Critical Source Areas Research Articles

Projections of Watershed Pollutant Loads Using a Spatially Explicit, Agent-Based Land Use Conversion Model: A Case Study of Berkeley County, West Virginia

This research presents a methodology to make projections of land use conversions in Berkeley County, West Virginia and then utilizes these projections to estimate water quality impacts on the Opequon Creek in Berkeley County. Empirical estimates for factors that influence the land use conversion probability are captured using parameters from a spatial logistic regression (SLR) model. Then, an agent-based, probabilistic land use conversion (APLUC) model is used to explore the impacts of policies on land use conversion decisions using estimates from actual land use change from 2001 to 2011 in SLR model. Three policy scenarios are developed: (1) no policy implementation, (2) a 15.24 m (50 ft) buffer zone policy of no development applied to all streams, and (3) 15.24 m buffer policy applied only on critical source area (CSA) watersheds. The projected land use patterns in the APLUC model are driven by individual land conversion decisions over 50 model runs of 10 iterations each under each policy scenario. The results show that with no policy scenario, most conversions occurred near existing residential land use and urban centers. Residential land use conversions are greatly reduced in a 15.24 m buffer policy around all streams in watershed. Spatial patterns generated under a 15.24 m buffer policy in CSAs only showed that future projected land use changes occurred close to major highways and shifted the residential development to the northern part of the Opequon Creek. Finally, the impacts of these three policies on water quality are estimated using an ArcSWAT model, a graphical user interface for SWAT (Soil and Water Assessment Tool). This model indicates that the 15.24 m buffer policy in CSAs is most effective among the three policies in reducing the pollutant loads. This study suggests that carefully designed policies which discourage residential land use conversions in CSAs, result in less pollutant loads by shifting the location of residential conversions to less critical areas where agricultural land is dominant in the watershed.

Effect of best management practice implementation on sediment and phosphorus load reductions at subwatershed and watershed scale using SWAT model

Identification of areas contributing disproportionately high amount of pollutants (i.e., critical source areas (CSAs)) to streams is important to efficiently and effectively target best management practices (BMPs). Process-based models are commonly used to identify CSAs and evaluate the impact of alternative management practices on pollutant load reductions. The objective of this study was to use the Soil and Watershed Assessment Tool (SWAT) to identify CSAs at the subwatershed level and evaluate the impact of alternative BMPs on sediment and total phosphorus (TP) load reductions in the Pleasant Valley watershed (50km2) in South Central Wisconsin (USA). The Nash-Sutcliffe efficiency, percent bias, and coefficient of determination ranged from 0.58 to 0.71, −12.87 to 38.33, and 0.67 to 0.79, respectively, indicating that SWAT was able to predict stream flow, sediment and TP loadings at a monthly time-step with sufficient accuracy. Based on the SWAT simulation results, annual average (2006–2012) subwatershed yield for sediment and TP ranged from 0.06 to 3.14tonsha−1yr−1 and 0.04 to 1.9kgha−1yr−1, respectively. The croplands were the major source of sediment and TP in this watershed (≥84%). Reduction in sediment and TP loading ranged from 66% to 99% at the subwatershed level after conversion of croplands to Conservation Reserve Program (CRP) grasslands in subwatersheds identified as CSAs. On the other hand, reduction in sediment and TP loading with implementation of no-till practices ranged from only 14% to 25%. At the watershed outlet, sediment and TP loading reduction was ≤15% after conversion of croplands to CRP grasslands and implementation of no-till practices because only about 8% of the watershed area was targeted for BMPs and/or resuspension of sediment deposited on the stream bed masked the downstream improvements in water quality.

