Agricultural Headwater Catchment Research Articles

Interpreting the seasonal and long-term trend of nitrate in both groundwater and spring water in a typical headwater wetland with well-defined groundwater flow pathways

Timescale problems of nitrate behavior (i.e. seasonal variation and long-term trend) in headwater are closely related to its hydrological process. In a typical agricultural headwater catchment in the Chiba prefecture, Japan, the groundwater nitrate concentration showed an increasing trend, while for spring water, a substantial decreasing trend was observed during our monitoring period. Two key issues, (1) identification of multiple flow pathways and (2) evaluation of the residence time for different pathways, were emphasized to reveal the factors controlling the different patterns of nitrate trend. Three major flow pathways including vertical soil water flow (VF), lateral groundwater flow (LG) and deep groundwater flow (DG) along the upland-slope-valley were differentiated. Different timescales of three flow pathways were identified. The residence time of VF was calculated as 9–10 years based on the soil water budget equation and the apparent age of LG was estimated as 41 years by chlorofluorocarbon (CFC) traces. The increasing trend of NO3− in groundwater agreed well with the historical nitrate loading, and the decreasing trend of NO3− in spring was mainly influenced by nitrate behavior of LF, which substantially decreased due to reduction of nitrogen fertilizer loadings since 2000.

Nitrate sinks and sources as controls of spatio-temporal water quality dynamics in an agricultural headwater catchment

Abstract. Several controls are known to affect water quality of stream networks during flow recession periods, such as solute leaching processes, surface water–groundwater interactions as well as biogeochemical in-stream turnover processes. Throughout the stream network, combinations of specific water and solute export rates and local in-stream conditions overlay the biogeochemical signals from upstream sections. Therefore, upstream sections can be considered functional units which could be distinguished and ordered regarding their relative contribution to nutrient dynamics at the catchment outlet. Based on snapshot sampling of flow and nitrate concentrations along the stream in an agricultural headwater during the summer flow recession period, we determined spatial and temporal patterns of water quality for the whole stream. A data-driven, in-stream-mixing-and-removal model was developed and applied for analysing the spatio-temporal in-stream retention processes and their effect on the spatio-temporal fluxes of nitrate from subcatchments. Thereby, we have been able to distinguish quantitatively between nitrate sinks, sources per stream reaches, and subcatchments, and thus we could disentangle the overlay of nitrate sink and source signals. For nitrate sources, we determined their permanent and temporal impact on stream water quality and for nitrate sinks, we found increasing nitrate removal efficiencies from upstream to downstream. Our results highlight the importance of distinct nitrate source locations within the watershed for in-stream concentrations and in-stream removal processes, respectively. Thus, our findings contribute to the development of a more dynamic perception of water quality in streams and rivers concerning ecological and sustainable water resource management.

Characterisation of agricultural drainage ditch sediments along the phosphorus transfer continuum in two contrasting headwater catchments

This study investigated the phosphorus (P) source, mobilisation and transport potential of ditch bed sediments as well as surrounding field and bank soils in two agricultural headwater catchments with contrasting soil drainage capacities. This information is important for discerning the potential for ditches to attenuate or augment transfers of P from upstream sources and thus for developing appropriate management strategies for these features. Phosphorus sources were characterised using the Mehlich3-P, water-soluble P and total P tests. Phosphorus mobilisation potential was characterised using the Mehlich3-AL/P, Mehlich3-Ca/P and DESPRAL P tests. Phosphorus transport potential was characterised using data collected on the presence/absence of surface water in ditches during field surveys and downstream turbidity data. Ditch sediments had similar P source contents (Mehlich3-P, water-soluble P and total P) to the surrounding field soils and higher P contents than bank soils. However, calcium contents of sediments in the poorly drained catchment reflected the deep sub-soils rather than the surrounding field and bank soils. Mehlich3-Al/P and Mehlich3-Ca/P contents of ditch sediments in the well (non-calcareous) and poorly (calcareous) drained catchments respectively indicated potential for P retention (above thresholds of 11.7 and 74, respectively). However, sediments were less aggregated than field soils and may mobilise more particulate P (PP) during rain events. Nevertheless, the majority of surveyed ditches dried out from March to September 2011; thus, their potential to mobilise PP may be less important than their capacity to attenuate soluble and PP during this time. In these and similar catchments, soluble P attenuation and particulate P mobilisation should be maximised and minimised, respectively, for example, by cleaning out the sediments before they become saturated with P and encouraging vegetation growth on ditch beds. This study also highlighted the influence of deep sub-soils on soluble P retention in ditches and thus the utility of characterising soils below depths normally included in soil classifications.

