Agricultural Catchments Research Articles

Impacts of sugarcane (Saccharum sp.) soil and fertiliser management practices on nutrients and sediment in plot-scale runoff from simulated rainfall

Runoff of nutrients and sediment from agricultural catchments can impair the health and resilience of aquatic ecosystems in receiving waterbodies. The sugarcane (Saccharum sp.) industry in Australia has adopted strategies to improve farm runoff water quality. Rainfall simulation trials were conducted to quantify the relative effectiveness of some sugarcane soil and fertiliser management practices on nutrient and sediment loss in runoff. The trials were conducted within the Herbert River catchment in the Wet Tropics of Queensland. Simulated rainfall (~76 mm h–1) was used to generate runoff from 1.70 m2 plots in first ratoon sugarcane on well-drained Ferralsols at Abergowrie and from 1.55 m2 plots in sugarcane planted into furrows and mound beds on an imperfectly drained Luvisol at Trebonne. At Abergowrie, the practices studied were; crop residue retention after harvest (green cane trash blanketing), applying a liquid rather than a granular fertiliser, and applying fertiliser to the sub-surface rather than to the surface on ratoon phase sugarcane. At Trebonne, sub-surface fertiliser placement was studied on plant cane and after varying amounts of time and repeated rainfall after fertiliser application. Comparisons were made with a similar study in plant cane at Macknade. Particulate nitrogen (N), particulate phosphorus (P), and suspended sediment losses in runoff were reduced by crop residue retention, which was expected. Dissolved inorganic N (DIN) loads (but not concentrations) were also reduced by residue retention. Dissolved inorganic N and filterable reactive P concentrations in runoff were reduced by sub-surface fertiliser placement, as expected, and by the liquid fertiliser that was studied. In contrast to expectations, DIN concentrations increased in mound beds (but not in furrows) with increasing time and rainfall after fertiliser application. The increase was attributed mainly to increases in soil mineral N and runoff volume. The practice that was most effective at reducing DIN loads in runoff (64–85% reduction) was sub-surface rather than surface placement of fertiliser.

Floating microplastic debris in a rural river in Germany: Distribution, types and potential sources and sinks

Microplastic debris affects marine as well as freshwater ecosystems and an increasing number of studies have documented the contamination in aquatic environments worldwide. However, while the research focuses on oceans and larger rivers, little is known about the situation in smaller rivers within rural catchments. Since microplastics pose various risks to ecosystems, wildlife and human health, it is important to identify potential sources, sinks and transport patterns, which are probably different for small rivers. In this study, we investigate the contamination with microplastic debris of the river Ems, representing a smaller river in Northwest Germany with an agricultural catchment. We hypothesised that with increasing river length the plastic concentration increases, especially downstream of towns, waste water treatment plant (WWTP) effluents and major tributaries as they may be important point sources of microplastics. We collected 36 surface water samples at 18 sampling sites within the first 70 km using manual driftnets. We sampled every 7 km and upstream and downstream of three larger towns, four major tributaries and four WWTP effluents. Overall, we found 1.54 ± 1.54 items m−3, which corresponds to the plastic concentrations in larger streams. However, the shape of the detected items differed as we did not find potential primary microplastic. Furthermore, the pattern contradicts our assumption, that the contamination increased with distance to the river's source. Downstream of towns, we found significantly less floating microplastic indicating possible sinks due to sedimentation at sites with slowing flow velocity caused by weirs in towns. Hence, the non-linear distribution pattern of microplastics indicates potential sinks of microplastics due to flow alterations on the river course.This should be considered in future studies modelling microplastic distribution and transport. Furthermore, studies especially in smaller rivers are urgently needed to identify and quantify sources and sinks and to find applicable solutions to reduce microplastic loads.