Current patterns and future perspectives of best management practices research: A bibliometric analysis

Best management practices (BMPs), as pollution control systems to improve the water environment, were proposed by the Federal Water Pollution Control Act in 1972 (Ice et al. 2010). Best management practices were initially designed for the purpose of controlling soil erosion (Logan 1993). As nonpoint source (NPS) pollution has caused major water quality degradation and threatened the safety of water resources worldwide, BMPs have been recommended as an effective tool used widely to reduce NPS pollution (D'Arcy and Frost 2001; Giri et al. 2012; Ice 2004). Countries, including the United States and the United Kingdom, first initiated BMPs and have had successful experience in agricultural NPS pollution control since the 1970s (Krivak 1978; Mueller et al. 1981). Over the span of a few decades, the application of BMPs has accelerated, and new practices (e.g., stormwater management, watershed management, low impact development, and critical source areas [CSAs]) have been continuously put forward (Martin-Mikle et al. 2015; Niraula et al. 2013; Sage et al. 2015; Zhang et al. 2015). In recent years, several in-depth studies of BMPs have been carried out, such as the Conservation Effect Assessment Project in the United States (Simon and Klimetz 2008), the Watershed Evaluation of Beneficial Management…

USING REMOTE SENSING AND GIS TECHNIQUES TO DETECT CHANGES TO THE PRINCE ALFRED HAMLET CONSERVATION AREA IN THE WESTERN CAPE, SOUTH AFRICA

Understanding and identifying the spatial-temporal changes in the natural environment is crucial for monitoring and evaluating conservation efforts, as well as understanding the impact of human activities on natural resources, informing responsible land management, and promoting better decision-making. Conservation areas are often under pressure from expanding farming and related industry, invasive alien vegetation, and an ever-increasing human settlement footprint. This study focuses on detecting changes to the Prince Alfred Hamlet commonage, near Ceres in the Cape Floral Kingdom. It was chosen for its high conservation value and significance as a critical water source area. The study area includes a fast-growing human settlement footprint in a highly productive farming landscape. There are conflicting development needs as well as risks to agricultural production, and both of these threaten the integrity of the ecosystems which supply underlying services to both demands on the land. Using a multi-disciplinary approach and high-resolution satellite imagery, land use and land cover changes can be detected and classified, and the results used to support the conservation of biodiversity and wildlife, and protect our natural resources. The aim of this research is to study the efficacy of using remote sensing and GIS techniques to detect changes to critical conservation areas where disturbances can be understood, and therefore better managed and mitigated before these areas are degraded beyond repair.

USING REMOTE SENSING AND GIS TECHNIQUES TO DETECT CHANGES TO THE PRINCE ALFRED HAMLET CONSERVATION AREA IN THE WESTERN CAPE, SOUTH AFRICA

Abstract. Understanding and identifying the spatial-temporal changes in the natural environment is crucial for monitoring and evaluating conservation efforts, as well as understanding the impact of human activities on natural resources, informing responsible land management, and promoting better decision-making. Conservation areas are often under pressure from expanding farming and related industry, invasive alien vegetation, and an ever-increasing human settlement footprint. This study focuses on detecting changes to the Prince Alfred Hamlet commonage, near Ceres in the Cape Floral Kingdom. It was chosen for its high conservation value and significance as a critical water source area. The study area includes a fast-growing human settlement footprint in a highly productive farming landscape. There are conflicting development needs as well as risks to agricultural production, and both of these threaten the integrity of the ecosystems which supply underlying services to both demands on the land. Using a multi-disciplinary approach and high-resolution satellite imagery, land use and land cover changes can be detected and classified, and the results used to support the conservation of biodiversity and wildlife, and protect our natural resources. The aim of this research is to study the efficacy of using remote sensing and GIS techniques to detect changes to critical conservation areas where disturbances can be understood, and therefore better managed and mitigated before these areas are degraded beyond repair.

