Small Agricultural Catchment Research Articles

Evaluation of the stable isotope signatures of nitrate to detect denitrification in a shallow groundwater system in New Zealand

Denitrification in the groundwater systems of agricultural catchments can provide a substantial ‘ecosystem service’ by attenuating leached nitrate (NO3−) before it reaches surface water bodies. Samples along a groundwater flow path with low dissolved oxygen and declining NO3− concentrations can indicate the occurrence of denitrification. Isotopic analysis of this NO3− can potentially identify and quantify denitrification activity. In this study, shallow groundwater samples (maximum 5m below ground surface) were taken from three locations within a small agricultural catchment in the Waikato region of New Zealand. The δ15N and δ18O values of NO3− were analysed to try to determine where denitrification was occurring and at what rate. Results indicated that denitrification rates varied spatially, but interpretation was confounded by insufficiently understood flow paths and extremely low concentrations of NO3− in reduced groundwater. Seasonal denitrification was observed at a Gley soil site where the soil profile was periodically saturated to near the ground surface and δ15N-NO3− values reached +28.5‰ and δ18O-NO3− values up to +19.6‰. In contrast to expectations, NO3− in well-oxidised groundwater samples showed substantial variability in its δ15N and δ18O isotopic signature. This indicated that the NO3− originated from multiple sources, which restricted the quantification of denitrification.

Study on Effect of Planting Measure on Red Soil Runoff Reduction in Small Agricultural Catchment Using Spatially Distributed Sediment Runoff Model

Study on Effect of Planting Measure on Red Soil Runoff Reduction in Small Agricultural Catchment Using Spatially Distributed Sediment Runoff Model

Evaluating the Impact of the Spatial Distribution of Land Management Practices on Water Erosion: Case Study of a Mediterranean Catchment

Abstract: The spatial distribution of land management practices (LMPs), such as the use of vegetated filters, may have a strong impact on their efficiency in trapping sediments and pollutants. Distributed water erosion models help managers, planners, and policymakers optimize the efficiency of these LMPs regarding their location relative to water and sediment pathways. In this work, the authors analyzed the impact of the spatial distribution of LMPs using an existing distributed model and sensitivity analysis procedures. The distributed model that was used is a distributed single-event physically based water erosion model developed to calculate erosion rates and sediment flow for small (less than 10 km2) agricultural catchments. To measure the impact of the spatial distribution of LMPs, the authors developed a stochastic model that generates LMP locations over the entire catchment. The stochastic model has three input parameters: the density of LMPs, their downslope/ upslope location probability, and the probability density function shape controller. Because of its ability to account for the cross effects between parameters, the variance-based Sobol method was used to calculate the sensitivity of the soil loss ratio of a typical Mediterranean agricultural catchment (Roujan, southern France) to the LMP location model parameters. Three measurement points (two subcatchment outlets and the main outlet) were used to examine the spatially distributed effects of the LMP locations. The simulation results indicated that 70% of the variation of the net erosion is explained by variations in LMP density for the main outlet catchment, making LMP density the most sensitive parameter. However, the total Sobol sensitivity indices indicate a strong interaction among the three parameters when the density values are low (few LMPs are applied). Thus, although the density of the LMPs is the most sensitive parameter, their location may influence their global trapping efficiency in (real) cases where few LMPs are applied.

Establishing a sediment budget for a small agricultural catchment in southern Brazil, to support the development of effective sediment management strategies

Summary The rapid expansion of agriculture in Brazil has increased erosion rates and sediment yields, causing many negative environmental and economic impacts, both on- and off-site. However, to date, very few catchment-scale sediment budget investigations have been carried out in Brazil. Given the need to reduce the negative off-site impacts of increasing agricultural activity, there is an important need for such investigations in order to inform the development of effective sediment management strategies. Against this background, 137Cs measurements have been combined with measurements of sediment yield and fingerprinting the source of the fine sediment output, to establish a provisional sediment budget for a small (1.19 km2) agricultural catchment in southern Brazil. The catchment is located in an area of steep highly erodible basaltic terrain, which has been intensively cultivated with tobacco. An ongoing monitoring programme provided information on the sediment yield from the catchment and existing suspended sediment source fingerprinting investigations provided information on the main sediment sources contributing to the sediment load at the catchment outlet. 137Cs measurements have been used to estimate medium-term erosion and deposition rates along 17 transects across the cultivated slopes and to quantify sedimentation rates within valley floor sediment sinks. These data have been used to estimate sediment redistribution rates within the cultivated areas of the study catchment and sediment accumulation in the valley floor sinks. The information provided by the three primary data sources has been integrated to establish the sediment budget for the catchment over the past 57 years. The individual terms of the budget necessarily involve much uncertainty, but its closure adds confidence to the final result. The budget calculations indicate that the study catchment has a sediment delivery ratio of ∼15%. The implications of the key features of the budget for developing effective sediment management strategies are discussed.