Nutrient dynamics in stream water and groundwater in riparian zones of a mesoscale agricultural catchment with intense seasonal pumping

Water and soil in agricultural areas are usually enriched with nutrients from excessive fertilizations. Riparian zones, the interfaces between terrestrial and aquatic systems, play crucial roles in delivery of the nutrients to groundwater and surface water. We evaluated spatiotemporal variations of water flow and nutrients in a mesoscale agricultural catchment along the riparian zones of the main stream for dry and rainy season by hydrologic monitoring and measurements of hydrochemical parameters and water stable isotopes. Gaining stream conditions were dominant in the mountainous upper reaches with steep topographic gradient while groundwater level in the lower basin considerably varied seasonally due to intense groundwater pumping to aid in heating greenhouses in winter season, which resulted in losing stream conditions during the dry season. Compared to shallow groundwater, deep groundwater had higher nitrate concentration derived from dry farmland in hilly terrains and induced recharge due to the groundwater pumping. Nitrate concentration was significantly higher in groundwater than that in stream water, indicating that groundwater is a major source of nitrate in stream water considering mainly gaining stream conditions. Compared to stream water, dissolved phosphorus concentration was higher in shallow groundwater recharged from paddy fields and floodplain areas with reducing conditions, evaporation signature of water stable isotopes, and lower nitrate concentration. Nutrient flux estimates for the catchment revealed that nitrate in stream water is considerably contributed by groundwater discharge and shallow groundwater in reducing conditions is a significant source of dissolved phosphorus in stream water, which was more pronounced in the rainy season. This study demonstrated that intense agricultural activities in riparian zones strongly affect stream-aquifer interactions and nutrient delivery to the stream, as well as groundwater. These findings can contribute to understanding hydrological and biogeochemical processes of nutrients in agricultural catchments and establishing an effective management of water use and nutrient application.

New insight into the response and transport of nitrate in karst groundwater to rainfall events

Although numerous studies focused on nitrate source, transformation and transport of river water in karst area have been reported, it's still unclear in understanding nitrate main source and transformation in karst groundwater system and how nitrate transport from soil to water during rainfall events in karst critical zone. In order to explore the response and transport of nitrate in karst groundwater to rainfall events, different depths of well water before, during and after rainfall event were sampled, and hillslope runoff, surface runoff of different land-use types during rainfall event were sampled synchronously at a typical karst agricultural catchment in Southwest China. Results showed that fluctuations of EC, pH and DO in deep borehole well (W1) and artesian well (W2) were small, on the contrary, variations of EC and DO in shallow well (W3) were large during sampling period. The nitrate concentrations and isotopic values indicated that nitrate in karst groundwater mainly originated from chemical fertilizer (CF), and influenced by denitrification process. High intensity of denitrification was observed in deep groundwater (87%) and artesian well water (almost 100%). Extremely high dual nitrate isotope values up to 46.8 ± 1.5‰ and 24.7 ± 0.5‰ were found in the deep artesian well. The small variation of water chemistry (EC, DO and pH), nitrate concentration and dual nitrate isotope values in deep wells during sampling period suggested that newly supplied nitrogen in deep groundwater during rainfall events also comes from deep groundwater. Low nitrogen concentrations in hillslope subsurface flow and surface runoff suggests that nitrogen transport process leading to increase of water nitrogen content mainly occur in depression. Nitrogen in depression soil is mainly transported to groundwater through fissures, fractures and conduits, rather than through vertical migration processes in the soil during rainfall events.

Tracer elements revealed the soil organic carbon sources in a dam-controlled watershed

The dynamics of the soil organic carbon (SOC) pool and the mechanisms that underpin those dynamics are at the frontier of terrestrial carbon (C) cycling research. Soil erosion seriously affects the migration and redistribution of SOC, driving a critical part of the C cycle. In this study, soil samples were collected from a 14.69 m sediment profile formed between 1960 and 2017, which was located in a small agricultural catchment on the Loess Plateau in northwest China. Nuclear tracer elements (137Cs, 210Pb) and stable C isotopes (δ13C) from organic matter were used to quantitatively identify the SOC sources in different siltation stages. The method used assumes that the contribution of a source is proportional to the elemental concentration in that source. SOC in soil samples from different erosion source areas (farmland, forest land, grassland, shrub land and ditch wall) were compared against the deposit in the check dam. The concentration of SOC in the sediments of the check dam was between 1.21 g kg−1 and 4.03 g kg−1. Nearly half of the SOC in the check dam sediments was traced back to the ditch wall. Farmland contributed 24.8 %, while shrub land contributed the least SOC at 0.4 %. SOC erosion attributed to farmland decreased gradually, while the relative contribution from the ditch wall increased. Within the agricultural catchment area, there was a net migration of SOC from the various land cover types (the source) to the sedimentary area behind the dam (the sink). These results indicate that a dynamic balance exists between redistribution and heterotopic storage of SOC during sedimentation. This study provides an opinion for sediment source identification and the soil C cycle in watersheds on the Loess Plateau.

