Small Agricultural Catchment Research Articles

Evaluating the SWAT model to predict streamflow, nitrate loadings and crop yields in a small agricultural catchment

Abstract. This study aimed to evaluate the applicability of the Soil and Water Assessment Tool (SWAT) to predict streamflow, nitrate loadings and crop yields for a small agricultural catchment in northeastern Germany. To this end, a 167 ha catchment was delineated consisting of 10 hydrological response units. Daily data for streamflow and nitrate loadings from 2004 to 2015 were used to calibrate and validate the model, while annual values for crop yields (winter wheat, winter barley, rapeseed, maize silage) were available. In addition, the detailed field maps provided by the local farmer were used to implement exact crop rotations and nitrogen fertilization into the model. Nash-Sutcliffe-Efficiencies for streamflow were 0.54 during the calibration and 0.57 for the validation period. The modeling performance for nitrate loadings were lower with 0.31 for the calibration and 0.42 for the validation period. The average crop yields were reproduced well, while SWAT failed to reproduce the inter-annual crop yield variations. A scenario analysis revealed that a slight decrease of nitrogen fertilization leads to significant reductions in nitrate loadings, while crop yields remained on a high level. The outcome of the study may help practitioners to operate according to an economic and environmental optimal N management. Nevertheless, experimental studies with varying fertilization intensities at catchment scale are needed to underpin the modeling results.

Long-term nitrate response in shallow groundwater to agricultural N regulations in Denmark

Over the last 30 years, Denmark has implemented a series of environmental action plans involving regulation of agricultural nitrogen (N) use and management in order to minimize the N pollution of the environment. The local effects of these action plan initiatives depend on various factors such as the efficiency of the implemented measures, and in particular, the hydrogeological structures and geochemical conditions of the subsurface that control the transport and fate of N. In this study, the effects of the Danish agricultural N regulations on shallow oxic groundwater are compared among five small agricultural catchments underlain by two types of lithology (sandy vs. loamy). Long-term spatially dense groundwater monitoring data is compared with monitoring data of nitrate in the root zone leaching and in stream. The results show clear improvements in the environmental state of shallow oxic groundwater in the first two decades since 1989 where the number of monitoring points with significant decreasing nitrate trends gradually increased for both soil types. Such improvements can be attributed to the effects of N mitigation measures implemented as a general regulation all over the country. However, deteriorations have been recorded in the last decade until 2016 where 26–35% of the monitoring points exhibited significant upward nitrate trends in both types of catchments. It is noteworthy that for sandy soils, the major part (93%) of the monitoring points showing significant upward trends in the period 2009–2016, also had concentrations above the groundwater standard of 50 mg/l nitrate in 2016. Altogether, the oxic groundwater in the sandy catchments was more responsive to N regulations than that in the loamy catchments. This might be due to efficient N regulation through statutory norms for the utilization of N in manure, increasing the N use efficiency in areas with a relatively high livestock density. Another reason is the nitrate vulnerability of the aquifers in sandy areas, with widespread oxic conditions from the top soil to the saturated zone. In the loamy catchments, nitrate may be reduced in near-surface localized reduced zones, and the reaction is often fast and the travel time from the root zone to the stream often relatively short. Therefore, stream nitrate concentrations were higher in the loamy catchments than in the sandy catchments. This is attributed to different hydrogeological pathways. Thus, in sandy catchments, deep groundwater is an important pathway, while in loamy catchments tile drains deliver nitrate directly to the streams. Our results indicate that N mitigation measures will help improve groundwater quality in sandy soils, while in loamy soils they will help to reduce surface water N loads. This implies that to achieve optimal environmental N reduction, agricultural N regulations should be strategically implemented according to farming characteristics and site-specific hydrogeological and geochemical conditions of the subsurface.

