Critical Source Areas Research Articles

Land use and land cover in critical source areas on small dairy farms in the eastern United States

Rotational grazing (RG) is a livestock management practice that moves grazing cattle among small temporary pastures called paddocks. In dairy farming, RG has potential to improve soil and water conservation by situating well-managed perennial vegetation on erosion-prone parts of farm landscapes. Recently, geospatial technologies have enabled precise identification of “critical source areas” (CSAs). CSAs are sites where slope, soil, and water flow factors coincide to create the largest potential impacts on surface water quality from erosion within the landscape of a farm. Examination of land-use and land-cover (LULC) practices on CSAs allows nuanced comparisons of effects on water resources among agricultural production systems, enabling us to test the hypothesis that RG dairy farms manage CSAs so as to have lesser effects on water resources than comparable-sized nongrazing dairy farms. We assessed LULC on CSAs in 53 small dairy farms in New York, Pennsylvania, and Wisconsin, United States, using satellite imagery and other spatial data. Among these farms, 22 used some form of RG, while the remainder did not use grazing. We also did on-farm observations of most CSAs to assess the frequency of features that may particularly affect water resources, such as unvegetated areas within pastures. RG dairy farms differed significantly from nongrazing dairy farms in the total allocation of CSA area to hayfields and rotational pastures—two LULC types that are likely to reduce runoff and emissions of sediment and nutrients into waterways—but this difference varied across the states and was pronounced only in the Pennsylvania farms, where high-intensity grazing farms allocated these perennial LULC types to over 60% of CSA lands, while nongrazing farms allocated these LULC types to about 20% of CSAs. In New York and Pennsylvania, RG farms allocated significantly less erosion-prone annual cropland to CSA sites than nongrazing farms, and allocated less of the total farm area to LULC practices that are relatively vulnerable to soil erosion. However, these differences between farm types were pronounced only in Pennsylvania farms. Our results do not support the hypothesis that RG dairy farms are generally superior to comparably sized nongrazing dairy farms in their management of CSAs. It is likely that targeted technical and cost-share support could improve management of CSAs in RG farms, allowing them to more fully realize the potential of RG systems to improve soil and water conservation.

Physico-chemical characteristics affect the spatial distribution of pesticide and transformation product loss to an agricultural brook

Diffuse entry of pesticide residues from agriculture into rivers is spatially unevenly distributed. Therefore, the identification of critical source areas (CSAs) may support water quality management in agricultural catchments. In contrast to former studies, we followed the hypothesis that not only hydrological and topographical characteristics but also physico-chemical properties of pesticide residues have a major influence on their loss to rivers and on corresponding formation of CSAs. We designed a virtual experiment, i.e. a numerical experiment as close as possible to environmental conditions, in a headwater catchment where pronounced spatial differences in hydrological transport processes were identified in the past. 144 scenarios with different combinations of adsorption coefficients (KOC=10–1000ml/g) and transformation half-lives (DT50=3–60days) for pesticide parent compounds (PCs) and their transformation products (TPs) were simulated using the catchment-scale spatially distributed reactive transport model ZIN-AgriTra.Export fractions of substances in the virtual experiment ranged from 0.001–15% for pesticides and 0.001–1.8% for TPs. The results of the scenario investigations suggest that more of the calculated export mass variability could be attributed to KOC than to DT50 for both PCs and TPs. CSAs for TPs were spatially more equally distributed in the catchment than for PC export which was likely an effect of changing physico-chemical properties during transformation. The ranking of highest export fields was different between PCs and TPs for most of the investigated scenarios but six fields appeared among the top ten export fields in 95% of the scenarios, which shows the influence of site characteristics such as tile drains or soil properties in the catchment. Thus, the highest export fields were determined by a combination of site characteristics and substance characteristics. Therefore, despite the challenge of widely differing physico-chemical characteristics of pesticides on the market, these characteristics are an important consideration when delineating pesticide residue CSAs.