Continuous high-frequency pesticide monitoring to observe the unexpected and the overlooked

Synthetic Plant Protection Products (PPPs) are a key element for a large part of today's global food systems. However, the transport of PPPs and their transformation products (TPs) to water bodies has serious negative effects on aquatic ecosystems. Small streams in agricultural catchments may experience pronounced concentration peaks given the proximity to fields and poor dilution capacity. Traditional sampling approaches often prevent a comprehensive understanding of PPPs and TPs concentration patterns being limited by trade-offs between temporal resolution and duration of the observation period. These limitations result in a knowledge gap for accurate ecotoxicological risk assessment and the achievement of optimal monitoring strategies for risk mitigation. We present here high-frequency PPPs and TPs concentration time-series measured with the autonomous MS2Field platform that combines continuous sampling and on-site measurements with a high-resolution mass spectrometer, which allows for overcoming temporal trade-offs. In a small agricultural catchment, we continuously measured 60 compounds at 20 minutes resolution for 41 days during the growing season. This observation period included 8 large and 15 small rain events and provided 2560 concentration values per compound.To identify similarities and differences among the compound-specific concentration time-series, we analysed the entire dataset with positive matrix factorisation. Six factors sufficiently captured the overall complexity in concentration dynamics. While one factor reflected dilution during rainfall, five factors identified PPPs groups that seemed to share a common history of recent applications. The investigation per event of the concentration time-series revealed a surprising complexity of dynamic patterns; physico-chemical properties of the compounds did not influence the (dis)similarity of chemographs. Some PPPs concentration peaks led while others lagged by several hours the water level peaks during large events. During small events, water level peaks always preceded concentration peaks, which were generally only observed when the water levels had almost receded to pre-event levels. Thus, monitoring schemes relying on rainfall or water level as proxies for triggering sampling may lead to systematic biases.The high temporal resolution revealed that the Swiss national monitoring integrating over 3.5 days underestimated critical concentration peaks by a factor of eight to more than 32, captured 3 out of 11 exceedances of legal acute quality standards (the relevant values in the Swiss Water Protection Law) and recorded 1 out of 9 exceedances of regulatory acceptable concentrations (the relevant values for the PPPs registration process).MS2Field allowed for observing unexpected and overlooked pesticide dynamics with consequences for further research but also for monitoring. The large variability in timing of concentration peaks relative to water level calls for more in-depth analyses regarding the respective transport mechanisms. To perform these analyses, spatially distributed sampling and time-series of geo-referenced PPPs application data are needed.

Contrasting responses of terrestrial and aquatic consumers in riparian – stream networks to local and landscape level drivers of environmental change

The biodiversity of streams and riparian zones is highly interlinked by multiple cross-habitat flows of organisms, nutrients and materials, and are thus increasingly recognized as forming an extended “meta-ecosystem” network spanning both within and across entire catchments. However, current understanding of how these extended networks respond to environmental change is limited by the lack of studies quantifying the local and larger-scale drivers of biodiversity in both terrestrial and aquatic systems simultaneously. Using high-resolution species and environmental data from eight boreal riparian – stream meta-ecosystems, half of which were situated in forest and half in agricultural catchments, we quantified land-use impacts and the importance of catchment and local (riparian, instream) scale variables on the diversity and community composition of three epigeal arthropod groups (spiders and staphylinid and carabid beetles) and aquatic macroinvertebrates. All four organism groups responded to quantifiable environmental variables. Staphylinid beetle and spider assemblages differed significantly between forested and agricultural sites and were strongly correlated with riparian variables such as vegetation type and soil properties, but also instream variables such as conductivity and floating macrophytes. By contrast, carabid beetle and aquatic macroinvertebrate assemblages did not differ between forested and agricultural sites and showed similar responses to catchment and local scale variables. Our results indicate that measures that only address local scale drivers of terrestrial biodiversity might be ineffective if the catchment-scale variables regulating aquatic biodiversity and aquatic – terrestrial linkages are not adequately addressed in ecosystem management.