Assessing temporal variability and controlling factors of the sediment budget of a small agricultural catchment in Northern France (the Pommeroye)

A high-frequency monitoring station was implemented at the outlet of the small catchment of the Pommeroye (0.54 km2) in Northern France to study erosion by runoff and hydro-sedimentological responses to heavy rainfall events in the context of Quaternary loess deposits. The aim of this experimental work is to assess the temporal variability of sediment yield and to identify the factors controlling the hydro-sedimentary response. To achieve this goal, statistical and hydro-sedimentary dynamic analyses were performed. During two years of monitoring (April 2016–April 2018), 48 flood events were recorded. The specific sediment yield (SSY) is highly variable and was evaluated to 29.4–70 t km-2 yr−1 which is conventional for the study region. Most of the sediment yield was produced in winter (55%) and autumn (42%). Only 3% of SSY were produced during spring and summer periods. According to our results, only 6% of the erosive events are responsible for the transport of more than 40% of the sediment flux recorded at the outlet. This underlines the temporal variability of the hydro-sedimentary production in small agricultural catchments for which most of the hydro-sedimentary flux is produced during a limited number of events. The results of statistical analyses show that the total amount of rainfall and the duration of a rainfall episode are the main controlling factors on the hydro-sedimentary response. Our results also suggest that the rainfall kinetic energy better reflects the sediment detachment, and that the 48 h-antecedent rainfall is not linked to the hydro-sedimentary response.

Prioritising areas for soil conservation measures in small agricultural catchments in Norway, using a connectivity index

Measures designed to control erosion serve two purposes: on site (reduce soil loss) and off site (reduce sediment delivery to streams and lakes). While these objectives often coincide or at least are complementary, they could result in different priority areas when spatial planning is concerned. Prioritising for soil loss reduction at the field level will single out areas with high erosion risk. When sediment flux at the catchment scale is concerned, sediment pathways need to be identified in ex ante analyses of soil conservation plans. In Norway, different subsidy schemes are in place to reduce the influx of solutes and sediments to the freshwater system. Financial support is given to agronomic measures, the most important of which is reduced autumn tillage where areas with higher erosion risk receive higher subsidies. The objectives of this study are (1) to assess the use of an index of connectivity to estimate specific sediment yields, and (2) to test whether conservation measures taken in critical source areas are more effective than those taken at where erosion risk levels are the highest. Different modelling approaches are combined to assess soil loss at catchment level from sheet and gully erosion and soil losses through the drainage system. A calibration on two parameters gave reasonable results for annual soil loss. This model calibration was then used to quantify the effectiveness of three strategies for spatial prioritisation: according to hydrological connectivity, sheet erosion risk level and estimated specific sediment yield. The latter two strategies resulted in a maximum reduction in total soil loss due to reduced autumn tillage of 10%. Both model performance and the effectiveness of the different prioritisation strategies varied between the study catchments.

Impact of runoff prediction on grey water footprint in a small agricultural catchment

Impact of runoff prediction on grey water footprint in a small agricultural catchment

Effect of interflow and baseflow on nutrient runoff characteristics in agricultural area

The most common way of reducing non-point source pollutants from agricultural areas is the installation of reservoirs. However, this method is only effective for surface runoff of settleable pollutants. This study was conducted to estimate the effect of interflow, baseflow, and surface runoff on pollutant runoff in a small agricultural catchment. Runoff of organic matters, SS, and T-P were directly proportional to the rainfall variation, while ammonia and nitrate were inversely proportional to the amount of rainfall. The interflow and baseflow was only 46% of the total stream flow, but the nitrate load reached 78%. The interflow as a nutrient transport pathway should be considered for managing a stream water quality. It requires careful attention and appropriate control methodology such as vegetation to consider the influence by interflow. The reservoir as a dry extended detention pond (DEDP) has function of nutrient captor.

