in the purple soil area of southwestern China. The study took place during the rainy season, and the plots were situated on a 15° slope. The results showed that rainstorms and heavy rainstorms generated runoff and sediment yield. Rainfall intensity had a significantly positive power relationship with runoff rate and sediment yield rate in artificially disturbed plots but not in naturally restored plots. Plot length had a significant effect on runoff rate under artificial disturbance but not natural restoration. Within the same land disturbance category, there was no significant effect of plot length on sediment yield rate but there was a significant effect on sediment concentration. Overall, runoff rate, sediment yield rate, and sediment concentration showed remarkable effects of land disturbance across all plot lengths: naturally restored plots had 62.8-77.5% less runoff, 95.1-96.3% less sediment yield, and 63.1-73.5% lower sediment concentration than artificially disturbed plots. The relationship between runoff rate and sediment rate under the different land disturbances could be described by an exponential function. The results not only demonstrate the effectiveness of natural restoration for controlling runoff and sediment yield but also provide useful information for the design of field studies, taking into consideration the complexity of terrestrial systems.

Improving the identification of hydrologically sensitive areas using LiDAR DEMs for the delineation and mitigation of critical source areas of diffuse pollution

Identifying critical source areas (CSAs) of diffuse pollution in agricultural catchments requires the accurate identification of hydrologically sensitive areas (HSAs) at highest propensity for generating surface runoff and transporting pollutants. A new GIS-based HSA Index is presented that improves the identification of HSAs at the sub-field scale by accounting for microtopographic controls. The Index is based on high resolution LiDAR data and a soil topographic index (STI) and also considers the hydrological disconnection of overland flow via topographic impediment from flow sinks. The HSA Index was applied to four intensive agricultural catchments (~7.5-12km(2)) with contrasting topography and soil types, and validated using rainfall-quickflow measurements during saturated winter storm events in 2009-2014. Total flow sink volume capacities ranged from 8298 to 59,584m(3) and caused 8.5-24.2% of overland-flow-generating-areas and 16.8-33.4% of catchment areas to become hydrologically disconnected from the open drainage channel network. HSA maps identified 'breakthrough points' and 'delivery points' along surface runoff pathways as vulnerable points where diffuse pollutants could be transported between fields or delivered to the open drainage network, respectively. Using these as proposed locations for targeting mitigation measures such as riparian buffer strips reduced potential costs compared to blanket implementation within an example agri-environment scheme by 66% and 91% over 1 and 5years respectively, which included LiDAR DEM acquisition costs. The HSA Index can be used as a hydrologically realistic transport component within a fully evolved sub-field scale CSA model, and can also be used to guide the implementation of 'treatment-train' mitigation strategies concurrent with sustainable agricultural intensification.

Combined impacts of precipitation and temperature on diffuse phosphorus pollution loading and critical source area identification in a freeze-thaw area

The loss of diffuse phosphorus (P) presented different characteristics in the freeze-thaw area due to the combined impacts of precipitation and temperature, which caused spatiotemporal variations of the critical source area of diffuse P (CSAP). The temperature and precipitation classification (TPC) method was proposed to identify the spatiotemporal characteristics of the CSAP in the cold area, and each year was divided into a freeze-thaw season and a growing season according to the average monthly temperature. The Soil and Water Assessment Tool (SWAT) provided the spatiotemporal patterns of the diffuse P loads. The years were also reclassified into dry, normal and wet years according to the annual precipitation levels. The CSAP with the 1st cumulative load level shared 9.68% of the same area between the two seasons, which had dry land as the dominant land use with direct P fertilization. The spatial distributions of the potential areas and the CSAP with the 2nd cumulative load level were more sensitive to the variation in temperature, which had 30.8%-46.1% of unvaried area between seasons. The cumulative load level analysis indicated that 14 subbasins in the freeze-thaw season and 7 subbasins in the growing season, which covered 61.2% and 48.6% of the total basin area, respectively, changed with the traditional CSAP identification among dry, normal and wet years. The fluctuation level analysis was carried out to compare the distributional difference of the CSAP and the potential areas between the TPC method and the traditional method, which highlighted the advantages of the TPC method. The results would be useful in identifying the distribution of the CSAP in cold areas, which improved the efficiency of diffuse pollution control.

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.

Reduced nutrient pollution in a rural stream following septic tank upgrade and installation of runoff retention measures

Surface water quality in the UK and much of Western Europe has improved in recent decades, in response to better point source controls and the regulation of fertilizer, manure and slurry use. However, diffuse sources of pollution, such as leaching or runoff of nutrients from agricultural fields, and micro-point sources including farmyards, manure heaps and septic tank sewerage systems, particularly systems without soil adsorption beds, are now hypothesised to contribute a significant proportion of the nutrients delivered to surface watercourses. Tackling such sources in an integrated manner is vital, if improvements in freshwater quality are to continue. In this research, we consider the combined effect of constructing small field wetlands and improving a septic tank system on stream water quality within an agricultural catchment in Cumbria, UK. Water quality in the ditch-wetland system was monitored by manual sampling at fortnightly intervals (April-October 2011 and February-October 2012), with the septic tank improvement taking place in February 2012. Reductions in nutrient concentrations were observed through the catchment, by up to 60% when considering total phosphorus (TP) entering and leaving a wetland with a long residence time. Average fluxes of TP, soluble reactive phosphorus (SRP) and ammonium-N (NH4-N) at the head of the ditch system in 2011 (before septic tank improvement) compared to 2012 (after septic tank improvement) were reduced by 28%, 9% and 37% respectively. However, TP concentration data continue to show a clear dilution with increasing flow, indicating that the system remained point source dominated even after the septic tank improvement.