A high-resolution map of direct and indirect connectivity of erosion risk areas to surface waters in Switzerland—A risk assessment tool for planning and policy-making

Off-site effects of soil erosion are becoming increasingly important, particularly the pollution of surface waters. In order to develop environmentally efficient and cost effective mitigation options it is essential to identify areas that bear both a high erosion risk and high connectivity to surface waters. This paper introduces a simple risk assessment tool that allows the delineation of potential critical source areas (CSA) of sediment input into surface waters concerning the agricultural areas of Switzerland. The basis are the erosion risk map with a 2m resolution (ERM2) and the drainage network, which is extended by drained roads, farm tracks, and slope depressions. The probability of hydrological and sedimentological connectivity is assessed by combining soil erosion risk and extended drainage network with flow distance calculation. A GIS-environment with multiple-flow accumulation algorithms is used for routing runoff generation and flow pathways. The result is a high resolution connectivity map of the agricultural area of Switzerland (888,050ha). Fifty-five percent of the computed agricultural area is potentially connected with surface waters, 45% is not connected. Surprisingly, the larger part of 34% (62% of the connected area) is indirectly connected with surface waters through drained roads, and only 21% are directly connected. The reason is the topographic complexity and patchiness of the landscape due to a dense road and drainage network. A total of 24% of the connected area and 13% of the computed agricultural area, respectively, are rated with a high connectivity probability. On these CSA an adapted land use is recommended, supported by vegetated buffer strips preventing sediment load. Even areas that are far away from open water bodies can be indirectly connected and need to be included in planning of mitigation measures. Thus, the connectivity map presented is an important decision-making tool for policy-makers and extension services. The map is published on the web and thus available for application.

Using SWAT for sub-field identification of phosphorus critical source areas in a saturation excess runoff region

The identification of phosphorus critical source areas (CSAs) is an important step in reducing nonpoint-source phosphorus contributions to lakes and streams, providing the basis for targeted mitigation and efficient utilization of limited resources. A high-resolution SWAT model, consisting of sub-field level hydrologic response units, was applied to Lake Champlain’s Missisquoi Bay Watershed, located along the United States–Canadian border. Because this region of the northeastern United States is dominated by saturation excess runoff, a novel approach for adjusting SCS (Soil Conservation Service) curve numbers based on the local compound topographic index was developed. After calibration and validation, the SWAT model was run for a 30-year period to identify the phosphorus CSAs throughout the watershed, determining that 20% of the watershed produces 74% of the total phosphorus load. The resulting sub-field level CSA map is currently being hosted online, providing a valuable tool for natural resources planners, land owners, and scientists.Editor Z.W. Kundzewicz; Guest editor V. Krysanova

Assessing BMP Effectiveness and Guiding BMP Planning Using Process‐Based Modeling

AbstractThere is an increasing need for improved process‐based planning tools to assist watershed managers in the selection and placement of effective best management practices (BMPs). In this article, we present an approach, based on the Water Erosion Prediction Project model and a pesticide transport model, to identify dominant hydrologic flow paths and critical source areas for a variety of pollutant types. We use this approach to compare the relative impacts of BMPs on hydrology, erosion, sediment, and pollutant delivery within different landscapes. Specifically, we focus on using this approach to understand what factors promoted and/or hindered BMP effectiveness at three Conservation Effects Assessment Project watersheds: Paradise Creek Watershed in Idaho, Walnut Creek Watershed in Iowa, and Goodwater Creek Experimental Watershed in Missouri. These watersheds were first broken down into unique land types based on soil and topographic characteristics. We used the model to assess BMP effectiveness in each of these land types. This simple process‐based modeling approach provided valuable insights that are not generally available to planners when selecting and locating BMPs and helped explain fundamental reasons why long‐term improvement in water quality of these three watersheds has yet to be completely realized.

Distributed, high-resolution modelling of critical source areas for erosion and phosphorus losses.

Phosphorus losses from arable land need to be reduced to prevent eutrophication of surrounding waters. Owing to the high spatial variability of P losses, cost-effective countermeasures need to target parts of the catchment that are most susceptible to P losses. Field surveys identified critical source areas for overland flow and erosion amounting to only 0.4–2.6 % of total arable land in four different catchments in southern Sweden. Distributed modelling using high-resolution digital elevation data identified 72–96 % of these observed erosion and overland flow features. The modelling results were also successfully used to predict occurrence of overland flow and rill and gully erosion in a catchment in central Sweden. Such exact high-resolution modelling allows for accurate placement of planned countermeasures. However, current legislative and environmental subsidy programmes need to change their approach from income-loss compensation to rewarding high cost effectiveness of implemented countermeasures.