Influence of relief characteristics and landscape connectivity on sediment redistribution in small agricultural catchments in the forest-steppe landscape zone of the Russian Plain within European Russia

Sheet, rill and gully erosion occurring in the snowmelt period (March–April) and rainfall season (May–September) is the main factor of soil degradation and mobilized hillslope sediment redistribution within cultivated lands of the Russian Plain. The evaluation of sediment redistribution for the period since 1986 within catchment sediment cascades was done based on an integrated approach for some representative dry valley catchments located in the western (the Plava River basin, Tula Oblast, Russian Federation) and eastern (the Temeva Rechka Creek catchment, the Myosha River basin, Republic of Tatarstan, Russian Federation) sectors of the forest-steppe landscape zone of the Russian Plain. All the catchments studied are characterized by a high proportion (within the range of 60–80%) of cultivated lands. The modified version of the Universal Soil Loss Equation (USLE), the Russian State Hydrological Institute's erosion model, and the LandSoil erosion model were applied to calculate soil losses within the cultivated lands. The morphological classification of interfluve slopes and hollow slope catchments, in combination with the sediment delivery ratio (SDR) assessment for slopes and hollow slope catchments of different configuration, were used to assess sediment transfer from the cultivated lands. The Chernobyl-derived 137Cs isotope was applied as a chronomarker for sediment dating in different sediment sinks located along pathways from cultivated slopes to river valley bottoms. We found that the morphological features of the dry valley catchments, including a pattern of the dry valleys of different Hortonian orders, dry valley and hollow density, dry valley incision depth, and proportion of slopes and hollow slope catchments of different configurations are the main parameters that determined a proportional input of the different sediment sinks to the sediment interception along the pathways from the cultivated slopes to the river valley bottoms. The land use/cover features are mostly responsible for the pattern of buffer zones within the interfluve parts of the catchments. The quantitative assessment of the sediment budget allowed us to conclude that the mean SDR coefficients for the dry valley catchments of second, third, and fourth Hortonian orders are 0.56, 0.33, and 0.07 respectively. The extrapolation of the study results of sediment redistribution evaluation, obtained from the studied catchments located within the Plava River basin, to the entire basin of the river also allowed us to evaluate the mean value of hillslope-to-river-valley-bottom SDR = 0.27.

Optimal location of set-aside areas to reduce nitrogen pollution: a modelling study

AbstractDistributed models and a good knowledge of the catchment studied are required to assess mitigation measures for nitrogen (N) pollution. A set of alternative scenarios (change of crop management practices and different strategies of landscape management, especially different sizes and distribution of set-aside areas) were simulated with a fully distributed model in a small agricultural catchment. The results show that current practices are close to complying with current regulations, which results in a limited effect of the implementation of best crop management practices. The location of set-aside zones is more important than their size in decreasing nitrate fluxes in stream water. The most efficient location is the lower parts of hillslopes, combining the dilution effect due to the decrease of N input per unit of land and the interception of nitrate transferred by sub-surface flows. The main process responsible for the interception effect is probably uptake by grassland and retention in soils since the denitrification load tends to decrease proportionally to N input and, for the scenarios considered, is lower in the interception scenarios than in the corresponding dilution zones.

Comparison of two concepts for assessment of sediment transport in small agricultural catchments

Abstract The erosion, transport and deposition of sediments in small valley reservoirs represent a significant impact on their operations, mainly with regard to reducing the volume of their accumulation. The aim of this study is a comparison and uncertainty analysis of two modelling concepts for assessment of soil loss and sediment transport in a small agricultural catchment, with an emphasis on estimating the off-site effects of soil erosion resulted in sedimentation of a small water reservoir. The small water reservoir (polder) of Svacenicky Creek which was built in 2012, is a part of the flood protection measures in Turá Lúka and is located in the western part of Slovakia, close to the town of Myjava. The town of Myjava in recent years has been threatened by frequent floods, which have caused heavy material losses and significantly limited the quality of life of the local residents. To estimate the amount of soil loss and sediments transported from the basin, we applied two modelling concepts based on the USLE/SDR and WaTEM/SEDEM erosion models and validated the results with the actual bathymetry of the polder. The measurements were provided by a modern Autonomous Underwater Vehicle (AUV) hydrographic instrument. From the sediment data measured and the original geodetic survey of the terrain conducted at the time of the construction of the polder, we calculated changes in the storage volume of the polder during its four years of operation. The results show that in the given area, there has been a gradual clogging of the bottom of the polder caused by water erosion. We estimate that within the four years of the acceptance run, 10,494 m3 of bottom sediments on the Svacenicky Creek polder have accumulated. It therefore follows that repeated surveying of the sedimentation is very important for the management of the water reservoir.