Long-term monitoring of nutrient losses from agricultural catchments in the Nordic–Baltic region – A discussion of methods, uncertainties and future needs

Monitoring of nutrient losses to surface waters at the small agricultural catchment scale has been carried out in seven Nordic and Baltic countries for the past two decades, with the aim of obtaining information on agricultural activities and their impact on surrounding waters. The monitoring methods have been harmonised and in almost all 35 catchments currently being monitored, water samples are taken flow-proportionally, water discharge is measured continuously and information on crops grown and crop management is collected yearly. Here, we evaluate and discuss uncertainties in data and their consequences for fulfilling the aim of the monitoring systems. We also suggest methods and priorities for future monitoring.Although the catchments altogether cover major variations in climate, soil texture and farm type, there is a bias towards more intensive agricultural production compared with the surrounding agricultural regions. For applicability of the results to larger agricultural areas, the representativity of the selected catchments should be stated. The determination of nutrient loads is affected by uncertainties in water discharge measurements, water sampling and chemical analyses, but these uncertainties may decrease over time due to improvements in the methods used. This would improve the possibility to detect trends in the naturally large variation in nutrient loads. For wider applications, the level of nutrient loads at the catchment stream outlet must be related to catchment characteristics such as proportion of agricultural land and contribution from other land uses and point sources. These sources are often estimated based on limited information, but should be included regularly in monitoring schemes. Also data on agricultural activities are related to uncertainties such as coverage of crop management information and nutrient content in manure and harvested products. When data are missing, standard values are used, further contributing to uncertainties. However, intensive monitoring increases awareness of sources of uncertainty in data, and hence the possibility to reduce these uncertainties. Therefore, the intensive monitoring approach should be maintained and rather extended, for example by using sensors for high resolution water quality measurements so that the variations can be identified and related to catchment activities and to processes in streams and groundwater. To further understand these processes and crop management systems, modelling should be closely related to monitoring. With harmonised monitoring programmes where differences in methods are known, focus can be put on the evaluation of data so that further knowledge on the impact of agriculture on the environment can be obtained.

Small agricultural monitoring catchments in Sweden representing environmental impact

Nutrient losses to surface waters have been monitored at the small agricultural catchment scale (2–35km2) for 20 years in Sweden. Eight of the 21 catchments have been more intensively monitored, with flow-proportional stream water sampling, analysis of groundwater quality, yearly crop management surveys and soil characterisation. Annual losses of total nitrogen (N) at catchment stream outlet vary from 6 to 32kgha−1, with the largest losses from sandy loam soils in south-west Sweden, where precipitation is high. Losses of total phosphorus (P) vary from 0.1 to 2.0kgha−1year−1 and are largest in catchments with clay soils. Compared with surrounding agricultural areas, crop production is more intensive in most of the monitoring catchments, e.g. the production of annual crops for the market constitutes a larger share of arable land than production of ley in 15 out of 21 monitoring catchments. A more intensive crop production is a consequence of a preference for a high proportion of arable land in the monitoring area which coincides with more productive agricultural areas in the regions. Knowing how the catchments relate to other agricultural areas is important when the catchments are used as indicators of agricultural impacts on surface waters. For detection of the success of implemented mitigation measures, small monitoring catchments are suitable since the response on stream water quality is faster than in larger river catchments where the contribution from other sources is larger and retention in streams and lakes occurs to a larger extent. The catchment information also enhances validation of models used for estimating losses of nutrients from other agricultural areas where information on crops, soils and climate exist but data on agricultural management and water quality is scarce.