Are spray drift losses to agricultural roads more important for surface water contamination than direct drift to surface waters?

Spray drift is considered a major pesticide transport pathway to surface waters. Current research and legislation usually only considers direct spray drift. However, also spray drift on roads and subsequent wash-off to surface waters was identified as a possible transport pathway. Hydraulic shortcuts (storm drainage inlets, channel drains, ditches) have been shown to connect roads to surface waters, thus increasing the risk of drift wash-off to surface waters. However, the importance of this pathway has never been assessed on larger scales. To address this knowledge gap, we studied 26 agricultural catchments with a predominance of arable cropping (n = 17) and vineyards (n = 9). In these study sites, we assessed the occurrence of shortcuts by field mapping. Afterwards, we modelled the areas of roads drained to surface waters using a high-resolution digital elevation model (0.5 m resolution) and a multiple flow algorithm. Finally, we modelled drift deposition to drained roads and surface waters using a spatially explicit, georeferenced spray drift model. Our results show that for most sites, the drift to drained roads is much larger than the direct drift to surface waters. In arable land sites, drift to roads exceeds the direct drift by a factor of 4.5 to 18, and in vineyard sites by 35 to 140. In arable land sites, drift to drained roads is rather small (0.0015% to 0.0049% of applied amount) compared to typical total pesticide losses to surface waters. However, substantial drift to drained roads in vineyard sites was found (0.063% to 0.20% of applied amount). Current literature suggests that major fractions of the drift deposited on roads can be washed off during rain events, especially for pesticides with low soil adsorption coefficients. For such pesticides and particularly in vineyards, spray drift wash-off from drained roads is therefore expected to be a major transport pathway to surface waters.

Impact of river dams on phosphorus migration: a case of the Pubugou Reservoir on the Dadu River in China

Reservoirs in agricultural catchments retain large proportions of inflowing phosphorus (P). However, the effects of reservoirs on the P cycle and related biogeochemical processes remain unclear. Therefore, this study investigated the degree to which a typical river-transition-reservoir in Southwest China retains both inflowing particulate phosphorus (PP) and dissolved total phosphorus (DTP) and various forms of P in sediments over different water seasons [normal-water season (NWS), low-water season (LWS), and high-water season (HWS)]. The proportions of inflowing PP and DTP retained were 37% and 27%, respectively. This result could be attributed to the absorption of DTP by the large load of intercepted sediment in the dam and the interception of PP itself. The rank of water seasons in terms of the proportion and load of inflowing TP retained was LWS (79%, 336 t P yr−1) > NWS (21%, 43 t P yr−1) > HWS (4%, 27 t P yr−1), which might be due to the high P concentration 0.78 mg L−1 and long hydraulic retention time (HRT) 780 d during the LWS. In the long-term, there was a high rate of retention of bioavailable phosphorus (BAP) in sediments (63%). This result could be attributed to the combined effect of fine sediment particles and organic matter (OM). In addition, HRT (R2 = 0.89, p < 0.05) affected the retention of P more significantly than P concentration (R2 = 0.56, p < 0.05). Dam interception during the LWS resulted in high BAP contents (280 mg kg−1) in sediments, high P concentrations (0.78 mg L−1), and weak hydrodynamics (HRT: 780 d) in overlying water. Therefore, further regulatory measures are urgently demanded during the LWS to prevent reservoir algal blooms.

Can the Morphological Quality Index (MQI) be used to determine the ecological status of lowland rivers?