Effect of riparian vegetation on stream bank stability in small agricultural catchments

The hydrological processes associated with vegetation and their effect on slope stability are complex and so difficult to quantify, especially because of their transient effects (e.g. changes throughout the vegetation life cycle). Additionally, there is very limited amount of field based research focusing on investigation of coupled hydrological and mechanical influence of vegetation on stream bank behavior, accounting for both seasonal time scale and different vegetation types, and none dedicated to marine clay soils (typically soil type for Norway).In order to fill this gap we established hydrological and mechanical monitoring of selected test plots within a stream bank, covered with different types of vegetation, typical for Norwegian agricultural areas (grass, shrubs and trees). The soil moisture, groundwater level and stream water level were continuously monitored. Additionally, soil porosity and shear strength were measured regularly. Observed hydrological trends and differences between three plots (grass, tree and shrub) were analysed and formed the input base for stream bank stability modeling. We did not find particular differences between the grass and shrub plot but we did observe a significantly lower soil moisture content, lower soil porosity and higher shear strength within the tree plot. All three plots were stable during the monitoring period, however modeling scenarios made it possible to analyse potential differences in stream bank stability under different vegetation cover depending on root reinforcement and slope angle.

Assessment of soil erosion rate trends in two agricultural regions of European Russia for the last 60 years

Forest–steppe and the southern forest ecotones of European Russia (ER) are the most productive agricultural areas in Russia. Both climate and land use changes have occurred within the ER during last 30 years. These changes can lead to changes in the timing, magnitude, and spatial distribution of soil erosion rates on cultivated lands. The objective of this research was to assess the trends in soil erosion rates since the 1960s for two agricultural regions of ER. Rates of soil erosion were estimated for two time windows (1963–1986 and 1986–2015) within the two agricultural regions. Both regions are characterized by a high proportion of cropland (> 60%), and within each region, one river basin and one 1st–3rd-order agricultural catchment were selected for a detailed assessment of soil erosion rates. Erosion models and visual interpretation of satellite images were used for the evaluation of the erosion rates for the river basins. Sediment budget assessments, 137Cs dating, geomorphologic mapping, and erosion models were used for the evaluation of the sediment redistribution for the two time windows in agricultural catchments. At the river basin scale, the mean annual erosion rate did not change in the western part of forest–steppe ecotone; however, there was a weak negative trend in the mean annual erosion rate for the eastern part of the southern forest ecotone. A large negative trend in the erosion rate was found for both small agricultural catchments. In all cases, the reduction in the erosion rates was mainly associated with a decrease of surface runoff during snowmelt, as a result of an increase in both the air and soil temperatures during winter season. The soil loss reduction during snowmelt was counteracted by an equal increase in rainfall erosion due to increase of rainfall intensity in western part of forest–steppe ecotone. Reduction of surface runoff during spring snowmelt was the main reason the erosion rates declined on cultivated lands within the forest–steppe and southern forest ecotones of ER. Evaluation of ephemeral gully erosion rate was not incorporated into State Hydrological Institute erosion model used for the evaluation of the soil losses during snowmelt. This has led to an underestimation of the total soil losses for the 1963–1986 time window for all study sites.

Contrasting suspended sediment export in two small agricultural catchments: Cross‐influence of hydrological behaviour and landscape degradation or stream bank management