Retention and Curve Number Variability in a Small Agricultural Catchment: The Probabilistic Approach

The variability of the curve number (CN) and the retention parameter (S) of the Soil Conservation Service (SCS)-CN method in a small agricultural, lowland watershed (23.4 km2 to the gauging station) in central Poland has been assessed using the probabilistic approach: distribution fitting and confidence intervals (CIs). Empirical CNs and Ss were computed directly from recorded rainfall depths and direct runoff volumes. Two measures of the goodness of fit were used as selection criteria in the identification of the parent distribution function. The measures specified the generalized extreme value (GEV), normal and general logistic (GLO) distributions for 100-CN and GLO, lognormal and GEV distributions for S. The characteristics estimated from theoretical distribution (median, quantiles) were compared to the tabulated CN and to the antecedent runoff conditions of Hawkins and Hjelmfelt. The distribution fitting for the whole sample revealed a good agreement between the tabulated CN and the median and between the antecedent runoff conditions (ARCs) of Hawkins and Hjelmfelt, which certified a good calibration of the model. However, the division of the CN sample due to heavy and moderate rainfall depths revealed a serious inconsistency between the parameters mentioned. This analysis proves that the application of the SCS-CN method should rely on deep insight into the probabilistic properties of CN and S.

Temporal trends in nitrogen concentrations and losses from agricultural catchments in the Nordic and Baltic countries

Long-term monitoring data from catchments with relatively uniform land use is important in order to cover management needs such as implementation of various EU Directives. This paper in a uniform fashion examines the temporal trends in nitrogen (N) concentrations and losses from agricultural catchments in the Nordic and Baltic countries. Thirty-five (35) catchments (range 0.1–33km2) in Norway (9), Denmark (5), Sweden (8), Finland (4), Estonia (3), Latvia (3) and Lithuania (3) were selected for this study. The longest time series were 23 years (1988–2010), while the shortest one was 10 years (2002–2011). The monthly nitrogen concentration and loss data series were tested for statistical trends (p<0.05; two-sided test) using the partial Mann–Kendall (PMK) test with stream discharge as an explanatory variable.The study results show a large variation in nitrogen concentrations and losses among the studied catchments, with a large interannual variability in all catchments. For nitrogen losses, 11 statistically significant trends were detected. Nine of these were downward (four out of five Danish catchments; four out of eight in Sweden; one out of three in Finland). Upward trends were detected in two catchments (one in Estonia and one in Latvia). For nitrogen concentrations, 13 statistically significant trends were detected. 10 of these were downward. Among the 11 catchments that showed trends in nitrogen losses, nine catchments also showed statistically significant trends in the in-stream concentration series. In addition, three more Swedish catchments showed downward trends in the concentration series, and one Estonian catchment showed an upward trend. These results indicate that targeted strategies towards reduced nitrogen losses from agricultural land (as in the case of Denmark and Sweden) may significantly improve nutrient surface water quality in small agricultural catchments

Nitrogen losses from small agricultural catchments in Lithuania

A study of precipitation, runoff and total annual nitrogen concentrations in streamflow was performed in three small stream catchments (A=1.66÷14.2km2) located in western, middle and southeastern parts of Lithuania during 1996–2010. The studied catchments belong to different geographic districts of Lithuania in which climate, soil, land use and farming conditions differ. The main objective was to present and analyse long-term data collected from three water-quality monitoring sites and to investigate the impact of various factors that may affect nitrogen losses.The results revealed the complex and dynamic nature of the various factors affecting nitrogen losses. The nitrogen contribution from wet deposition varied from 6.3 to 56.7kgha−1 per year and it was found to be significant factor controlling nitrogen export from the catchments. However, the proportions of arable land and pasture within the catchment, as well as the soil conditions were also found to be important.The annual runoff varied from 54 to 403mm and exhibited strong spatial patterns among the catchments. The highest runoff was observed in the stream in southeast Lithuania, where sandy soils and a larger groundwater supply prevail. High runoff was also found typical in the stream in western Lithuania, the catchment of which received greater precipitation, exhibited hilly topography and a large drained area. The smallest runoff and highest instream nitrogen concentration (flow-weighted annual average was 7.0mgl−1 compared with 3.3 and 2.8mgl−1 in the other streams) was observed in the stream in the middle Lithuanian lowland, where more intensive agricultural activity occurs. The average annual load of total nitrogen in the stream in mid-Lithuania was 15.3kgha−1, compared with 11.8 and 7.0kgha−1 for the streams in the southeast and west, respectively.The 15-year data did not show any statistically significant trend either in the dynamics of the annual nitrogen concentration or in the annual nitrogen load of the streams.