Conversion of dissolved phosphorus in runoff by ferric sulfate to a form less available to algae: Field performance and cost assessment.

Conversion of dissolved P by ferric sulfate into a particulate form sparingly available to algae was studied in 15 ditches in Finland using stand-alone dispensers for ferric sulfate administration. Ferric sulfate typically converted 60–70 % of dissolved P into iron-associated form, a process which required 250–650 kg per kg dissolved P. Mean cost was 160 EUR per kg P converted (range 20–400 EUR kg−1). The costs were lowest at sites characterized by high dissolved P concentrations and small catchment area. At best, the treatment was efficient and cost-effective, but to limit the costs and the risks, ferric sulfate dispensers should only be installed in small critical source areas.

The precipitation driven correlation based mapping method (PCM) for identifying the critical source areas of non-point source pollution

Summary Critical source areas (CSAs) are the areas that are relatively more erosion-prone and contribute significantly more pollutants per unit area. They have been widely recognized as optimal locations for the control of non-point source (NPS) pollution. Modeling approach has been frequently used to identify the CSAs of NPS pollution on a basin scale. In previous studies, CSAs were identified based on the simulated average annual nutrient yields for the simulation period at the levels of sub-basin or hydrologic response unit (HRU). However, this method did not consider the impact of uneven spatial distribution of precipitation, which is considered to be the driven force of NPS pollution. In many cases, due to limited length of qualified monitoring data collected, the simulation period may not cover a full spectrum of the precipitation characteristics so that some potential CSAs may be missed. In the present study, the precipitation driven correlation based mapping method (PCM) was proposed, which can reduce the impact of uncertain spatial–temporal distribution of precipitation and identify the CSAs of NPS pollution with a better coverage. This method was applied to the Zhang River Basin, a watershed in North China that occupies an area of 18,072 km 2 . The SWAT (Soil and Water Assessment Tool) was used for simulation purposes. By using PCM, the maps of CSAs for controlling total nitrogen (TN) and total phosphorus (TP) were produced. This study has found that the monthly precipitation is highly correlated with the TN and TP yields. It was observed that TN yields have slightly higher correlation value with the precipitation than TP yields. Hence, the precipitation has more impacts on TN yields than TP yields. The impact is more substantial in urban areas than other areas.

Estimating sediment and particulate organic nitrogen and particulate organic phosphorous yields from a volcanic watershed characterized by forest and agriculture using SWAT model

The study was conducted in the Shibetsu River watershed (SRW), Hokkaido, Japan, in order to examine the possibility of using the soil and water assessment tool (SWAT) to provide an understanding of sediment and particulate organic nitrogen (PON) and particulate organic phosphorous (POP) yields between 2003 and 2008. The SRW is a non-conservative catchment (the surface catchment lying on a continuous impervious horizon) and it is recognized that it receives external groundwater (EXT) from other watersheds. The EXT yield from each hydrologic response unit (HRU) was added to streamflow in the SWAT model. Simulated daily sediment and PON and POP yields from the SWAT model showed a strong agreement with the observed values. The simulated annual sediment yield ranged from 5 to 45 tonnes.km−2.yr−1 (annual mean of 24 tonnes.km−2.yr−1). Annual PON yield ranged from 0.1 to 0.3 tonnes.km−2.yr−1 (annual mean of 0.18 tonnes.km−2.yr−1). Annual POP yield ranged from 0.01 to 0.03 tonnes.km−2.yr−1 (annual mean of 0.02 tonnes.km−2.yr−1). Snowfall, snowmelt and rainfall seasons contributed about 10, 20 and 70% respectively to total sediment and associated PON and POP yields. The SWAT model identified that sub-basins located in the upper part of the watershed were critical source area of land surface erosion. This research demonstrates the ability of the SWAT model to estimate sediment and associated PON and POP yields, and to improve the understanding of soil erosion mechanisms at catchment scale receiving external water.