Stream assessment indices have become increasingly important in quantifying the overall status of river networks to define specific targets for restoration initiatives. Such an assessment is particularly needed in degraded environments, such as agricultural streams. Some of these evaluation tools, for instance the Qualitative Habitat Evaluation Index (QHEI), are resource intensive because they are field based. Indices that are less dependent on detailed field observations, such as the Morphological Quality Index (MQI), can provide a greater spatial coverage at a lower cost. The objectives of this study are to (1) verify whether a river's morphological quality, quantified using the MQI, can predict the fish habitat quality of a stream determined with the QHEI for eastern Canadian lowland streams, (2) compare the morphological quality, estimated solely from remotely sensed data (RMQI), to the standard MQI, (3) test whether a modified MQI (MMQI) is more appropriate than the original index for the eastern Canadian landscape when comparing with the QHEI fish habitat assessment, and (4) considering the near absence of dams in lowland eastern Canadian streams, compare an MQI where dam-related indicators are optional (MQI-OD) with the QHEI. The hydrogeomorphological and ecological conditions of 118 stream reaches, including 97 in agricultural areas, were assessed across Quebec and southern Ontario using the MQI and QHEI. Each stream was initially evaluated using remotely sensed data: 1-m LiDAR (Quebec) and 5-m DEMs (Ontario), historical aerial photography (1964–2010), and orthophotos. Field assessments were conducted to validate fish habitat and morphological data for both indices. A strong correlation was observed at the reach scale between the MQI and QHEI (r = 0.81) and MQI and RMQI (r = 0.95). Both modified MQI (MMQI and MQI-OD) showed stronger correlation than the standard MQI with the QHEI (r = 0.87), likely because the standard MQI slightly overestimates the current quality status at the habitat scale in the case of small agricultural catchments where main pressures are not always represented by artificial structures. However, because the standard MQI performs generally well in the study area, we suggest using it to ensure consistency and facilitate comparison with other regions. The results from this study demonstrate that the MQI can be used to provide an assessment of ecological (fish habitat) quality. In addition, our results indicate that a remote hydrogeomorphological assessment can be conducted to estimate the overall status of stream networks.

The combination of chemical, structural, and functional indicators to evaluate the anthropogenic impacts on agricultural stream ecosystems.

Freshwater contamination by pesticides in agricultural landscapes is of increasing concern worldwide, with strong pesticide impacts on biodiversity, ecosystem functions, and ultimately human health (drinking water, fishing). In addition, the excessively large number of substances, as well as their low - and temporally variable - concentrations in water, make the chemical monitoring by grab sampling very demanding and not fully representative of the actual contamination. Tools that integrate temporal variations and that are ecologically relevant are clearly needed to improve the monitoring of freshwater contamination and assess its biological effects. Here, we studied pesticide contamination and its biological impacts in 10 stream sections (sites) belonging to 3 agricultural catchments in France. In each site, we deployed a combination of pesticide integrative samplers, biocenotic indicators based on benthic macroinvertebrates, and functional indicators based on leaf litter decomposition and associated fungal communities. The 3 approaches largely proved complementary: structural and functional indicators did not respond equally to different agricultural impacts such as pesticide contamination (as revealed by integrative samplers), nutrients, or oxygen depletion. Combining chemical, structural, and functional indicators thus seems an excellent strategy to provide a comprehensive picture of agricultural impacts on stream ecosystems.

A Workflow to Extract the Geometry and Type of Vegetated Landscape Elements from Airborne LiDAR Point Clouds

Rural European landscapes are characterized by a variety of vegetated landscape elements. Although it is often not their main function, they have the potential to affect river discharge and the frequency, extent, depth and duration of floods downstream by creating both hydrological discontinuities and connections across the landscape. Information about the extent to which individual landscape elements and their spatial location affect peak river discharge and flood frequency and severity in agricultural catchments under specific meteorological conditions is limited. This knowledge gap can partly be explained by the lack of exhaustive inventories of the presence, geometry, and hydrological traits of vegetated landscape elements (vLEs), which in turn is due to the lack of appropriate techniques and source data to produce such inventories and keep them up to date. In this paper, a multi-step methodology is proposed to delineate and classify vLEs based on LiDAR point cloud data in three study areas in Flanders, Belgium. We classified the LiDAR point cloud data into the classes ‘vegetated landscape element point’ and ‘other’ using a Random Forest model with an accuracy classification score ranging between 0.92 and 0.97. The landscape element objects were further classified into the classes ‘tree object’ and ‘shrub object’ using a Logistic Regression model with an area-based accuracy ranging between 0.34 and 0.95.