Prediction of dissolved reactive phosphorus losses from small agricultural catchments: calibration and validation of a parsimonious model

Abstract. Eutrophication of surface waters due to diffuse phosphorus (P) losses continues to be a severe water quality problem worldwide, causing the loss of ecosystem functions of the respective water bodies. Phosphorus in runoff often originates from a small fraction of a catchment only. Targeting mitigation measures to these critical source areas (CSAs) is expected to be most efficient and cost-effective, but requires suitable tools. Here we investigated the capability of the parsimonious Rainfall-Runoff-Phosphorus (RRP) model to identify CSAs in grassland-dominated catchments based on readily available soil and topographic data. After simultaneous calibration on runoff data from four small hilly catchments on the Swiss Plateau, the model was validated on a different catchment in the same region without further calibration. The RRP model adequately simulated the discharge and dissolved reactive P (DRP) export from the validation catchment. Sensitivity analysis showed that the model predictions were robust with respect to the classification of soils into "poorly drained" and "well drained", based on the available soil map. Comparing spatial hydrological model predictions with field data from the validation catchment provided further evidence that the assumptions underlying the model are valid and that the model adequately accounts for the dominant P export processes in the target region. Thus, the parsimonious RRP model is a valuable tool that can be used to determine CSAs. Despite the considerable predictive uncertainty regarding the spatial extent of CSAs, the RRP can provide guidance for the implementation of mitigation measures. The model helps to identify those parts of a catchment where high DRP losses are expected or can be excluded with high confidence. Legacy P was predicted to be the dominant source for DRP losses and thus, in combination with hydrologic active areas, a high risk for water quality.

EIS Pesticides: An environmental information system to characterize agricultural activities and calculate agro-environmental indicators at embedded watershed scales

The French “Ecophyto 2018” program calls for a 50% reduction in pesticide use. Local authorities are required to design cost-effective measures to minimize the impact of farmers’ pesticides on water resources. A successful implementation of these new measures can only be achieved through a better understanding of the interactions between water, land use and the environment. One way of doing this is to calculate pesticide pressure and agro-environmental indicators (AEIs). However, this approach requires an effective information system that can process both the characteristics of the river basin and the agricultural activities using at least two embedded scales: the scale of the small agricultural catchment for action by farmers and the scale of a larger watershed for public decision making.To this end, an environmental information system (EIS Pesticide) for pesticide issues was created using spatial data warehouse technology. This system allowed qualifying agricultural activities along with river basin characteristics. Specific spatial objects were designed to characterize practices at the relevant scales. The axes of analysis allowed providing results at different levels of integration, for different dimensions e.g. time, sprayed surface area, or pesticide type. The system was tested using datasets collected in the Charente watershed and its sub-basins and calculated pesticide pressure indicators on demand for each aggregation level defined.

Tracing water flow from sloping farmland to streams using oxygen-18 isotope to study a small agricultural catchment in southwest China

Knowledge of the age and origins of water and its movement from hillslopes to streams is important in mapping aquifers, conserving water supplies, determining water-use policy, and controlling pollution. In a sloping farmland-dominated agricultural catchment in southwest China, a stable isotope tracer (18O) combined with hydrometric measurements was tested to determine the rainwater movement from a hillslope to a stream during three storm events. The results showed that underflow in finely fractured rocks accounted for the highest proportion of the total flow on the slope, indicating that vertical flow was the predominant water flow pattern on the hillslope. The surface flow had basically the same δ18O value as the coinstantaneous rainfall, indicating that it was a Hortonian overland flow. The two end-member separation model showed that pre-event water accounted for a large proportion of the hillslope subsurface flows and streamflow especially during the initial stage. The first sample of underflow and interflow water always contained significant rainwater, indicating that macropore flow was prevalent in this Entisol. Event water dominated the peak streamflow and accounted for 70.4%, 95.6% and 80.4% of the peakflow amount during the three storm events. In general, the results suggested that pre-event water dominated the initial stage of stormflow generation and that event water dominated the peakflow. During the recession period, hillslope subsurface flows were the main source of the streamflow, as indicated by the similar volume-weighted δ18O value in subsurface flows and stream water. In future studies, hydrological processes under various land uses should be monitored synchronously to better understand the mechanisms of stormflow generation and transport regarding non-point source pollution in agricultural catchments.