Multiscale Landscape Pattern Affecting on Stream Water Quality in Agricultural Watershed, SW Finland

Finnish arable land is typically located on flat areas, where the fields are mostly drained and sub-drained to control the water tables. These areas are highly susceptible to nutrient loss, which affects the water quality of rivers and lakes. Therefore, it is very important to understand current landscape pattern and processes controlling water quality, not only identifying factors affecting it, but also identifying strategies and restoring areas for mitigation. We studied linkage of 21 years (1990–2011) of water quality (WQ) data from 16 agricultural watersheds, using landscape indices at three functional scales: watershed-wide, saturation-excess zone, and riparian zone (of varying widths). The hydro-biogeochemical functional areas of watershed were obtained by digital terrain analysis. Statistical analyses by generalized linear model and multivariate redundancy analysis indicated that the fraction of watershed in agricultural use was linked to most of the studied water quality variables. The relationships varied across the seasons: they were strongest during high flow periods (spring and autumn) when also highest nutrient losses occur. Total suspended sediment concentrations were linked to critical source areas. Riparian vegetation index was important explaining nitrate concentrations in autumn. Terrain-based mapping of hydro-biogeochemical functional areas provides a rapid identification of potential sites to mitigate diffuse nutrient pollution, particularly in riparian zones.

Assessing winter cover crop nutrient uptake efficiency using a water quality simulation model

Abstract. Winter cover crops are an effective conservation management practice with potential to improve water quality. Throughout the Chesapeake Bay watershed (CBW), which is located in the mid-Atlantic US, winter cover crop use has been emphasized, and federal and state cost-share programs are available to farmers to subsidize the cost of cover crop establishment. The objective of this study was to assess the long-term effect of planting winter cover crops to improve water quality at the watershed scale (~ 50 km2) and to identify critical source areas of high nitrate export. A physically based watershed simulation model, Soil and Water Assessment Tool (SWAT), was calibrated and validated using water quality monitoring data to simulate hydrological processes and agricultural nutrient cycling over the period of 1990–2000. To accurately simulate winter cover crop biomass in relation to growing conditions, a new approach was developed to further calibrate plant growth parameters that control the leaf area development curve using multitemporal satellite-based measurements of species-specific winter cover crop performance. Multiple SWAT scenarios were developed to obtain baseline information on nitrate loading without winter cover crops and to investigate how nitrate loading could change under different winter cover crop planting scenarios, including different species, planting dates, and implementation areas. The simulation results indicate that winter cover crops have a negligible impact on the water budget but significantly reduce nitrate leaching to groundwater and delivery to the waterways. Without winter cover crops, annual nitrate loading from agricultural lands was approximately 14 kg ha−1, but decreased to 4.6–10.1 kg ha−1 with cover crops resulting in a reduction rate of 27–67% at the watershed scale. Rye was the most effective species, with a potential to reduce nitrate leaching by up to 93% with early planting at the field scale. Early planting of cover crops (~ 30 days of additional growing days) was crucial, as it lowered nitrate export by an additional ~ 2 kg ha−1 when compared to late planting scenarios. The effectiveness of cover cropping increased with increasing extent of cover crop implementation. Agricultural fields with well-drained soils and those that were more frequently used to grow corn had a higher potential for nitrate leaching and export to the waterways. This study supports the effective implementation of cover crop programs, in part by helping to target critical pollution source areas for cover crop implementation.

Effects of cultivation on soil and soil water under different fertiliser regimes

Nutrient exports from agriculture are an important environmental issue in many countries. In some production systems phosphorus (P) and nitrogen (N) accumulate at or near the soil surface and cultivation is one option for addressing the legacies of previous nutrient applications.This study aimed to compare the effects of cultivation (mouldboard ploughing), pasture type (ryegrass or ryegrass–clover) and P fertiliser application rate (10, 35 or 100kg P/ha per annum) on P and N in soil and soil water from dairy pastures over a three year period. The results are generally consistent with inversion of organic matter and P rich surface soil (i.e. 0–140mm). Cultivation lowered surface soil (0–20mm) P and N concentrations. Surface soil P concentrations also increased with increasing P fertiliser rate but organic matter and P buffering did not. The effects of vegetation were more equivocal. Ploughing lowered most P and N species in surface soil water. The concentrations of P in soil water increased with increasing fertiliser rates as did ammonia concentrations where pastures included clovers.Analyses of trends with time suggests that, for pasture re-establishment, cultivation reduced the risk of N exports through surface pathways. Moreover, analyses of P export risk measured as the dissolved reactive P concentration in surface soil water (0–20mm) suggests that where there is no predisposition to erosion, mouldboard ploughing not only reduces the risks of P exports, but that effect is likely to be more enduring where paddocks are in positive P balance. It follows that the use of cultivation as part of pasture renovation activities may reduce catchment-scale P exports, particularly in critical source areas where the risks of erosion and subsurface exports of N are low.