Impacts of DEM uncertainties on critical source areas identification for non-point source pollution control based on SWAT model

The impacts of different digital elevation model (DEM) resolutions, sources and resampling techniques on nutrient simulations using the Soil and Water Assessment Tool (SWAT) model have not been well studied. The objective of this study was to evaluate the sensitivities of DEM resolutions (from 30m to 1000m), sources (ASTER GDEM2, SRTM and Topo-DEM) and resampling techniques (nearest neighbor, bilinear interpolation, cubic convolution and majority) to identification of non-point source (NPS) critical source area (CSA) based on nutrient loads using the SWAT model. The Xiangxi River, one of the main tributaries of Three Gorges Reservoir in China, was selected as the study area. The following findings were obtained: (1) Elevation and slope extracted from the DEMs were more sensitive to DEM resolution changes. Compared with the results of the 30m DEM, 1000m DEM underestimated the elevation and slope by 104m and 41.57°, respectively; (2) The numbers of subwatersheds and hydrologic response units (HRUs) were considerably influenced by DEM resolutions, but the total nitrogen (TN) and total phosphorus (TP) loads of each subwatershed showed higher correlations with different DEM sources; (3) DEM resolutions and sources had larger effects on CSAs identifications, while TN and TP CSAs showed different response to DEM uncertainties. TN CSAs were more sensitive to resolution changes, exhibiting six distribution patterns at all DEM resolutions. TP CSAs were sensitive to source and resampling technique changes, exhibiting three distribution patterns for DEM sources and two distribution patterns for DEM resampling techniques. DEM resolutions and sources are the two most sensitive SWAT model DEM parameters that must be considered when nutrient CSAs are identified.

Estimating the effects of land use at different scales on high ecological status in Irish rivers

High ecological status at river sites is an indicator of minimal disturbance from anthropogenic activities and the presence of ecologically important species and communities. However, a lack of clarity on what factors cause sites to lose high ecological status is limiting the ability to maintain the quality of these sites. Examination of ecological status records at 508 high status river sites throughout the Republic of Ireland revealed that 337 had fallen below high status at some point between 2001 and 2012 due to changes in invertebrate communities. A geographical information system was used to characterise land use and environmental variables in the catchment, riparian and reach areas upstream of the sites. The relationships between these variables at the three spatial scales and whether or not river sites had maintained high ecological status were then estimated by multiple logistic regression and propensity modelling. The results indicated that grassland at either catchment or riparian scales had a greater negative impact on high ecological status than at the reach scale. This effect appeared to be strongest for upland, steeply sloping rivers that are subject to high rainfall, possibly due to the presence of sensitive biota and/or a greater potential for erosion. These results highlighted the need for better management of grassland upstream of the high status sites, with a focus on river alterations and critical source areas of nutrients, sediments and pesticides that are hydrologically connected to the river. Sustainable management practices and land use planning in those areas will need to be considered carefully if the aim of maintaining high ecological status at river sites is to be achieved.

Spatiotemporal modeling of watershed nutrient transport dynamics: Implications for eutrophication abatement

The main objective of this study was to quantify nutrient transport dynamics of a previously ungauged, temperate watershed (145km2) surrounding a shallow eutrophic lake and discern lake response to external nutrient loading, based on soil water assessment tool (SWAT) and the Organization of Economic Cooperation and Development (OECD) empirical lake models, respectively. A SWAT model was used to simulate baseline nutrient dynamics after its calibration and validation against daily tributary flow, total dissolved phosphorus (TDP), total phosphorus (TP), and nitrate (NO3) loads. On the watershed scale, median annual TDP, TP, and NO3 losses were 0.4, 1.1, and 2.0kgha−1, respectively. The highest median annual TP and NO3 losses were estimated at 3.7 and 7.7kgha−1 for pastureland and 1.7 and 3.8kgha−1 for cropland and mixed forests, respectively. Baseflow was the major nutrient transport pathway over a wide range of precipitation events (450 to 900mmyr−1). Erosion was the predominant surface process exporting P across the watershed. Critical source areas (CSAs) of TP and NO3 comprised 17% and 4% of the watershed, respectively. Annual mean TP, and mean and maximum chlorophyll content indicated a hyper-eutrophication risk for the lake. An external P load reduction by excess of 80% could be necessary to restore mesotrophy in the lake. Our results suggested that subsurface P transport should not be overlooked a priori when groundwater-dependent and extensively farmed watersheds are managed for eutrophication abatement.

