A hydrochemical modelling framework for combined assessment of spatial and temporal variability in stream chemistry: application to Plynlimon, Wales

Recent concern about the risk to biota from acidification in upland areas, due to air pollution and land-use change (such as the planting of coniferous forests), has generated a need to model catchment hydro-chemistry to assess environmental risk and define protection strategies. Previous approaches have tended to concentrate on quantifying either spatial variability at a regional scale or temporal variability at a given location. However, to protect biota from ‘acid episodes’, an assessment of both temporal and spatial variability of stream chemistry is required at a catchment scale. In addition, quantification of temporal variability needs to represent both episodic event response and long term variability caused by deposition and/or land-use change. Both spatial and temporal variability in streamwater chemistry are considered in a new modelling methodology based on application to the Plynlimon catchments, central Wales. A two-component End-Member Mixing Analysis (EMMA) is used whereby low and high flow chemistry are taken to represent ‘groundwater’ and ‘soil water’ end-members. The conventional EMMA method is extended to incorporate spatial variability in the two end-members across the catchments by quantifying the Acid Neutralisation Capacity (ANC) of each in terms of a statistical distribution. These are then input as stochastic variables to a two-component mixing model, thereby accounting for variability of ANC both spatially and temporally. The model is coupled to a long-term acidification model (MAGIC) to predict the evolution of the end members and, hence, the response to future scenarios. The results can be plotted as a function of time and space, which enables better assessment of the likely effects of pollution deposition or land-use changes in the future on the stream chemistry than current methods which use catchment average values. The model is also a useful basis for further research into linkage between hydrochemistry and intra-catchment biological diversity.</p> <p style='line-height: 20px;'><b>Keywords:</b> hydrochemistry, End-Member Mixing Analysis (EMMA), uplands, acidificatio

Characterising groundwater-dominated lowland catchments : the UK Lowland Catchment Research Programme (LOCAR)

This paper reports on a major UK initiative to address deficiencies in understanding the hydro-ecological response of groundwater-dominated lowland catchments. The scope and objectives of this national programme are introduced and focus on one of three sets of research basins – the Pang/Lambourn Chalk catchments, tributaries of the river Thames in southern England. The motivation for the research is the need to support integrated management of river systems that have high ecological value and are subject to pressures that include groundwater abstraction for water supply, diffuse pollution, and land use and climate change. An overview of the research programme is provided together with highlights of some current research findings concerning the hydrological functioning of these catchments. Despite the importance of the Chalk as a major UK aquifer, knowledge of the subsurface movement of water and solutes is poor. Solute transport in the dual porosity unsaturated zone depends on fracture/matrix interactions that are difficult to observe; current experimental and modelling research supports the predominance of matrix flow and suggests that slow migration of a time-history of decades of nutrient loading is occurring. Groundwater flows are complex; catchments vary seasonally and are ill-defined and karst features are locally important. Groundwater flow pathways are being investigated using natural and artificial geochemical tracers based on experimental borehole arrays; stream-aquifer interaction research is using a combination of geophysics, borehole array geochemistry and longitudinal profiles of stream flow and solutes. A complex picture of localised subsurface inflows, linked to geological controls and karst features, and significant longitudinal groundwater flow below the river channel is emerging. Management implications are discussed. Strategies to control surface application of nutrients are expected to have little effect on groundwater quality for several decades, and new modelling tools for decision support have been developed to represent these effects. Conventional modelling approaches are limited by the complexities of the subsurface system; catchment areas are difficult to define, hence tracking pollutant pathways to stream receptors is also problematic. Conventional distributed groundwater models have difficulty in capturing key aspects of the groundwater system. This raises important questions concerning the confidence that can be placed in models as routinely used for decision support and the level of knowledge required for catchmen

Assessing the relative importance of parameter and forcing uncertainty and their interactions in conceptual hydrological model simulations

Predictions of river flow dynamics provide vital information for many aspects of water management including water resource planning, climate adaptation, and flood and drought assessments. Many of the subjective choices that modellers make including model and criteria selection can have a significant impact on the magnitude and distribution of the output uncertainty. Hydrological modellers are tasked with understanding and minimising the uncertainty surrounding streamflow predictions before communicating the overall uncertainty to decision makers. Parameter uncertainty in conceptual rainfall-runoff models has been widely investigated, and model structural uncertainty and forcing data have been receiving increasing attention. This study aimed to assess uncertainties in streamflow predictions due to forcing data and the identification of behavioural parameter sets in 31 Irish catchments. By combining stochastic rainfall ensembles and multiple parameter sets for three conceptual rainfall-runoff models, an analysis of variance model was used to decompose the total uncertainty in streamflow simulations into contributions from (i) forcing data, (ii) identification of model parameters and (iii) interactions between the two. The analysis illustrates that, for our subjective choices, hydrological model selection had a greater contribution to overall uncertainty, while performance criteria selection influenced the relative intra-annual uncertainties in streamflow predictions. Uncertainties in streamflow predictions due to the method of determining parameters were relatively lower for wetter catchments, and more evenly distributed throughout the year when the Nash-Sutcliffe Efficiency of logarithmic values of flow (lnNSE) was the evaluation criterion

