Prediction of storm transfers and annual loads with data-based mechanistic models using high-frequency data

Excess nutrients in surface waters, such as phosphorus (P) from agriculture, result in poor water quality, with adverse effects on ecological health and costs for remediation. However, understanding and prediction of P transfers in catchments have been limited by inadequate data and over-parameterised models with high uncertainty. We show that, with high temporal resolution data, we are able to identify simple dynamic models that capture the P load dynamics in three contrasting agricultural catchments in the UK. For a flashy catchment, a linear, second-order (two pathways) model for discharge gave high simulation efficiencies for short-term storm sequences and was useful in highlighting uncertainties in out-of-bank flows. A model with nonlinear rainfall input was appropriate for predicting seasonal or annual cumulative P loads where antecedent conditions affected the catchment response. For second-order models, the time constant for the fast pathway varied between 2 and 15 h for all three catchments and for both discharge and P, confirming that high temporal resolution data are necessary to capture the dynamic responses in small catchments (10–50 km2/. The models led to a better understanding of the dominant nutrient transfer modes, which will be helpful in determining phosphorus transfers following changes in precipitation patterns in the future

Identification of catchment runoff processes as a basis for defining water quality protection zones

"Protection zones‟ for water quality are often defined as areas within the landscape where potentially polluting activities are excluded or restricted, with the aim of reducing the quantity of pollutants reaching the waterways. Such protection zones are needed in relation to agrochemical pollution from farming. This study uses a combination of hydrological, hydrochemical and modelling approaches to identify the nature and likelihood of generation of different hydrological flow pathways and to recommend hydrology-based protection zones for overland flow within an agricultural catchment. The study is based in the 616 km2 Upper Eden catchment in Cumbria, and one of its subcatchments, Blind Beck (8.8 km2). Transfer function modelling of rainfall-flow is used to investigate how the dominant mode(s) of stream response varies for different scale catchments (1 km2 to 616 km2). This indicates more water taking slower pathways as the proportion of the catchment on permeable sandstone increases. Chemical sampling, high temporal resolution measurements of specific conductivity, stream water temperature and diurnal variations of stream ions show results consistent with the rainfall-flow modelling in identifying a higher proportion of water on slower pathways in a small catchment on sandstone. Spatial distributions of surface soil moisture at 10 plots in Blind Beck investigate how topography, slope and land use affect the local distribution of surface saturation that produces saturation overland flow. At the plot scale, wetness distributions predicted by the Kirkby topographic index show significant discrepancies with the observed soil moisture distributions. A conceptual model for defining hydrology-based protection zones for overland flow combines areas of highest pollutant source risk with areas likely to generate overland flow. Overall, this study shows that while the factors controlling the location of saturated areas are complex, identification of the dominant hydrological pathways is fundamental to the design of water protection zones

Quantifying the differential contributions of deep groundwater to streamflow in nested basins, using both water quality characteristics and water balance

This paper describes use of a hydro-chemical mixing model and a water budget to investigate the presence of deep runoff pathways in two small, nested sub-catchments of the Eden basin, UK (8.8 km2 Blind Beck and 1.0 km2 Low Hall stream). A linear relationship between bicarbonate concentration and electrical conductivity was used in a two-component mixing model. End-members were identified as a high-solute, deep groundwater and a low-solute, soil-water. The mixing model indicated 69% ±10% deep groundwater in Low Hall for September–December 2008 and 46% ±8% in Blind Beck for the same period. The water budget also indicated more deep groundwater in Low Hall stream. These results were consistent with the findings of rainfall–runoff models which also indicated the presence of high storage, deeper pathways

The effect of topography, subsurface strata and land-use on observed distributions of soil moisture within a sub-catchment of the River Eden, Cumbria.

The location of saturated areas, generating saturation overland flow by return-flow and direct rainfall, affects the rates and pathways of nutrients and sediment moving into streams, thereby having implications for river water quality. The prediction of saturated areas is difficult as it depends on many factors such as topography, subsurface strata, land-use and aspect. This study, in the Blind Beck sub-catchment of the Eden River (Cumbria), investigates how these factors affect soil moisture patterns. Intensive spatial measurements of soil moisture are made during different seasons across research plots affected by different contributing factors. Geostatistical analysis is used to quantify differences in spatial patterns observed. Measured soil moisture distributions are to be compared to spatial distributions of relative wetness predicted by existing topography-only indices. This paper presents the results of a preliminary set of soil moisture plots

Nutrient reduction in field wetlands: do they work for dissolved nutrients?