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.

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 (~7.5–12km2) 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 59,584m3 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.

Evaluating Phosphorus Loss for Watershed Management: Integrating a Weighting Scheme of Watershed Heterogeneity into Export Coefficient Model

The identification of critical source areas (CSAs) and critical source periods (CSPs) are essential prerequisites for cost-effective practices of non-point source (NPS) pollution control. A simple empirical tool combining Export Coefficient Model (ECM) and a Geographic Information Systems (GIS)-based weighting scheme of watershed heterogeneity was proposed to estimate annual and monthly phosphorus loss, to identify critical source areas and periods, and to assess pollution control practices. The GIS-based weighting scheme was developed to represent the transport potential of runoff to move phosphorus from the land surfaces to waters, as a supplement to the source-based ECM. The empirical tool was applied to the Dianchi Lake watershed of China. The results showed that the total phosphorus loss from NPS in 2008 was 352.3 tons. The agricultural land was recognized as the largest and the most spatially various source type. The lakeside plain and the terraces of the watershed were identified as CSAs, which generated more than 90 % of non-point phosphorus. The early part of wet season (from May to August) was the CSPs, when about 70 % of non-point phosphorus was lost. The reduction of phosphorus fertilizers and the vegetated buffer strips (VBS) were effective in controlling phosphorus loss from NPS in the watershed. A reduction of 20 % in phosphorus fertilizer application combined with the set-up of VBS in both riparian area of the main watercourses and the lakeside areas would decrease 25 % of phosphorus loss.

RUSLE and SDR Model Based Sediment Yield Assessment in a GIS and Remote Sensing Environment; A Case Study of Koga Watershed, Upper Blue Nile Basin, Ethiopia

Soil erosion and the subsequent sedimentation are the major watershed problems in Ethiopia. Removal of top fertile soil, siltation of Koga irrigation reservoir, clogging of irrigation canal by sediment and reduction of irrigated land are the major threat of Koga watershed. Hence, this study was attempted to assess and map the spatial distribution of sediment yield of Koga watershed in a GIS and remote sensing environment. Sediment yield is dependent on factors of soil erosion such as rainfall erosivity, soil erodibilty, land use land cover (C and P) and topography (LS) and sediment delivery ratio of the drainage basin to the total amount of sediment yield by sheet and channel erosion. RUSLE framed with GIS and Remote sensing technique was therefore employed to assess the amount of soil loss existed in KW. Main stream channel slope based sediment delivery ratio analysis was also carried out. Soil map (1:250,000), Aster DEM (30 × 30 m), Thematic Mapper (TM) image (30 m × 30 m) of the year 2013, thirteen years (2000-2013) rainfall records from four rain gauge stations and topographic map (1:50,000) were the major data used. The estimated mean annual SY delivered to the out let of KW was found to be 25 t ha-1year-1. Most critical sediment source areas are situated in the steepest upper part of the watershed due to very high computed soil loss and sediment delivery ratio in this part. It could be therefore difficult to attain the intended goal of Koga irrigation reservoir positioned at lower part of the watershed. Sustainable land management practices have to be conducted in the upper part of the watershed by taking each stream order as a management unit to increase the storage capacity, and/or lessen the transportation capacity of the watershed. Proper drainage construction and stream bank stabilization via vegetative cover have to widely implement to safely dispose the eroded sediment.