Contrasting controls on the phosphorus concentration of suspended particulate matter under baseflow and storm event conditions in agricultural headwater streams

Whilst the processes involved in the cycling of dissolved phosphorus (P) in rivers have been extensively studied, less is known about the mechanisms controlling particulate P concentrations during small and large flows. This deficiency is addressed through an analysis of large numbers of suspended particulate matter (SPM) samples collected under baseflow (n=222) and storm event (n=721) conditions over a 23-month period across three agricultural headwater catchments of the River Wensum, UK. Relationships between clay mineral and metal oxyhydroxide associated elements were assessed and multiple linear regression models for the prediction of SPM P concentration under baseflow and storm event conditions were formulated. These models, which explained 71–96% of the variation in SPM P concentration, revealed a pronounced shift in P association from iron (Fe) dominated during baseflow conditions to particulate organic carbon (POC) dominated during storm events. It is hypothesised this pronounced transition in P control mechanism, which is consistent across the three study catchments, is driven by changes in SPM source area under differing hydrological conditions. In particular, changes in SPM Fe–P ratios between small and large flows suggest there are three distinct sources of SPM Fe; surface soils, subsurface sediments and streambed iron sulphide. Further examination of weekly baseflow data also revealed seasonality in the Fe–P and aluminium oxalate–dithionate (Alox–Aldi) ratios of SPM, indicating temporal variability in sediment P sorption capacity. The results presented here significantly enhance our understanding of SPM P associations with soil derived organic and inorganic fractions under different flow regimes and has implications for the mitigation of P originating from different sources in agricultural catchments.

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

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

Groundwater flow path dynamics and nitrogen transport potential in the riparian zone of an agricultural headwater catchment

summary Shallow groundwater dynamics play a critical role in determining the chemistry and movement of nitrogen (N) in the riparian zone. In this study, we characterized N concentration variability and hydrologic transport pathways in shallow groundwater draining areas of a riparian area with and without emergent groundwater seeps. The study was conducted in FD36, an agricultural headwater catchment in the Ridge and Valley physiographic region of central Pennsylvania, USA. Three seep and adjacent non-seep areas were each instrumented with a field of 40 piezometers installed in a grid pattern (1.5-m spacing) at both 20- and 60-cm depths. Piezometers were monitored seasonally for approximately two years (October 2010–May 2012). Results showed that hydraulic head within seep areas was variable and some regions exhibited upward vertical hydraulic gradients of 0.18–0.27. Non-seep areas were characterized by uniform hydraulic head levels and were relatively hydrostatic. Nitrate-N (NO3-N) concentrations in seep areas were significantly greater than those in the non-seep areas at two of the three study sites. A two-component mixing model using chloride as a conservative tracer indicated that shallow groundwater in seep areas was primarily (53–75%) comprised of water from a shallow fractured aquifer, which had elevated NO3-N concentrations (5.7 mg L � 1 ). Shallow groundwater in non-seep areas, however, was comprised (58–82%) of perched water on top of the fragipan that was likely recharged locally in the riparian zone and had low NO3-N concentrations (0.6 mg L � 1 ). Higher NO3-N concentrations, variable hydraulic head, and groundwater emergence onto the land surface in seep areas provided evidence for preferential flow paths as an important conduit for water and N movement in these areas of the riparian zone. We conclude that the potential for N delivery to the stream in FD36 was much greater from seep areas compared to non-seep areas. Targeted management of seeps should be a priority in efforts to reduce NO3-N levels in riparian zones within headwater agricultural catchments. Published by Elsevier B.V.