The influence of water erosion processes on sediment and nutriet transport from a small agricultural catchment area

Erosion processes in catchment areas cause serious ecologic and economic problems because of their negative consequences in terms of soil and water deterioration as well as for the environment as a whole. The soil particles washed down by water erosion are the biggest pollution factor due to their amount and volume. Sediments are the product of a selective process in which smaller and lighter particles are separated from eroded soil and taken away by water first. This means that the sediments contain a higher amount of organic, clay, and silt particles than the the original soils. Washed down sediments consist mainly of particles smaller than 0.05 mm (40–90% of the mixture). Other studies in the Czech Republic have focused on the assessment of soil erosion, based upon principles and parameters defined in the Universal Soil Loss Equation, but none of them has dealt with nutrient transport assessment as a consequence of water erosion. This paper presents a summary concerning the nutrient content in erosion sediment in a selected catchment area. Research work was conducted to identify and quantify the sediment load associated with nutrient transport especially from arable land on different soil types.

Significance of floods in metal dynamics and export in a small agricultural catchment

High-resolution monitoring of water discharge and water sampling were performed between early October 2006 and late September 2007 in the Montousse River, a permanent stream draining an experimental agricultural catchment in Gascogne region (SW France). Dissolved and particulate concentrations of major elements and trace metals (i.e. Al, Fe, Mn, As, Cd, Cr, Cu, Ni, Pb, Sc and Zn) were examined. Our results showed that contamination levels were deficient to moderate, as a result of sustainable agricultural practices. Regarding dynamics, metal partitioning between particulate and dissolved phases was altered during flood conditions: the particulate phase was diluted by coarser and less contaminated particles from river bottom and banks, whereas the liquid phase was rapidly enriched owing to desorption mechanisms. Soluble/reactive elements were washed-off from soils at the beginning of the rain episode. The contribution of the flood event of May 2007 (by far the most significant episode over the study period) to the annual metal export was considerable for particulate forms (72–82%) and moderate for dissolved elements (0–20%). The hydrological functioning of the Montousse stream poses dual threat on ecosystems, the consequences of which differ from both temporal and spatial scales: (i) desorption processes at the beginning of floods induce locally a rapid enrichment (up to 3.4-fold the pre-flood signatures on average for the event of May 2007) of waters in bioavailable metals, and (ii) labile metals – enriched by anthropogenic sources – associated to particles (mainly via carbonates and Fe/Mn oxides), were predominantly transferred during floods into downstream-connected rivers.

Dinámica de la humedad del suelo en una cuenca agrícola del sector central de la cuenca del Duero

In this work the soil moisture dynamics was estudied in a small agricultural catchment in the central sector of the Duero basin. The experience took place in the 1 km2 Villamor experimental catchment, located in the southeastern part of the Zamora province. The evolution of the soil water contenr and its availability for plants was analyzed from the data measured in eleven soil profiles equipped with TDR (Time Domain Reflectometry) probes at different depths, between 2002 and 2006. The soil moisture data recorded in Villamor which were compared against the data simultaneously registered in the Network of Soil Moisture Measurement Stations (REMEDHUS) in the agricultural region of La Guarena over 1300 km2. This comparison allowed to assess the repre-sentativeness of the experimental catchment as an expression of the hydrological dynamics of a territory larger of which is part. The results of the study carried out in Villamor have shown the seasonal water content changes in the soil, as it is expected under Mediterranean conditions. However, these changes were much more attenuated than those observed in other nearby areas. The use of the Soil Water Index (IDH) allows the analysis in terms of availability of water for the plants and enables the comparison between different areas. IDH data have shown much more reduced water deficit conditions of what might be expected, given the semi-arid conditions in the area. The comparison with series of IDH of two forested basins and the identification of deficit situations much more intense in them, leads to reflection on the very prominent role of vegetation cover in the hydrological response of certain territories.