The use of alum to decrease phosphorus losses in runoff from grassland soils.

Phosphorus (P) loss from land can impair surface water quality. Aluminum sulfate (alum)-treated, compared with untreated, manure or slurry decreases P loss when applied to land; our hypothesis was that alum may also decrease P loss when directly applied to grassland grazed by dairy cows. A rainfall simulation showed that alum decreased mean concentrations of filterable reactive P (FRP) by 25 to 70% and total P (TP) by 20 to 40%, depending on soil P, Al, and Fe concentration and alum application rate. Using these factors, we predicted that FRP losses would be significantly less from alum-treated grasslands than from untreated grasslands for 70 to 96 d. A 14-mo field trial compared runoff P losses from plots that received 0, 25, and 50 kg Al ha applied within a week of grazing by dairy cattle in spring. Runoff-weighted concentrations (and loads) of FRP and TP decreased in alum-treated plots by 47 to 52% and 25 to 34%, respectively. At US$157 to US$944 kg P mitigated, cost-effectiveness was estimated as medium to low compared with existing strategies for mitigating P loss in dairy farms but could be improved if applied to critical source areas of P loss. However, additional work, such as determining the need for repeat applications, is required before alum can be recommended to decrease P losses from grazed grassland.

A comparison of three simple approaches to identify critical areas for runoff and dissolved reactive phosphorus losses

Abstract. Diffuse phosphorus (P) losses are the main cause for eutrophication of surface waters in many regions. Implementing mitigation measures on critical source areas (CSAs) is seen to be the most effective way to reduce P losses. Thus, tools are needed that delineate CSAs on the basis of available data. We compared three models based on different approaches and sets of input data: the rainfall-runoff-phosphorus (RRP) model, the dominant runoff processes (DoRP) model, and the Sensitive Catchment Integrated Modeling Analysis Platform (SCIMAP). The RRP model is a parsimonious dynamic model using the topographic index and a binary soil classification to simulate discharge and P losses. The DoRP model distinguishes eight soil classes based on soil and geological maps. It does not account for topography when calculating runoff. SCIMAP assesses runoff risks solely on the basis of topography using the network index. Compared to surface runoff and soil moisture data available from a catchment in Switzerland, the RRP model and SCIMAP made better predictions than the DoRP model, suggesting that in our study area topography was more important for CSA delineation than soil data. Based on the results, we suggest improvements of SCIMAP to enable average risk predictions and the comparison of risk predictions between catchments.

Evaluating the critical source area concept of phosphorus loss from soils to water-bodies in agricultural catchments

Using data collected from six basins located across two hydrologically contrasting agricultural catchments, this study investigated whether transport metrics alone provide better estimates of storm phosphorus (P) loss from basins than critical source area (CSA) metrics which combine source factors as well. Concentrations and loads of P in quickflow (QF) were measured at basin outlets during four storm events and were compared with dynamic (QF magnitude) and static (extent of highly-connected, poorly-drained soils) transport metrics and a CSA metric (extent of highly-connected, poorly-drained soils with excess plant-available P).Pairwise comparisons between basins with similar CSA risks but contrasting QF magnitudes showed that QF flow-weighted mean TRP (total molybdate-reactive P) concentrations and loads were frequently (at least 11 of 14 comparisons) more than 40% higher in basins with the highest QF magnitudes. Furthermore, static transport metrics reliably discerned relative QF magnitudes between these basins. However, particulate P (PP) concentrations were often (6 of 14 comparisons) higher in basins with the lowest QF magnitudes, most likely due to soil-management activities (e.g. ploughing), in these predominantly arable basins at these times. Pairwise comparisons between basins with contrasting CSA risks and similar QF magnitudes showed that TRP and PP concentrations and loads did not reflect trends in CSA risk or QF magnitude. Static transport metrics did not discern relative QF magnitudes between these basins.In basins with contrasting transport risks, storm TRP concentrations and loads were well differentiated by dynamic or static transport metrics alone, regardless of differences in soil P. In basins with similar transport risks, dynamic transport metrics and P source information additional to soil P may be required to predict relative storm TRP concentrations and loads. Regardless of differences in transport risk, information on land use and management, may be required to predict relative differences in storm PP concentrations between these agricultural basins.