The contribution of a drainage network to the spatial and temporal patterns of macroinvertebrate diversity across an agricultural headwater catchment

The contribution of a drainage network to the spatial and temporal patterns of macroinvertebrate diversity across an agricultural headwater catchment

The contribution of a drainage network to the spatial and temporal patterns of macroinvertebrate diversity across an agricultural headwater catchment

The headwaters of lowland agricultural catchments in Europe often consist of extensive networks of small field drainage ditches. In comparison to larger freshwater systems, these small habitats are likely to be highly impacted by man, both physically and chemically. Little research has been conducted on drainage ditch ecosystems despite their high percentage contribution to the total length of running water habitat in a catchment and their potential contribution to total catchment freshwater biodiversity. Here, we report on an investigation into the macroinvertebrate species richness patterns of drainage ditches in an intensive agricultural landscape in south-west Ireland. Despite high anthropogenic disturbance in the form of nutrient and fine sediment inputs this study revealed dynamic and diverse community assemblages. Our results highlight the large beta diversity associated with these small waterbodies. The ditch habitats were found to support high numbers of unique and rare taxa, including nationally rare and threatened species.

Effect of climate change and increased atmospheric CO2 on hydrological and nitrogen cycling in an intensive agricultural headwater catchment in western France

Climate change and increased atmospheric CO2 concentration can impact hydrological and nitrogen cycling at the catchment scale. The objective of this study is to assess these impacts in an intensive agricultural headwater catchment in western France. A calibrated and validated agro-hydrological model was driven by output of the climate model ARPEGE under the A1B emission scenario over 30-year simulation periods. Our study indicated that with climate warming and increased atmospheric CO2, the main trends in water balance were a decrease in annual actual evapotranspiration (AET), a decrease in annual discharge and wetland extent, and a decrease in spring and summer of groundwater recharge and soil-water content. Not considering the effects of increased atmospheric CO2 in the agro-hydrological model led to overestimating discharge decrease and underestimating AET decrease and wetland extent. Climate change could influence N cycling by increasing soil N mineralisation, increasing soil denitrification in wetlands and upstream areas, and decreasing NO3–N load to streams. Since wetlands appear to be sensitive to climate change, improving modelling to better predict their responses is an important issue, especially to help plan sustainable management of these vulnerable areas.

Effect of a novel constructed drainage ditch on the phosphorus sorption capacity of ditch soils in an agricultural headwater catchment in subtropical central China

Plant vegetation has been reported to be one of the best management practices to control the transport of nutrient pollutants in agricultural drainage. In this study, soil samples were collected from a novel constructed drainage ditch (ND) vegetated with strips of Indica canna, Hydrocotyle vulgaris, Sparganium stoloniferum, Myriophyllum aquaticum, and Juncus effusus, from an upstream ordinary agricultural drainage ditch (OD), and from an adjacent paddy field (PD), at the 0–5 and 5–15cm layers, to compare phosphorus (P) sorption capacity. In the ND, there was a great difference in the properties of ditch soils between at the 0–5cm and 5–15cm layers. For instance, total organic carbon (TOC) at the 0–5cm layer increased rapidly with the growth of Indica canna, Hydrocotyle vulgaris, and Myriophyllum aquaticum in the ND for approximately 3 years. Correspondingly, those three plants elevated the P sorption maximums (Smax) of the ditch soil, which were determined to be 521, 612, and 552mgkg−1, respectively. The equilibrium phosphorus concentration (EPC0) and the degree of P saturation (DSP) of the ditch soils in the ND were also shown to be lower than those in the OD, which may be attributed to efficient ditch clearance in the ND. The Pearson's correlation analysis revealed significant correlations of TOC with the key parameters of the P sorption isothermal curves, such as the Freundlich adsorption constant (KF), Smax, EPC0, and DSP. Furthermore, the multivariate regression analysis showed that TOC explained more than 50% of the variability of Smax and DPS, suggesting that the vegetated drainage ditch played an important role in regulating the P adsorption capacity of the ditch soils. Our collective results indicated that plant vegetation and ditch clearance can be optimized to enhance the P retention capacity of ditch soils via robust and re-vegetated drainage ditch systems, such as the ND, and have a great potential to reduce the P loss from agricultural headwater catchments.