Using 137Cs tracing methods to estimate soil redistribution rates and to construct a sediment budget for a small agricultural catchment in the three gorges reservoir region, China

Soil erosion and associated off-site sedimentation are threatening the sustainable use of the Three Gorges Dam. To initiate management intervention to reduce sediment yields, there is an increasing need for reliable information on soil erosion in the Three Gorges Reservoir Region (TGRR). The purpose of this study is to use Cs-137 tracing methods to construct a sediment budget for a small agricultural catchment in the TGRR. Cores were taken from a pond and from paddy fields, for Cs-137 measurements. The results show that the average sedimentation rate in the pond since 1963 is 1.50 g cm(-2) yr(-1) and the corresponding amount of sediment deposited is 1,553 t. The surface erosion rate for the sloping cultivated lands and the sedimentation rate in the paddy fields were estimated to be 3,770 t km(-2) yr(-1) and 2,600 t km(-2) yr(-1), respectively. Based on the estimated erosion and deposition rates, and the area of each unit, the post 1970 sediment budget for the catchment has been constructed. A sediment delivery ratio of 0.5 has been estimated for the past 42 years. The data indicate that the sloping cultivated lands are the primary sediment source areas, and that the paddy fields are deposition zones. The typical land use pattern (with the upper parts characterized by sloping cultivated land and the lower parts by paddy fields) plays an important role in reducing sediment yield from agricultural catchments in the TGRR. A Cs-137 profile for the sediment deposited in a pond is shown to provide an effective means of estimating the land surface erosion rate in the upstream catchment.

Advanced modelling of runoff and soil redistribution for agricultural systems: The SERT model

Hydrological and soil erosion models allow mapping and quantifying rates of runoff depths and soil redistribution for different land uses and climatic scenarios. Mediterranean soils are threatened by marked seasonal changes in the climatic, thus soil and vegetation parameters and modelling predictions at monthly scale are required. The semi-physically-based Soil Erosion and Redistribution Tool (SERT) model is presented together with the results of its application in a Mediterranean agro-ecosystem (NE Spain) with a detailed database. The hydrological module is based on the recently published DR2 (Distributed Rainfall-Runoff) water balance model and the effects of man-made infrastructures on the natural dynamics of runoff connectivity are added. The erosion module is built using, as the basis, the Revised Morgan, Morgan and Finney model, and the new Remaining runoff Transport Capacity (TCr) factor used to estimate the rates of soil loss and deposition. Predicted runoff depth varied in time and space, presenting areas without runoff production mainly in Rendzic Leptosols and Haplic Calcisols between November and April. Average soil erosion was high in cultivated and bare soils, ca. 20 and 10Mgha−1yr−1, whereas rangeland soils were affected by moderate and, in some areas, by limited erosion processes. Soil erosion was minimal in February (0.08Mgha−1month−1 on average) and 23 times higher in October. The SERT model allowed mapping the significant changes in the monthly values of soil redistribution quantifying the variability in the magnitude of the processes involved. Predicted values of average soil loss and deposition were validated against quantified values with 137Cs obtaining an average Nash–Sutcliffe efficiency of 0.48 (Pearson's r=0.709) and a sediment balance of −1.15Mgyr−1 for the whole catchment that is consistent with the karst processes of the study area. The new model is an easy-to-run, reliable, low-input-demanding management tool with valuable outputs for hydrological and soil erosion studies in small agricultural catchments.

Simulating the long term impact of nitrate mitigation scenarios in a pilot study basin

The agro-hydrological model TNT2 was used to explore for a period of 14 years (1987–2001) the likely consequences of mitigation scenarios on nitrate contamination of the stream water in a small agricultural catchment. The Best Management Practices (BMPs) historically designed and implemented in 1992 and two devised agricultural scenarios (catch crop (CC) implementation and a global reduction of N fertilizer) are evaluated in term of nitrate contamination in the environment. Two of the BMPs consist in implementing natural strips of Poplar and rye-grass strips (5 meters large) along stream and ditches and the third is a delay in the burial of wheat straws (from August to October). Simulations indicated that natural strips implementation would lead to a slight decrease of Nitrate Fluxes (NF) in river by respectively 3.3% and 6.6% for rye-grass and poplar strips: a benefit associated to the non fertilization of strips area. Denitrification has not been particularly disrupted in such areas. The delay in the burial of wheat straw in autumn decreases annual mineralization rate and annual plant uptake (by respectively 9 and 13kgNha−1y−1) but increases denitrification fluxes by 6kgNha−1y−1. This would lead to a slight decrease by 6% of NF in stream (equivalent to 3.3mgNO3−l−1) and an average decrease of the following sunflower yield by 27%. The global reduction of fertilization by 10% would decreased NF in stream by 13.8% (equivalent to 8mgNO3−l−1), with a global decrease by 8kgNha−1y−1 of plant uptake. The cumulative effect of BMPs and CC would have together lead to a decrease of nitrate concentration from 57.5 to 46.6mgNO3−l−1 reaching the UE environmental quality objectives (below 50mgNO3−l−1). Spring crops yield following CC would have been penalized and the decrease of NF is balanced by an increase of denitrification fluxes in the environment contributing to release of N2O, a greenhouse gas, into the atmosphere.