AbstractThe study aimed to identify hydrological and landscape factors that influence sediment transport in 2 agricultural catchments in north‐western France—Moulinet and Kervidy‐Naizin—with the same climate, size, and bedrock but with contrasting suspended sediment (SS) fluxes. Discharge and SS concentrations were continuously monitored at the catchment outlets for 9 hydrological years. Data were examined at annual, seasonal, and individual storm‐flow event scales. Storm events were classified into 3 types—single small, successive, and major events—whose effect on sediment transport was studied. At the annual scale, SS yields were higher in Moulinet (16–63 × 103 kg km−2) than in Kervidy‐Naizin (3–22 × 103 kg km−2) despite similar water fluxes. However, Kervidy‐Naizin had higher potential for hillslope erosion because of more frequent occurrence of saturation excess runoff, more frequent bare soil in cultivated area, and fewer hedgerows. Major storm events were also stronger in this catchment. Although high SS concentrations and fluxes occurred only during a few major events in Kervidy‐Naizin, SS export was always substantial throughout the year in Moulinet. We hypothesised that bank degradation due to cattle trampling generated the higher SS export in the Moulinet catchment. This hypothesis was confirmed by the large decrease in SS fluxes following construction of watering troughs in the middle of the study period. In the Kervidy‐Naizin catchment, natural woody vegetation or grass buffer strips along the stream effectively protect the stream from bank erosion or a possible influx of hillslope erosion particles, except during some major events in winter.

Nitrogen and phosphorus discharge from small agricultural catchments predicted from land use and hydroclimate

Excess nutrients cause eutrophication of freshwaters all over the world. Decision-support tools are needed to assess nutrient discharges from catchments. This paper used a 28-year nutrient-discharge, hydroclimate and land-use history of small rural catchments to calibrate a simple nitrogen (N) and phosphorus (P) runoff model. The N and P runoffs declined following the post-Soviet collapse of agriculture, and stabilised at low output during the 1990s and early 2000s. Introduction of the European Union Common Agricultural Policy (CAP) re-intensified the agriculture and somewhat rebounded the N and P discharges. Thus, the history of the catchment represents a broad range of land-management systems. Our objective was to explain annual nutrient runoffs from small rural catchments by five factors: hydroclimate, soil type, land-use type, fertilisation and the autumn soil-nutrient stock. Our model independently predicted the eight-year mean N and P losses from a test set of small agricultural catchments in Estonia. This shows the impact of political decisions on agricultural contamination of waters. We can suggest our robust model as a decision-making tool for land-use management in small agricultural catchments.

Water Retention in a Small Agricultural Catchment and its Potential Improvement by Design of Water Reservoirs – A Case Study of the Bílý Potok Catchment (Czechia)

Abstract Water retention in the landscape is discussed in the context of conservation and improvement of both its productive and non-productive functions. We analysed the retention potential of a small agricultural catchment associated with the Bílý potok brook, investigating the possibility to improve its retention capacity and slow down the surface runoff, thus increasing the underground water resources. Method of curve numbers was used for that purposes. From results, it emerged that present maximum water retention in the Bílý potok catchment is 96.2 mm. It could increase by 101.3 mm in case of grassing about 20% arable land threatened by soil erosion. As next possibility to retain water from precipitations in landscape, capacity and transformation effect of reservoirs designed in master plans was analysed. The latest programming tools working in the GIS environment were used to assess the retention capacity of both the catchment surface and the reservoirs. Analysing master plans in the catchment, it was found that 16 designed water reservoirs (from 31) have a good potential to intercept water and transform flood discharges. In the result, priority for building of reservoirs was recommended according to their pertinence and efficiency in the studied catchment. Presented complex approach can be widely implemented, especially for better effectivity and cohesion of landscape planning and land consolidations processes.

Exploring the Dynamics of Transit Times and Subsurface Mixing in a Small Agricultural Catchment

AbstractThe analysis of transit/residence time distributions (TTDs and RTDs) provides important insights into the dynamics of stream‐water ages and subsurface mixing. These insights have significant implications for water quality. For a small agricultural catchment in central Germany, we use a 3D fully coupled surface‐subsurface hydrological model to simulate water flow and perform particle tracking to determine flow paths and transit times. The TTDs of discharge, RTDs of storage and fractional StorAge Selection (fSAS) functions are computed and analyzed on daily basis for a period of 10 years. Results show strong seasonal fluctuations of the median transit time of discharge and the median residence time, with the former being strongly related to the catchment wetness. Computed fSAS functions suggest systematic shifts of the discharge selection preference over four main periods: In the wet period, the youngest water in storage is preferentially selected, and this preference shifts gradually toward older ages of stored water when the catchment transitions into the drying, dry and wetting periods. These changes are driven by distinct shifts in the dominance of deeper flow paths and fast shallow flow paths. Changes in the shape of the fSAS functions can be captured by changes in the two parameters of the approximating Beta distributions, allowing the generation of continuous fSAS functions representing the general catchment behavior. These results improve our understanding of the seasonal dynamics of TTDs and fSAS functions for a complex real‐world catchment and are important for interpreting solute export to the stream in a spatially implicit manner.