Stream water nutrient and organic carbon exports from tropical headwater catchments at a soil degradation gradient

Carbon and nutrient losses were quantified from four small headwater catchments in western Kenya in the year 2008. They include a forested catchment and three catchments under maize continuously cultivated for 5, 10 and 50 years following forest conversion. The C isotopic composition of dissolved organic C (DOC) in stream discharge suggested that soil organic C (SOC) derived from the original forest rather than OC from maize may have contributed to a large extent to watershed OC losses, even 50 years after the forest was removed. Flow-weighted stream water concentra- tions of DOC and coarse particulate OC, all N species, total P, K and Na significantly (P \ 0.05) increased in streams after forest conversion and long-term cultiva- tion. Solute concentrations increased despite the fact that soil contents decreased and total water flow increased indicating mobilization of C and N, P and K from soil with progressing cultivation. In contrast, Ca and Mg concentrations in stream water did not system- atically change after deforestation and cultivation, and may be controlled by geochemical weathering rather than by changing water flow paths or topsoil contents. All OC and nutrient exports increased with longer cultivation over decadal time scales (P \ 0.05) to the same or greater extent than through deforestation and the first years of cultivation. Fluvial OC and total N losses were 2 and 21 % of total SOC and total N decline, respectively, in the top 0.1 m over 50 years. Fluvial OC losses therefore played a minor role, and SOC losses were mainly a result of microbial mineralization. Resulting total N losses by stream discharge, however, were large with 31 kg ha -1 year -1 after 50 years of continuous cropping in comparison to fertilization of 40 kg N ha -1 year -1 . Most (91 %) of the N losses occurred as NO3 - . In contrast, P losses by stream discharge were negligible in comparison to plant uptake. Water losses should be managed to reduce soil fertility declines especially through large N export from agricultural headwater catchments. However, stream concentrations of both P (0.01-0.15 mg L -1 )

Source strengths, transport pathways and delivery mechanisms of nutrients, suspended solids and coliforms within a small agricultural headwater catchment

Analysis of water samples and accompanying flow data collected (on ~100 occasions) from well defined land drain outlets located in a small catchment in NE Scotland were made over a five year period. The complex relationship between individual sources that can exist even within a small (200ha) agriculturally managed headwater catchment was clearly evident. On average ~60% of the measured flow from the catchment outlet was accounted for, with ~50% originating from field drains and 10% from the farmyard. Certain field drains stopped flowing during the summer. Flow from the farmyard was continuous, and because livestock were present all year round also represented a renewable source of potential contaminants. The majority of nitrate and suspended sediment originated directly from field drainage. The variability in nitrate concentration between individual field drains was large and probably reflected differences in soil drainage properties. Farmyard drainage contributed a large proportion of the ammonium, phosphate and Faecal Indicator Organisms (FIO) measured as a flux from the catchment. On numerous sampling occasions the combined flux from individual sources was greater than the corresponding loss measured at the catchment outlet. This was attributed to result from the temporary storage/retention mechanisms (sedimentation, transformation or biological uptake/exchange) that can operate within the stream channel. Despite many fields being grazed and/or receiving regular applications of slurry/manure, the majority ~60% of the total flux of FIO still originated from the ‘farmyard’, with significant contributions from the field drains only occurring during the autumn. The presence of field drinkers and secure well maintained fencing denying cattle access to the open drainage channel (often a recommended best management practice) may well have contributed to this observation. Benefits to water quality that might arise from riparian management, such as buffer strips in this particular situation may be limited due to the dominant contribution originating from land drains and farmyard.