A review of contaminant losses to water from pastoral hill lands and mitigation options

Pastoral hill lands deliver a range of contaminants to receiving environments that are of concern to the wider sector stakeholder community: principally sediment, phosphorus, nitrogen and faecal microorganisms. Thermal energy may also be considered a contaminant. Pastoral waterways generally have higher concentrations of suspended sediments, nutrients, faecal micro-organisms, and water temperature relative to forested waterways. These effects can be quantitatively linked to animal stocking rates and management. The large variation in the micro-climates, parent materials, soil types and vegetation resources inherent in hill country is the major driver of spatial and temporal dynamics of contaminant losses. This variation is modified by animal behaviour and physiology. Stores of contaminants in surface or sub-surface flow paths create important temporal lags resulting from land use and management change. The concept of critical source areas has become a key focus for the development of mitigation options. A wide range of biophysical options are now available, covering multiple scales and levels of cost-benefit. The use of farm planning tools is critical in balancing the implementation of mitigations with farm system objectives to improve whole-system sustainability. More research is needed on long-term impacts, given spatial and temporal variation in drivers and known spatial and temporal lag effects. There will be ongoing demand for mitigations that have been developed through co-innovation processes. Keywords: environmental mitigations, erosion, hill country, nutrient loss, pastoral, sediment export

Are Floodplain Soils a Potential Phosphorus Source When Inundated That Can Be Effectively Managed?

Core Ideas Dissolved phosphorus is released from floodplain soils when inundated. Phosphorus release rates are positively correlated with soil test and water‐extractable P in soil. Water treatment residuals decrease P release rates, providing a potential mitigation option. The growing concern over phosphorus (P) and water quality has led to questions as to where the loads originate and how to control landscape inputs from the watershed. We collected soil cores from two locations in the Illinois River watershed to examine the relationship between stored soil P and the amount of soluble reactive P (SRP) released into the water when soils are flooded. After inundation, P flux calculations for durations of ∼24 h ranged from less than 0.1 to 9.3 mg SRP m−2 h−1. Soil test P (STP), measured as water‐extractable P and Mehlich‐III P, correlated with SRP flux, where higher STP resulted in increased P flux to the overlying water. Floodplain soils have the potential to be a P source, potentially releasing more than 1600 mg m−2 yr−1 (>16 kg ha−1 yr−1) depending on length and frequency of inundation and on STP content. Applying water treatment residuals (WTRs) significantly reduced P flux from the cores. Hence, WTR application to critical source areas provides a mitigation strategy to manage floodplain soils and potential SRP release.

Antecedent conditions, hydrological connectivity and anthropogenic inputs: Factors affecting nitrate and phosphorus transfers to agricultural headwater streams

This paper examines relationships between rainfall–runoff, catchment connectivity, antecedent moisture conditions and fertiliser application with nitrate-N and total phosphorus (TP) fluxes in an arable headwater catchment over three hydrological years (2012–2014). Annual precipitation totals did not vary substantially between years, yet the timing of rainfall strongly influenced runoff generation and subsequent nitrate-N and TP fluxes. The greatest nitrate-N (>250kgNday−1) and TP (>10kgTPday−1) fluxes only occurred when shallow groundwater was within 0.6m of the ground surface and runoff coefficients were greater than 0.1. These thresholds were reached less frequently in 2012 due to drought recovery resulting in lower annual nitrate-N (7.4kgNha−1) and TP (0.12kgPha−1) fluxes in comparison with 2013 (15.1kgNha−1; 0.21kgPha−1). The wet winter of 2013 with elevated shallow groundwater levels led to more frequent activation of sub-surface pathways and tile drain flow. Throughout the period, dry antecedent conditions had a temporary effect in elevating TP loads. Evidence of TP source exhaustion after consecutive storm events can be attributed to the repeated depletion of temporarily connected critical source areas to the river network via impermeable road surfaces. Fertiliser application varied considerably across three years due to differences in crop rotation between farms, with annual N and P fertiliser inputs varying by up to 21% and 41%, respectively. Proportional reductions in annual riverine nitrate-N and TP loadings were not observed at the sub-catchment outlet as loadings were largely influenced by annual runoff. Nitrate loadings were slightly higher during fertiliser application, but there was little relationship between P fertiliser application and riverine TP load. These data indicate that this intensive arable catchment may be in a state of biogeochemical stationarity, whereby legacy stores of nutrients buffer against changes in contemporary nutrient inputs.