Classification of rare land cover types: Distinguishing annual and perennial crops in an agricultural catchment in South Korea

Many environmental data are inherently imbalanced, with some majority land use and land cover types dominating over rare ones. In cultivated ecosystems minority classes are often the target as they might indicate a beginning land use change. Most standard classifiers perform best on a balanced distribution of classes, and fail to detect minority classes. We used the synthetic minority oversampling technique (smote) with Random Forest to classify land cover classes in a small agricultural catchment in South Korea using modis time series. This area faces a major soil erosion problem and policy measures encourage farmers to replace annual by perennial crops to mitigate this issue. Our major goal was therefore to improve the classification performance on annual and perennial crops. We compared four different classification scenarios on original imbalanced and synthetically oversampled balanced data to quantify the effect of smote on classification performance. smote substantially increased the true positive rate of all oversampled minority classes. However, the performance on minor classes remained lower than on the majority class. We attribute this result to a class overlap already present in the original data set that is not resolved by smote. Our results show that resampling algorithms could help to derive more accurate land use and land cover maps from freely available data. These maps can be used to provide information on the distribution of land use classes in heterogeneous agricultural areas and could potentially benefit decision making.

Tempo-spatial controls of total coliform and E. coli contamination in a subtropical hilly agricultural catchment

Waterborne microbial pollution remains poorly understood in developing countries, especially in rural areas. The transport of point (e.g. untreated domestic wastewater) and non-point sources of fecal bacteria such as Total coliform (TC) and Escherichia coli (E. coli) and their spatial and temporal patterns were identified in a typical small agricultural catchment in a hilly area of subtropical China. Over the sampling area, 460 water samples from 24 sites were collected during a period of 12 months, and a remediation option using an existing ecological ditch was evaluated. The results showed that 70% surface water samples exceeded the guideline of 1000 TC MPN 100 ml−1 in the catchment. Also, 90% of groundwater samples exceeded the guideline of 10 MPN E. coli 100 ml−1 for safe drinking water. Higher concentrations of TC and E. coli occurred both in the warm and rainy season in surface water and groundwater. TC and E. coli concentrations in surface water tributaries and groundwater near residential areas were greater than those in downstream water bodies. A 300 m long vegetated ecological ditch greatly reduced average concentrations TC of 97.7% and E.coli of 99.5%. TC and E. coli concentrations were significantly correlated with electrical conductivity and turbidity, both in surface water and groundwater, indicating that mixtures of organic colloids and sediment were the most important transport vector for pathogens. However, temperature only showed a controlling effect on bacteria concentration in groundwater in this particular catchment. The current research could help predict microbial risks and establish integrated management strategy to improve water quality in the hilly agricultural catchments in the subtropical region.

Multi-scale assessment of forest cover in an agricultural landscape of Southeastern Brazil: Implications for management and conservation of stream habitat and water quality

Forest cover has important functions for streams. Consequently, deforestation and forest degradation due to agricultural activities tend to have negative impacts on stream ecosystems. We related forest cover to stream variables, expecting to find better habitat and water quality conditions in catchments with better forest cover conditions in order to evaluate forest cover as indicator of stream health in agricultural landscapes. We sampled stream variables and quantified forest cover and physical variables in 60 small agricultural catchments in Southeast Brazil. We used redundancy and regression analysis to relate the landscape predictors to the channel responses. Percent forest cover had low to intermediate values in the spatial scales evaluated. Forest cover was fragmented and mostly located in riparian and steep slope areas. Redundancy analysis showed little influence of forest cover on the response variables, which were more influenced by catchment physical variables. Regression analysis showed that forest cover in the reach and forests located closer to the sampled reach are positively related to wood, habitat diversity, and dissolved oxygen, and negatively related to channel depth, volume, and temperature. We also found that forest cover fragmentation is negatively related to pH, potassium, water acidity, and temperature. Although many of these relationships were fairly weak, it appears that naturally regenerated forest cover is at least moderately effective in protecting streams in agricultural landscapes in the region.