Hydrology under change: long-term annual and seasonal changes in small agricultural catchments in Norway

Abstract In agricultural catchments, hydrological processes are highly linked to particle and nutrient loss and can lead to a degradation of the ecological status of the water. Global warming and land use changes influence the hydrological regime. This effect is especially strong in cold regions. In this study, we used long-term hydrological monitoring data (22–26 years) from small agricultural catchments in Norway. We applied a Mann–Kendall trend and wavelet coherence analysis to detect annual and seasonal changes and to evaluate the coupling between runoff, climate, and water sources. The trend analysis showed a significant increase in the annual and seasonal mean air temperature. In all sites, hydrological changes were more difficult to detect. Discharge increased in autumn and winter, but this trend did not hold for all catchments. We found a strong coherence between discharge and precipitation, between discharge and snow water equivalent and discharge and soil water storage capacity. We detected different hydrological regimes of rain and snow-dominated catchments. The catchments responded differently to changes due to their location and inherent characteristics. Our results highlight the importance of studying local annual and seasonal changes in hydrological regimes to understand the effect of climate and the importance for site-specific management plans.

Spatial and Temporal Variability in Concentration‐Discharge Relationships at the Event Scale

AbstractThe analysis of concentration‐discharge (C‐Q) relationships from low‐frequency observations is commonly used to assess solute sources, mobilization, and reactive transport processes at the catchment scale. High‐frequency concentration measurements are increasingly available and offer additional insights into event‐scale export dynamics. However, only few studies have integrated inter‐annual and event‐scale C‐Q relationships. Here, we analyze high‐frequency measurements of specific conductance (EC), nitrate (NO3‐N) concentrations and spectral absorbance at 254 nm (SAC254, as a proxy for dissolved organic carbon) over a two year period for four neighboring catchments in Germany ranging from more pristine forested to agriculturally managed settings. We apply an integrated method that adds a hysteresis term to the established power law C‐Q model so that concentration intercept, C‐Q slope and hysteresis can be characterized simultaneously. We found that inter‐event variability in C‐Q hysteresis and slope were most pronounced for SAC254 in all catchments and for NO3‐N in forested catchments. SAC254 and NO3‐N event responses in the smallest forested catchment were closely coupled and explainable by antecedent conditions that hint to a common near‐stream source. In contrast, the event‐scale C‐Q patterns of EC in all catchments and of NO3‐N in the agricultural catchment without buffer zones around streams were less variable and similar to the inter‐annual C‐Q relationship indicating a homogeneity of mobilization processes over time. Event‐scale C‐Q analysis thus added key insights into catchment functioning whenever the inter‐annual C‐Q relationship contrasted with event‐scale responses. Analyzing long‐term and event‐scale behavior in one coherent framework helps to disentangle these scattered C‐Q patterns.

Mapping the potential for Payments for Ecosystem Services schemes to improve water quality in agricultural catchments: A multi-criteria approach based on the supply and demand concept

Payments for Ecosystem Services (PES) schemes are an increasingly popular form of catchment management for improving surface water and groundwater quality. In these schemes, downstream water users who are impacted by agricultural diffuse pollution incentivise upstream farmers to adopt better practices. However, this type of scheme will not be successful in all situations, in part, due to a lack of potential for agriculture to improve the suuply of good water quality and/or a lack in demand from downstream users for good water quality. As such, this study aims to present a flexible approach to mapping the potential for PES schemes to improve water quality in agricultural catchments. The approach is based on multi-criteria analysis, with supply and demand as key criteria. It uses expert judgement or current guidance on PES to select supply and demand sub-criteria, expert judgement to weight all criteria through pairwise comparisons and readily available, national datasets to indicate criteria. Once indicator data are normalized, it combines them in a weighted sums analysis and presents results spatially at the national scale, all within a geographical information system. The approach can easily be applied to the country or region of interest by using locally relevant criteria, expert judgement and data. For example, when applied to the situation for river waterbodies in England, supply sub-criteria were the contribution of agriculture to loads of the major pollutants (nitrogen, phosphorus and sediments) and demand sub-criteria were the different downstream water users present (water companies and, tourist and local recreational users). Experts assigned equal weight to supply and demand criteria and the highest weights to sediments and water companies for sub-criteria, respectively. When national scale datasets to indicate these criteria were combined in a weighted sums analysis, it was possible to identify areas of high potential for PES. This would hopefully motivate more detailed research at the individual catchment level into the constraints in linking supply and demand. Three case-study schemes were also examined to show how some of these constraints are being identified and overcome. As such, the approach forms the first tier in a two-tier framework for establishing PES schemes to improve water quality in agricultural catchments.