Identifying non-point source critical source areas based on multi-factors at a basin scale with SWAT

The identification of critical source areas (CSAs) is a precondition for non-point source (NPS) pollution control at a basin scale, especially in areas with limited resources. Based on the Soil and Water Assessment Tool (SWAT), nutrient loads coupled with population density and water quality requirements are regarded as multi-factors for CSAs identification in Xiangxi river watershed, the first tributary of the Yangtze River. The results based on the calibrated model found that the subbasins heavily and seriously polluted by nutrient loads were different from the subbasins identified as CSAs, demonstrating integrating socio-economic factors like population density and water quality requirements to identify CSAs is of much necessity. The CSAs occupied 19.7% of the total subbasins, and accounted for 53% total nitrogen loads, 54% total phosphorus loads and 36% of the total population. Considering the model calibration and validation will take a long time as well as data deficiency in some subbasins, the influence of uncalibrated SWAT on CSAs identifications was discussed. The comparative results between CSAs identification with calibrated and uncalibrated SWAT model revealed that model calibration had little effect on nutrients distribution and CSAs locations in the study area. Uncalibrated SWAT model may be applied when the research objective is less related to model calibration. The results will be greatly effective for CSAs identification and NPS pollution control at a basin scale.

Modeling and Evaluating of Non‐Point Source Pollution in a Semi‐Arid Watershed: Implications for Watershed Management

Watershed models are cost‐effective and powerful tools for evaluating and controlling non‐point source (NPS) pollution. Generalized Watershed Loading Functions (GWLF) model, a relative parsimonious model, was applied to simulate watershed streamflow and nutrient loads in Liu River watershed, northern China. The model results were used to evaluate watershed NPS load, and the implications for NPS pollution mitigation measures were investigated. The temporal‐spatial distribution, the critical source areas (CSAs) at sub‐watershed level, and the best management practices (BMPs) efficiencies at CSAs were investigated by GWLF model. Due to the contrasting hydrological conditions, the NPS loads of wet year and flood season were much more than those of dry year and non‐flood season. Four sub‐watersheds were identified as CSAs, and the results showed that CSAs had high level of load intensities due to the high percentage of agricultural land and residential land. Through evaluating the efficiency and cost of seven BMPs, conservation tillage and vegetative buffers were selected as the measures to implement in the CSAs, and they can reduce 14.2% and 12.5% of total nitrogen and total phosphorus NPS loads in Liu river watershed. These findings can provide valuable information for implementing more effective and reasonable management measures to control NPS pollution in Liu River watershed and indicate the utility of GWLF with modest data requirements for such application.

The use of a mass balance phosphorus budget for informing nutrient management in shallow coastal lakes

Eutrophication has degraded ecosystem, cultural and recreational values in Lake Forsyth, a small, shallow, coastal lake in New Zealand. To inform catchment management decisions designed to prevent algal blooms and improve water quality, a sub-catchment scale, mass-balance approach to understanding the behaviour of the critical nutrient, phosphorous (P), has been taken. To determine a P budget for the lake, and identify key P reservoirs, hydrological inflows and outflows were measured over a 15 month period. These were combined with total (TP) and dissolved reactive P (DRP) concentrations in these flows, to determine the external load of P transported to the lake. Lake water was also analysed for TP and DRP concentrations, and chemical extractions were used to determine the mass and mobility of P in the lake sediments. Biomass surveys and chemical digestions were used to quantify the mass of P contained in lake macrophytes. Changes in the lake water P reservoirs were then used to assess the contribution of external P loading relative to fluxes of P from the sediment to the lake water column (internal P loading). More than 7000 kg P per year was delivered to the lake, 68% of which came from a single sub-catchment. P associated with suspended particulate material accounted for 80% of the external P load transported into the lake and 61% of the load delivered over the study period was transported during a single flood event. A reduction of 53% in the external P load is necessary to achieve a recommended areal loading guideline. As the lake has no permanent outflow, this external load and the low flushing rates have created a large legacy reservoir of P in the lake sediments with 70% of external P loads retained in the lake. It is the release of P from these lake sediments rather than fluctuations in external loading that control P concentrations in the lake water column during the blooms of nitrogen-fixing cyanobacteria. The results indicate the importance of targeting both external and internal loading processes in the catchment. The sub-catchment scale, mass-balance approach to determining a P budget and quantifying P reservoirs enable critical source areas for external P loads to be identified, and the potential efficacy of targeted interventions to reduce P sources and minimise P transport, such as wetlands and sediment retention basins, to be assessed.