Pollution of natural waters arises from both point sources (direct inputs) and diffuse inputs (many small sources entering the waterways by numerous pathways). Legislation has ensured that pollution from point sources has been reduced, thus increasing the significance of diffuse sources, and the contribution from agriculture in particular. Field wetlands (small sediment and nutrient trapping features, < 500 m2), constructed along runoff pathways, are one set of options for diffuse pollution mitigation. Polluted surface runoff as well as subsurface drainage is slowed down by passage through the field wetland, allowing more opportunity for sediment and associated nutrients to settle out. However, the nutrients transported from soil to water are not only attached to the sediment but also in soluble form, and field drains have been identified as a fast pathway for dissolved nutrients to reach the waterways. The soluble form is critical in the short term because it is readily available for aquatic organisms. On the other hand, the particulate form (associated with sediment) is a reservoir for growth and development of aquatic organisms and represents a problem in the long term. The ability of field wetlands to reduce both the particulate and dissolved nutrient loads is being tested in the UK as part of a project on Mitigation Options for Phosphorus and Sediment. Ten field wetlands have been built on farms in the UK, capturing surface runoff and subsurface field drainage, to quantify the sediment and nutrient retention under a range of different conditions. At Whinton Hill in Cumbria (sandy soil), samples from the inlet and outlet of a field wetland system showed an average decrease in the concentration of total solids of 11%. Total phosphorus (TP) was reduced by an average of 43%. However, soluble reactive phosphorus, which accounted for approximately 50% of the TP at the inlet, was reduced by an average of 74%, showing that this wetland made a significant difference to both the particulate and dissolved fractions of phosphorus. For nitrogen, at least 80% of the total nitrogen was in dissolved form at the inlet, with most of this being in the form of ammonium. Concentrations of ammonium were reduced by an average of 72% between the inlet and outlet. We conclude that the long hydraulic residence times in this field wetland, due to very limited flow at the outlet of the wetland, maximise the opportunity for biological activity and uptake of soluble nutrients

Improving water quality in agricultural catchments: sediment and nutrient retention in field wetlands

A recent update of Water Framework Directive classifications in the UK indicates that only 28% of water bodies currently achieve good ecological status and that agriculture is one of the main sectors responsible for the pressures contributed by sediment and nutrients. The use of edge-of-field features, such as field wetlands - small sediment and pollutant trapping features (<500 m2) constructed along runoff pathways, is one set of mitigation options available to farmers. Before reaching the waterways, polluted runoff is slowed down by passage through the field wetland, allowing some sediment and nutrients to settle out. Although the principle of field wetlands is well accepted, and they are widely used in Scandinavia for diffuse pollution mitigation, there is little quantitative evidence of their capability for water quality improvement in the UK. Ten field wetlands have been constructed in the UK agricultural landscape in order to quantify the potential for sediment and nutrient retention and to provide guidelines on their likely effectiveness under various conditions. The ten sites covered different combinations of soil type, field wetland design, wetland size relative to catchment area and runoff source. Sediment and nutrient retention was measured by annual sediment surveys at each field wetland. Sediment trapping rates of 0.5 – 6 t ha-1 yr-1 were recorded on a sandy soil site, compared to 0.02 – 0.4 t ha-1 yr-1 on a silty soil site and 0.01 – 0.07 t ha-1 yr-1 on a clay soil site, although rainfall was a confounding factor, with much lower rainfall at the clay site during the monitoring period. Concentrations of total phosphorus, total nitrogen and total carbon in the sediments trapped were highest at the sandy site, where there was a wastewater input in addition to the agricultural runoff. The importance of land use, and ground cover in particular, was highlighted by a ten-fold increase in the sediment trapped in one field wetland from one year to the next when pasture was ploughed up for an arable crop. These multi-functional edge-of-field features have shown good potential for reduction of sediment and nutrient input to the waterways. In addition, field wetlands provide biodiversity benefits and in some circumstances may also contribute to flood attenuation, and should be considered alongside in-field measures as part of an integrated solution for catchment management

Keeping agricultural soil out of rivers:evidence of sediment and nutrient accumulation within field wetlands in the UK

Intensification of agriculture has resulted in increased soil degradation and erosion, with associated pollution of surface waters. Small field wetlands, constructed along runoff pathways, offer one option for slowing down and storing runoff in order to allow more time for sedimentation and for nutrients to be taken up by plants or micro-organisms. This paper describes research to provide quantitative evidence for the effectiveness of small field wetlands in the UK landscape. Ten wetlands were built on four farms in Cumbria and Leicestershire, UK. Annual surveys of sediment and nutrient accumulation in 2010, 2011 and 2012 indicated that most sediment was trapped at a sandy site (70 tonnes over 3 years), compared to a silty site (40 tonnes over 3 years) and a clay site (2 tonnes over 3 years). The timing of rainfall was more important than total annual rainfall for sediment accumulation, with most sediment transported in a few intense rainfall events, especially when these coincided with bare soil or poor crop cover. Nutrient concentration within sediments was inversely related to median particle size, but the total mass of nutrients trapped was dependent on the total mass of sediment trapped. Ratios of nutrient elements in the wetland sediments were consistent between sites, despite different catchment characteristics across the individual wetlands. The nutrient value of sediment collected from the wetlands was similar to that of soil in the surrounding fields; dredged sediment was considered to have value as soil replacement but not as fertiliser. Overall, small field wetlands can make a valuable contribution to keeping soil out of rivers