A catchment study of sources and sinks of nutrients and sediments in south-east Australia

Summary A study of nutrient and sediment generation was carried out on a dairy farm in south eastern Australia over a period of two years. The study collected nitrogen (N), phosphorus (P) and total suspended sediment (TSS) samples from three headwater subcatchments totalling 4.4 km2 in area, which had streamflow measurement sites located at their outlets. In addition to these three sites, samples were collected from farm tracks, the discharge from a farm dam (water storage pond) and a rainfall sampler. Over 20 runoff events were monitored in addition to background flows in the catchment. The data enabled nutrient exports to be estimated from the total catchment and two of the three subcatchments. The atmospheric nutrient loads were also estimated and found to be similar to other studies in the region, and were low compared with the nutrient export. The nutrient load estimates indicated that a large proportion of the catchment Total Nitrogen (TN) and Total Phosphorus (TP) loads originated from critical source areas on the farm associated with dairy operations, and that the catchment TN and TP export rates were high compared to other studies. The sediment data also indicated that TP concentrations were closely related to TSS in samples collected from surface runoff. Catchment TN loads in baseflow were high relative to the total load, probably due to indirect effluent and direct groundwater discharges into watercourses, with high concentrations of soluble N (nitrate plus nitrite and ammonium). The farm dam was found to trap a high percentage (>60%) of the influent TP load; however, the retention of TN by the dam was seasonally varying and close to net neutral.

Critical source areas for herbicides can change location depending on rain events

During rain events, herbicides can be transported from their point of application to surface waters, where they may harm aquatic organisms. Since the spatial variability of herbicide losses to streams can be large, the identification of critical source areas could help to target mitigation measures efficiently to those locations where they reduce herbicide pollution the most. We performed a controlled herbicide application on wheat (isoproturon) and corn fields (atrazine, S-metolachlor and sulcotrione) in an agricultural head-water catchment (about 1km2) in the Swiss Plateau to investigate the spatial variability of herbicide losses. We performed spatially distributed discharge measurements and high-resolution water sampling during rain events after herbicide application to determine herbicide loads and loss rates for individual fields. The dry weather conditions after the wheat herbicide application resulted in a very low isoproturon loss rate (0.005% of the applied amount). In contrast, the corn herbicide application was followed by several rain events with varying intensities and magnitudes causing loss rates of 0.26, 0.16 and 0.26% for atrazine, S-metolachlor and sulcotrione, respectively. The spatial differences in loss rates between fields were about a factor of three in most events. However, the spatial loss pattern varied between events implying that being a critical source area is not a temporally stable field property. No correlations were observed with several field characteristics (connectivity of the fields to the stream, the tendency for topsoil saturation, field-specific herbicide dissipation rates and sorption affinities). However, the data suggest that critical source areas may depend on the type of rain event because infiltration and saturation-excess runoff affects different parts of the catchment.

Predicting phosphorus dynamics in complex terrains using a variable source area hydrology model