Controls on the phosphorus content of fine stream bed sediments in agricultural headwater catchments at the landscape-scale

There have been no landscape-scale assessments which quantify the relative importance of the organic and mineral properties of BS (bed sediment) and associated catchment characteristics (geology, land cover and mean topsoil phosphorus (P) content) for BSP concentration. Mid infra red diffuse reflectance spectrometry was applied to estimate the quantities of organic matter, dithionite extractable aluminium- (Al d ) and iron (Fe d ), kaolinite, dioctahedral clay and mica (D&M) minerals in 1052 snapshot samples of fine (<150 μm) BS in small to medium-sized (5–55 km 2) agricultural headwater catchments across a large area (15 400 km 2) of central England. Analyses included estimates of BS specific surface area, cerium (Ce) concentrations (enriched in P-bearing apatite and P-fertilsers), and catchment average topsoil P content. Simple linear regression demonstrated that the proportion of variance in BSP explained by specific components of BS across all catchments declined in the following order: Al d > Fe d > topsoil P = kaolinite = residual iron> organic matter = Ce> D&M > mineral SSA. No single component accounted for more than 36% of the variance in BSP. Multiple regression – including a classification of bedrock lithology and proportions of arable and grassland by area – accounted for 61.9% of the variance in BSP. The proportion of arable and grassland by area in each catchment was also a statistically significant predictor of BSP. Across this large region – with widely differing geology and soils – Fe d in BS is more strongly associated with kaolinite than D&M minerals because iron-oxyhydroxides and kaolinite form contemporaneously during pedogenesis. The SSA of fine bed sediments is largely determined by catchment area, fitted accurately using a power function.

Role of water table dynamics on stream nitrate export and concentration in agricultural headwater catchment (France)

The objective of this work is to identify hydrological processes controlling nitrate export and base flow concentration at the year scale in agricultural headwater catchment streams fed by an unconfined aquifer. The study is based on the hydrological and hydrochemical monitoring of the stream and shallow groundwater of three headwater catchments (0.1­5 km2) in Western France over three to five water years. Results show that at the year scale nitrate export from the three catchments is a transport-limited process. The stream nitrate flux depends on how much water flows in the stream and not on the distribution of the flow over the year. Seasonal variations of concentration over the water year were complex. Variations were different for a given year from one catchment to another, and also different for a given catchment from one year to another. We show that the seasonal variations are controlled by water table depth dynamics along hillslope associated with spatially distributed nitrate concentration in the groundwater. The groundwater displays high nitrate concentrations, from 6 to 22 mg, in upland and in the deeper zones of bottom lands. Persistence of such high concentrations results from the geochemical and mineralogical properties of the aquifers that consist of old, very oxidised and strongly weathered material, such that any primary electron donors have been leached. In riparian zones, concentrations are close to zero due to denitrification with oxidation of organic matter. In winter, stream nitrate concentration is controlled by the nitrate rich groundwater flow from upland. In summer, groundwater flow from upland decreases and stream concentration is controlled by bottom land hydrological and biogeochemical processes. The shift between winter and summer control depends on the water table dynamics along hillslopes. As long as the water table remains deep in upland, summer controls prevail. As soon as water table rises in upland, hydraulic gradient and groundwater flow from this zone increase, leading to an increase in the stream nitrate concentration. The shift from summer to winter control can be considered as the result of a connection between the upland nitrate rich groundwater and the stream, connection triggered by upland water table rise.