Reducing diffuse pollution in agricultural catchments: retention of sediment and nutrients in field wetlands

Diffuse water pollution continues to be an important environmental concern, with only 28% of surface waters in the UK currently classified as ‘good ecological status’ in a recent update of Water Framework Directive classifications. Agriculture has been identified as the largest sector associated with diffuse sources of nutrients and sediment. Field wetlands have been used for mitigation of diffuse pollution in Scandinavia but there is currently little quantitative evidence of their effectiveness in the UK. Ten field wetlands have been built at four sites in the UK, covering a range of soil types, runoff sources, wetland sizes and wetland designs. The wetlands, small (< 350 m2), unlined ponds constructed along runoff pathways, slow the connection between the pollution source and the waterways, and provide more opportunity for sediment and nutrients to settle out or be taken up by aquatic organisms. This paper describes sediment retention in all ten wetlands and nutrient retention at one highly polluted site. Sediment retention was estimated from annual surveys of the sediment build-up in each wetland. Sediment trapping rates were highest on a sandy soil site (0.5 – 6 t ha-1 yr-1), compared to a silty soil site (0.02 – 0.4 t ha-1 yr-1) and a clay soil site (0.01 – 0.07 t ha-1 yr-1). Nutrient retention was estimated from samples collected at the inlet and outlet of each wetland. At Whinton Hill, Cumbria, a wetland system with a long hydraulic residence time, concentrations of both particulate and dissolved nutrients were observed to decrease between the inlet and outlet of the wetland system. The average concentration of some nutrients was observed to reduce by up to 80%. Overall, small field wetlands have shown good potential for mitigation of diffuse pollution and should be considered alongside in-field measures as part of an integrated solution for catchment management

Reduced nutrient pollution in a rural stream following septic tank upgrade and installation of runoff retention measures

Surface water quality in the UK and much of Western Europe has improved in recent decades, in response to better point source controls and the regulation of fertilizer, manure and slurry use. However, diffuse sources of pollution, such as leaching or runoff of nutrients from agricultural fields, and micro-point sources including farmyards, manure heaps and septic tank sewerage systems, particularly systems without soil adsorption beds, are now hypothesised to contribute a significant proportion of the nutrients delivered to surface watercourses. Tackling such sources in an integrated manner is vital, if improvements in freshwater quality are to continue. In this research, we consider the combined effect of constructing small field wetlands and improving a septic tank system on stream water quality within an agricultural catchment in Cumbria, UK. Water quality in the ditch-wetland system was monitored by manual sampling at fortnightly intervals (April - October 2011 and February - October 2012), with the septic tank improvement taking place in February 2012. Reductions in nutrient concentrations were observed through the catchment, by up to 60% when considering total phosphorus (TP) entering and leaving a wetland with a long residence time. Average fluxes of TP, soluble reactive phosphorus (SRP) and ammonium-N (NH4-N) at the head of the ditch system in 2011 (before septic tank improvement) compared to 2012 (after septic tank improvement) were reduced by 28%, 9% and 37% respectively. However, TP concentration data continue to show a clear dilution with increasing flow, indicating that the system remained point source dominated even after the septic tank improvement

Sediment and nutrient trapping in field wetlands.

Agriculture has been identified as the largest sector contributing to diffuse water pollution. Field drains have been identified as a fast pathway for dissolved nutrients to reach the waterways, potentially bypassing conventional buffer zones. Field wetlands, (small, unlined ponds (< 500 m2) constructed in unproductive agricultural areas and designed to intercept runoff), are a simple and effective option for diffuse pollution mitigation, addressing both sediment-associated and dissolved pollutants. This paper describes the results of sediment and nutrient trapping from ten field wetlands built at four sites in the UK, covering a range of soil types and runoff sources. Sediment and nutrient retention in the wetlands was estimated from samples collected at the inlet and outlet of each wetland, as well as from an annual survey of the sediment retained in each wetland. Sediment masses trapped within the wetlands ranged from 0.3 – 1 t yr-1 at a clay soil site to 25 – 40 t yr-1 at a sandy soil site. At the sandy site, 30 – 70 kg yr-1 total phosphorus (TP) and 60 – 200 kg yr-1 total nitrogen (TN) was trapped with the sediment. Rainfall was a confounding factor, with much lower rainfall at the clay site during the monitoring period. Concentrations of both particulate and dissolved nutrients were observed to decrease between the inlet and outlet of wetland systems. At a sandy soil site, the average concentration of some nutrients was reduced by up to 80% in a wetland system with a long hydraulic residence time. Overall, these multi-functional edge-of-field features have shown good potential for reduction of sediment and nutrient input to the waterways and should be considered alongside in-field measures as part of an integrated solution for catchment management