Hydro‐sedimentary Dynamics of a Drained Agricultural Headwater Catchment: A Nested Monitoring Approach

Core Ideas Discharge and sediment concentrations were measured in a drained catchment. The impact of tile drainage at the catchment scale was analyzed with a nested approach. The major part of sediment fluxes occurred during winter floods. Sediment transfer in the tile drain was episodic although significant. Soil erosion and sediment transfer are extensive in lowland agricultural catchments. In these environments, high‐frequency datasets of both water discharge and suspended sediment concentrations are often lacking. In particular, the impact of tile drainage networks on sediment fluxes in these catchments is poorly understood. Our research quantified sediment fluxes between 2013 and 2016 at five nested stations including a tile drain outlet across a small (25‐km2) agricultural catchment (located in the Loire River basin, France) representative of lowland cultivated environments of the middle part of this basin. Sediment fluxes varied from 1 to 38 t km−2 yr−1 across the monitored subcatchments. Most of sediment fluxes (79 ± 9%) were measured during flood events (n = 44), mostly occurring in winter (75%) and spring (20%). Seasonality controlled most of the variations of sediment fluxes. Sediment transfers in tile drains occurred episodically. Flows were measured during 11.4% of the monitoring time. A mean time lag of 80 min was recorded between the peak discharge at the drain outlet and the downstream river station. At the event scale, sediment fluxes exported from the tile drain varied between 1.1 × 10−4 and 2.5 t km−2 and remained of the same order of magnitude as those measured at the downstream river station, although the latter were on average 53% lower. Results suggest significant storage of sediment material in the river channel and its export during the most intense flood events. Our research emphasizes the need for continuous monitoring of water and sediment transfers in agricultural catchments equipped with tile drains.

Modelling spatial and temporal variations of annual suspended sediment yields from small agricultural catchments

Estimates of sediment yield are important for ecological and geomorphological assessment of fluvial systems and for assessment of soil erosion within a catchment. Many regulatory frameworks, such as the Convention for the Protection of the Marine Environment of the North-East Atlantic, derived from the Oslo and Paris Commissions (OSPAR) require reporting of annual sediment fluxes. While they may be measured in large rivers, sediment flux is rarely measured in smaller rivers. Measurements of sediment transport at a national scale can be also challenging and therefore, sediment yield models are often utilised by water resource managers for the predictions of sediment yields in the ungauged catchments. Regression based models, calibrated to field measurements, can offer an advantage over complex and computational models due to their simplicity, easy access to input data and due to the additional insights into factors controlling sediment export in the study sites. While traditionally calibrated to long-term average values of sediment yields such predictions cannot represent temporal variations. This study addresses this issue in a novel way by taking account of the variation from year to year in hydrological variables in the developed models (using annual mean runoff, annual mean flow, flows exceeded in five percentage of the time (Q5) and seasonal rainfall estimated separately for each year of observations). Other parameters included in the models represent spatial differences influenced by factors such as soil properties (% poorly drained soils and % peaty soils), land-use (% pasture or % arable lands), channel slope (S1085) and drainage network properties (drainage density). Catchment descriptors together with year-specific hydrological variables can explain both spatial differences and inter-annual variability of suspended sediment yields. The methodology is demonstrated by deriving equations from Irish data-sets (compiled in this study) with the best model efficiency of 0.84 and best model fit of adjusted R2 of 0.82. Presented approach shows the potential for regression based models to model contemporary suspended sediment yields in small river systems.