Evaluation of connectivity indices applied to a Mediterranean agricultural catchment

Connectivity analysis has proved a very valuable approach in understanding runoff and sediment fluxes in catchments. Many analyses use Connectivity Indices to model the patterns, but various versions of the Index have been developed. This paper tests the effects of these variants of the Index as applied to an agricultural-terraced catchment in the semi-arid Mediterranean region where occurrence of connectivity is relatively well documented to validate the modelling. This paper compares three versions of the Connectivity Index that use vegetation/land use (C), topography (RI) and roughness (n) as the weighting factors. Two different slope algorithms are also tested. The weightings are derived from a UAV (drone) survey, with simultaneous ground field mapping. The UAV imagery is found to provide a good, high resolution basis for determining detailed topography and vegetation cover. The slope limit present in some modelling is found to be unsuitable in this landscape where steep terrace embankments that are vegetated in places, and also partially vegetated uplands, occur. Subsequent modelling removed the slope limit. The major patterns of flow produced are similar between the models and are largely related to the overall topography and major drainage lines but considerably modified by the terracing. The model outcomes differ in detail, with the C factor model, incorporating vegetation, coming nearer to the field situation. The main deficiency in all the indices is the weakness in modelling ponding and sedimentation created by the terraces, and therefore the disconnectivity in the landscape. Comparison with field data confirms the variable connectivity in rainstorms, the effects of features such as tracks, and the effect of management actions. Evidence from field mapping during the last two decades indicates that the models probably represent the maximum possible functional connectivity.

Hydroclimatic variability and riparian wetland restoration control the hydrology and nutrient fluxes in a lowland agricultural catchment

Climate change and riparian management are major drivers of hydrological change, with important implications for nutrient fluxes in lowland streams. However, the coupling of hydrological and nutrient dynamics is complex due to time-variant flow paths, interacting biogeochemical processes and variation in land use, fertilization levels and management measures. Here, we assessed the long-term (30 year) changes in climate, discharge, groundwater levels and stream water quality in a mixed land use catchment (Demnitzer Mill Creek catchment, DMC) in northern Germany. The catchment was subject to wetland restoration and subsequent beaver (Castor fiber) recolonization over the last 15 years. Climatic variability is the primary driver of the hydrological regime, with discharge and groundwater variations closely coupled to increases in annual air temperature and fluctuations of annual rainfall. Over the past 8 years, stream flows have been greatly reduced during a prolonged drought period. Meanwhile, beaver dams have also moderated flow regimes and facilitated groundwater recharge. In terms of water quality regimes, the concentration-discharge (C-Q) relationship for nitrate (NO3-N) and soluble reactive phosphate (SRP) showed positive and negative chemodynamic behavior respectively, whilst dissolved organic carbon (DOC) exhibited chemostatic behavior. These differences were dictated by the intrinsic properties of each solute in terms of contrasting catchment storage and immobilization processes. Moreover, the changing hydrological conditions regulated such patterns and strengthened the C-Q relationships for nutrients during the drought. As an integrated result of drought and wetland restoration, nutrient concentrations and fluxes in stream water have been reduced in recent years. Headwater catchments like DMC, where much of the hydrochemical signature of larger, downstream river networks are generated, are critical observatories for integrating understanding of the linkages between water quality and land management in an era of climate change.