Hydro-ecological functioning of the Pang and Lambourn catchments : an introduction to the special issue

Introduction to thematic issu

Sources and controls of calcium and magnesium in storm runoff: the role of groundwater and ion exchange reactions along water flowpaths

A combined hydrological and chemical investigation was undertaken in a small moorland catchment at Plynlimon to determine the processes controlling storm runoff chemistry. Flow from natural soil pipes, overland flow from peat soils, throughflow from a mineral horizon and streamflow were gauged and sampled intensively during seven storms. Stormflow Ca and Mg concentrations in stream water consistently exceeded those observed in overland flow, pipeflow and throughflow. The response of Ca and Mg to increases in streamflow varied between the storms and could not be explained readily by the mixing of the dominant source waters monitored within the catchment. Intensive sampling of pipe water along a major stormflow pathway revealed a large and consistent increase in the concentration of dissolved Ca and Mg accompanied by a corresponding decrease in acidity, the magnitude of which was strongly influenced by antecedent conditions. Analyses of soil exchangeable cations along the stormflow pathway revealed soils enriched in divalent cations probably derived from a groundwater source. Laboratory leaching experiments confirmed that rapid cation exchange reactions could explain the changes in pipe water chemistry along the stormflow pathway. The relative contribution of flow from pathways where these ion exchange reactions occur strongly influences the stormflow response of Ca and Mg in the stream. The results also highlight a potentially important, indirect role for base-rich groundwater in modifying storm runoff chemistry along water flowpaths

New nuclear build: adaptation options over the full life-cycle

Nuclear power is an important part of the UK Government's plan to improve energy security, and build a more balanced, decarbonised economy. Recent legislation establishes a framework for selecting potential sites for new nuclear build (NNB) and the terms for meeting the full costs of decommissioning and waste management. In this context, there are specific requirements to consider flood risk and to adapt to climate change. However, there is uncertainty about how climate risks might evolve and how these could be managed within each phase of a plant's life-cycle: design, operation, decommissioning and fuel storage. Given that sites will need to be secure for at least 160 years, the possibility of rising sea level ighe sea temperatur s, and more extreme weather events cannot be ignored. This paper describes the engineering and non-structural adaptation options for NNB sites, illustrated with reference to the east coast of England. Despite large uncertainty about climate scenarios for the 2200s, we explain how flexibility of design and safety margins can be incorporated from outset and, when combined with routine environmental monitoring, how sites could be adaptively managed throughout their life-cycl

Water quality of lowland, permeable Chalk rivers: the Frome and Piddle catchments, west Dorset, UK

This paper provides a baseline interpretation of extensive hydrochemical characteristics of UK Chalk catchments based on the Frome/Piddle (west Dorset), a major component of the LOCAR (Lowland Catchment Research) programme. It examines a wide range of chemical determinands using data from two long-term (1979–2000) river water monitoring sites. The water quality of the Frome and Piddle is very similar to that of agricultural catchments associated with Chalk aquifers. Waters are of a calcium bicarbonate type and nutrient bearing. In general, there are poor correlations between major ion determinands and between chemical concentrations and flow due to the large groundwater storage which buffers chemical variability. Hence, there are no major chemical gradients within the aquifer to provide distinct water end-member chemistries for contrasting hydrochemical environments to allow chemical hydrograph separation on the basis of flow. NO3-N and soluble reactive phosphorus (SRP) are the dominant species of dissolved N and P, respectively. NO3-N sources both within the aquifer and within the soils contribute to relatively high concentrations, with average values close to those of other agriculturally impacted catchments. SRP concentrations are generally lower than for high agriculturally, urban and industrially impacted catchments. High correlations between Al, Cu, Fe, Mn and Zn occur due to the association of these trace metals with the soil-forming oxide and hydroxide ions present in suspended solids. Biological process influence SiO2, pH and EpCO2 levels. Average levels of EpCO2 at 4.15 and 5.52 times atmospheric pressure in the Frome and Piddle, respectively, indicates that the river waters have not fully degassed to equilibrium with the atmosphere, suggesting a short residence time of spring waters in the river channel

Nitrate transport in Chalk catchments : monitoring, modelling and policy implications

Rising nitrate levels have been observed in UK Chalk catchments in recent decades, with concentrations now approaching or exceeding legislated maximum values in many areas. In response, strategies seeking to contain concentrations through appropriate land management are now in place. However, there is an increasing consensus that Chalk systems, a predominant landscape type over England and indeed northwest Europe, can retard decades of prior nitrate loading within their deep unsaturated zones. Current levels may not fully reflect the long-term impact of present-day practices, and stringent land management controls may not be enough to avert further medium-term rises. This paper discusses these issues in the context of the EU Water Framework Directive, drawing on data from recent experimental work and a new model (INCA-Chalk) that allows the impacts of different land use management practices to be explored. Results strongly imply that timelines for water quality improvement demanded by the Water Framework directive are not realistic for the Chalk, and give an indication of time-scales over which improvements might be achieved. However, important unresolved scientific issues remain, and further monitoring and targeted data collection is recommended to reduce prediction uncertainties and allow cost effective strategies for mitigation to be designed and implemented