A review of the policies and implementation of practices to decrease water quality impairment by phosphorus in New Zealand, the UK, and the US

The improper use of phosphorus (P) on agricultural land in developed countries is related to P losses that impair surface water quality. We outline policy in New Zealand, the UK, and the US who have imposed limits for P measured as ecological status, but in some cases, also as chemical concentrations or loads. We contrast the strategies used in each country and discuss their likelihood of being able to decrease P losses and improve surface water quality. All three countries have focused on understanding pathways and catchment processes so that cause and effect can be traced across spatial and temporal scales. A poor understanding of catchment processes and critical source areas of P loss has resulted in some areas where regulation has had minimal effect on P discharges. Furthermore, while biophysical science can inform policy, we give several examples where social and economic challenges are of equal if not greater relevance to P discharges (e.g. subsidies). Some evidence shows that these challenges can be overcome at the farm to small catchment scale with a mix of mandatory and voluntary rules in targeted areas. Other policy instruments (e.g. trading schemes) may be needed at larger scales, but should be flexible and encourage innovation over a culture of dependence. There is increasing recognition among all three countries that while targeting good management practices can substantially decrease P losses from existing land use, to achieve ‘good’ water quality in catchment, policy may have to consider land use change.

Spatial discretization of large watersheds and its influence on the estimation of hillslope sediment yield

The combined use of water erosion models and geographic information systems has facilitated soil loss estimation at the watershed scale. Tools such as the Geo-spatial interface for the Water Erosion Prediction Project (GeoWEPP) model provide a convenient spatially distributed soil loss estimate but require discretization to identify hillslopes and channels. In GeoWEPP, the TOpographic PArameteriZation (TOPAZ) model is used as an automated procedure to extract a watershed boundary, hillslopes and channels from a digital elevation model (DEM). Previous studies in small watersheds have shown that the size of the hillslopes and the channel distribution affect the model estimates, but in large watersheds, the effects on the soil loss estimates have yet to be tested. Therefore, the objective of this study was to evaluate the effect of discretization on the hillslope sediment yield estimates using GeoWEPP in two large watersheds (>10 km2). The watersheds were selected and discretized varying the TOPAZ parameters [critical source area (CSA) and minimum source channel length (MSCL)] in a 30-m resolution digital elevation model. The drainage networks built with TOPAZ were compared with each other using the drainage density index. The results showed that the discretization affected hillslope sediment yield estimates and their spatial distribution more than the total runoff. The drainage density index and the hillslope sediment yield were proportional but inversely related; thus, soil loss estimates were highly affected by the spatial discretization. As a result of this analysis, a method to choose the CSA and MSCL values that generates the greatest fraction of hillslopes having profile lengths less than 200 m was developed. This slope length condition is particularly crucial when using the WEPP and GeoWEPP models, in order for them to produce realistic estimates of sheet and rill erosion. Finally, and as a result of this analysis, a more reliable method was developed for selecting the TOPAZ channel network parameters (CSA and MSCL). Copyright © 2015 John Wiley & Sons, Ltd.