Using nitrate as a tracer to constrain age selection preferences in catchments with strong seasonality

Excess export of nitrate to streams has been an environmental issue attracting attention across the world. Lumped models based on StorAge Selection (SAS) functions have been used widely to investigate this export processes in terms of hydrological and chemical responses in catchment outflux – both in discharge (Q) and in evapotranspiration (ET). The validity of these models depends on the appropriate choice of water age selection preferences, i.e. the parameterization of the SAS functions of Q and ET, which are often calibrated against observed in-stream tracer data. This study attempted to assess the usefulness of in-stream nitrate (N-NO3) concentrations for resolving the SAS functions in terms of plausibility (being physically meaningful) and parameter uncertainty, in a small agricultural catchment in central Germany that experiences strong seasonality. A spatially implicit model was built to simulate the dynamics of nitrate export. The data sets of in-stream nitrate concentrations, nitrogen surplus in the soil, and the median age of Q were used to constrain the model. The best-fit (to all data sets) results indicate high plausibility of the calibrated, temporally dynamic age selection preferences for Q for most time of a year, however, not for ET. The uncertainty analysis using a Monte Carlo approach suggests that lower uncertainty is associated with the SAS functions for Q than that for ET. The uncertainties of the SAS functions can be reduced by 60% combining the three datasets instead of using only in-stream concentration data. This study demonstrated the usefulness of combined data sets for adequately determining dynamic age selection preferences for Q, but not for ET. Better constraining selection preferences will require better in-stream concentration data for the dry period and additional data sets for ET (e.g. age of ET).

Pathways and composition of dissolved organic carbon in a small agricultural catchment during base flow conditions

The amount and composition of organic carbon are major controls on water quality and ecological processes in streams. In this study we explored the fate of the quantity of dissolved organic carbon (DOC) and the composition of dissolved organic matter (DOM) in an agricultural hillslope - stream network system. We conducted our study in the 66 ha HOAL (Hydrological Open Air Laboratory) in Lower Austria. We measured DOC of the soil eluates from different land use units, water samples from the stream and from seven tributaries, and estimated DOM components by fluorescent spectrophotometry and PARAFAC analyses. Soil DOC shows the highest concentrations in summer, but DOC concentrations in the tributaries are lower in summer than in winter by between 19% and 31%. DOM composition of the soil eluate differs between land use units. The forest site exhibits the largest fractions of humic-like fluorophores and less labile DOM. DOM composition in the tributaries is, in addition to DOC, controlled by soil moisture. We estimated the DOC import from the tributaries into the stream as 125 kg during base flow conditions in the period February to December 2017 and the instream DOC production as 38 kg, considering mass balance and exchange with groundwater. Six out of seven DOM components have a positive net production along the stream, only aliphatic DOM with low molecular weight is consumed (65 % of its input). These findings suggest that agricultural land use increases DOC input into streams and alters their DOM quality. Instream processes modify DOM quality over short distances.

Factors affecting the placement of agricultural best management practices in the agricultural conservation planning framework (ACPF) toolbox in the mid-Atlantic region.

There has been a recent push to conduct spatially explicit landscape planning at finer hydrologic unit scales to mitigate diffuse pollution. The Agricultural Conservation and Planning Framework (ACPF) helps identify potential locations for agricultural conservation practices by using high-resolution soils and elevation data. This spatially explicit approach attempts to identify runoff and nutrient pathways, but output may be influenced by user-specified parameters and the properties of the digital elevation model (DEM) being used. Here we assess differences in the density and location of conservation practices sited by the ACPF toolbox across three DEM resolutions in three agricultural catchments, each in distinct physiographies (Ridge and Valley, Piedmont, Coastal Plain) of the United States mid-Atlantic region. Output frequency did not vary much for contour buffer strips or water and sediment control basins (WASCOBs) across DEM resolution, particularly compared with landscape type. The DEM resolution was crucial for the density of grassed waterways but of little consequence for contour buffer strips. Placement density of WASCOBs and contour buffer strips varied by region. Grassed waterways are sited based on either discrete values or statistical distributions of stream power index (SPI). A higher density of grassed waterways was placed in lower relief landscapes when a single standard deviation threshold was applied. Using discrete SPI values for the grassed waterway tool generated more consistent output across watersheds than output based on statistical distributions. These and other reported findings can help guide user decisions in future applications of the ACPF toolbox, particularly across different areas of study.