Identification of critical source areas for erosion and phosphorus losses in small agricultural catchment in central Sweden

Identification of main contributing sources and critical source areas is a precondition for a cost-effective abatement strategy. This task is even more complicated in small ungauged catchments where inadequate amount of input, calibration, and validation data exists and thereby limit the application of process-based, parameter-heavy models. In this study, we present methodology to assess spatial variations in erosion and P losses, and based on source-apportionment and erosion modeling, to develop a base for source- and site-specific abatement countermeasures. Water quality monitoring program was developed to cover spatial variability in erosion and phosphorus (P) losses. Conducted soil survey detected high variations in soil P content within the catchment and strong correlation between soil P test and easily soluble P. A simple water balance model, calibrated for the comparable catchment with existing water flow measurements, was applied to determine water discharge, providing necessary input data for calculations of loads and source-apportionment. Source-apportionment modeling based on results from water discharge modeling, water quality monitoring program and soil survey identified P losses from arable fields as main source of P to two small lakes situated in the catchment, contributing roughly a half of total P load to the lakes. Inclusion of an assumed but previously undocumented point source considerably improved the performance of source-apportionment model. Existence of such a point-source was afterwards confirmed by results from detailed water quality monitoring program. Erosion modeling based on high-resolution digital elevation data identified areas most prone to erosion, and model results were compared to farmers' observations and analyses of 137Cs. Use of a spectrum of models and assessment tools formed a broad support for further steps in abatement efforts, and created concrete discussion base for implementation of detailed site-specific countermeasures.

A suspended sediment budget for the agricultural Can Revull catchment (Mallorca, Spain)

Extensive rainfed herbaceous crops are one of the most representative agricultural elements of Mediterranean region, which should be one of the major factors affecting erosion processes. Although land use is commonly seen as resulting in increased sediment yields, the implementation of soil and water conservation practices can have the reverse effect. A sediment budget has been established in Can Revull, a small agricultural catchment (1.03 km2) on the island of Mallorca, by using 137Cs measurements, sediment source fi ngerprinting and more traditional monitoring techniques to quantify the individual components of the budget. A large proportion of the material mobilized from cultivated fi elds without conservation practices (gross erosion of 775 t yr 1, 1,270 t km 2 yr 1) was subsequently deposited either within the fi eld of origin (112 t yr 1, 180 t km 2 yr 1) or at intermediate locations between the source fi eld and the channel network (fi eld-to-channel conveyance loss was 454 t yr 1, 744 t km 2 yr 1). The estimates of sediment accumulation rates on the fl oodplain in the lower reaches of the catchment indicate that the mean sedimentation rate was 0.14 g cm-2 yr-1. This value was extrapolated to the total area of the fl oodplain to estimate a total annual conveyance loss or storage of 445 t yr 1. Monitoring at the catchment outlet over the study period (2004–2008) indicated a mean annual suspended sediment yield of 7 t km-2 yr-1. The sum of the estimates of sediment yield and fl oodplain storage (452 t yr-1) was taken to represent the total annual input of suspended sediment to the channel system. This value was subsequently apportioned using the information provided by the fi ngerprinting investigation, to estimate the mass of sediment reaching the channel network from cultivated fi elds and from eroding channel banks. Thus the annual contribution from channel banks was estimated to be 242 t yr-1. In the case of the contributions from cultivated fi elds, the estimates obtained were, as expected, signifi cantly less than the values of net soil loss from these zones provided by the 137Cs measurements due to conveyance losses associated to fi eld-to-channel conveyance loss. The overall sediment delivery ratios (< 1 %) indicate that approximately 99 % of the sediment mobilized by erosion within the Can Revull catchment is subsequently deposited before reaching the monitoring station. As such, the low sediment outputs from the study catchment should be seen as refl ecting the importance of conveyance losses and storage rather than a lack of sediment mobilization from the catchment surface, although part of the catchment headwaters was modifi ed historically by means of terraces and transverse walls to prevent erosion.