Phosphorus (P) loss from agricultural watersheds has long been a critical water quality problem, the control of which has been the focus of considerable research and investment. Preventing P loss depends on accurately representing the hydrological and chemical processes governing P mobilization and transport. The Soil and Water Assessment Tool (SWAT) is a watershed model commonly used to predict run-off and non-point source pollution transport. SWAT simulates run-off employing either the curve number (CN) or the Green and Ampt methods, both assume infiltration-excess run-off, although shallow soils underlain by a restricting layer commonly generate saturation-excess run-off from variable source areas (VSA). In this study, we compared traditional SWAT with a re-conceptualized version, SWAT-VSA, that represents VSA hydrology, in a complex agricultural watershed in east central Pennsylvania. The objectives of this research were to provide further evidence of SWAT-VSA's integrated and distributed predictive capabilities against measured surface run-off and stream P loads and to highlight the model's ability to drive sub-field management of P. Thus, we relied on a detailed field management database to parameterize the models. SWAT and SWAT-VSA predicted discharge similarly well (daily Nash–Sutcliffe efficiencies of 0.61 and 0.66, respectively), but SWAT-VSA outperformed SWAT in predicting P export from the watershed. SWAT estimated lower P loss (0.0–0.25 kg ha−1) from agricultural fields than SWAT-VSA (0.0–1.0+ kg ha−1), which also identified critical source areas – those areas generating large run-off and P losses at the sub-field level. These results support the use of SWAT-VSA in predicting watershed-scale P losses and identifying critical source areas of P loss in landscapes with VSA hydrology. Copyright © 2014 John Wiley & Sons, Ltd.

Predicting erosion and sediment yield in a mesoscale basin in the semiarid monsoon region Miyun/China

IntroductionChanges in land use and agricultural management have caused a strong increase in sediment and particulate phosphorus input into the Miyun reservoir. The simulation of the relevant runoff and sediment fluxes at the mesoscale needs adequate procedures, adapted to a semiarid monsoon climate and regional land-use pattern.ResultsThe modified Universal Soil Loss Equation (USLE-M) was integrated into the STOFFBILANZ model to calculate daily soil losses in the entire Miyun catchment area. The results are well within the range of monitoring data from Wischmeier plots that are located inside the investigation area. For the reference year 2009, soil erosion was estimated at an average of 952 kg ha−1 year−1. Daily based simulation of sediment input into surface waters was done in the next step, taking into account the likeliness of sediment input and sediment delivery ratio. Average sediment inputs are by 113 kg ha−1 year−1. Critical source areas are located on cropland and are particularly relevant on the former lakebed of the Miyun reservoir which has recently been used for corn cropping.ConclusionsCompared with observation data from the Bai and Chao rivers, the simulation of sediment loads showed a satisfying agreement that underlines the suitability of the selected approach to substitute recently used one-year balances based on the Universal Soil Loss Equation (USLE) approach. Conservation tillage was selected as a possible mitigation option to reduce soil erosion and sediment loads.Owing to the limited availability of spatial and monitoring data, the simulations are approximate and only a first step in contributing to an integrated river basin management that should be improved in future.

An integrated pressure and pathway approach to the spatial analysis of groundwater nitrate: A case study from the southeast of Ireland

Excess nitrogen in soil, aquatic and atmospheric environments is an escalating global problem. Eutrophication is the principal threat to surface water quality in the Republic of Ireland. European Union Water Framework Directive (2000/60/EC) water quality status assessments found that 16% of Irish groundwater bodies were ‘at risk’ of poor status due to the potential deterioration of associated estuarine and coastal water quality by nitrate from groundwater. This paper presents a methodology for evaluating pressure and pathway parameters affecting the spatial distribution of groundwater nitrate, investigated at a regional scale using existing national spatial datasets. The potential for nitrate transfer to groundwater was rated based on the introduced concepts of Pressure Loading and Pathway Connectivity Rating, each based on a combination of selected pressure and pathway parameters respectively. In the region studied, the South Eastern River Basin District of Ireland, this methodology identified that pathway parameters were more important than pressure parameters in understanding the spatial distribution of groundwater nitrate. Statistical analyses supported these findings and further demonstrated that the proportion of poorly drained soils, arable land, karstic flow regimes, regionally important bedrock aquifers and high vulnerability groundwater within the zones of contribution of the monitoring points are statistically significantly related to groundwater nitrate concentrations. Soil type was found to be the most important parameter. Analysis of variance showed that a number of the pressure and pathway parameters are interrelated.The parameters identified by the presented methodology may provide useful insights into the best way to manage and mitigate the influence of nitrate contamination of groundwater in this region. It is suggested that the identification of critical source areas based on the identified parameters would be an appropriate management tool, enabling planning and enforcement resources to be focussed on areas which will yield most benefit.