No description available.

No description available.

No description available.

No description available.

No description available.

Clear links are known to exist between the terrestrial characteristics of catchments and the chemistry of their surface waters. During the last decade it has been established that atmospheric pollution, in the form of acidic deposition, can also influence the chemistry of waters draining sensitive upland sites. Furthermore, recent studies have suggested that reductions in these anion loads can have marked consequences for the surface-water quality of acidified catchments. However, many of these field experiments and model estimates have neglected other, potentially important realms of atmospheric influence. This thesis proposes that climate change over periods of up to one century can alter the hydrogen-ion budget of a catchment in two ways. First, variations in the relative frequency of large-scale synoptic features may significantly modify catchment chemical budgets by changing existing spatial and temporal patterns of acidic deposition. Secondly, as each major class of weather type is characterised by distinct precipitation and temperature regimes, the seasonal magnitude and frequency of acidic episodes may also be affected by long-term adjustments to the catchment water-balance. A hydtological perspective was employed in order to investigate these potential hydrochemical relationships. This involved hydrological modelling and hydrogen-ion budgeting, statistical analyses of climatic trends, the application of weather classification schemes, and the generation of synthetic input data from observed and historic meteorological data. These elements were combined by the development of a robust and comprehensive computer package (the Shifting Climate and Catchment Acidification Model, or SeAM) which enables the manipulation of a wide range of atmospheric and catchment properties. The model was calibrated and validated against data obtained from the Beacon experimental catchment in the East Midlands and then transferred to three contrasting watersheds in the Llyn Brianne region, Mid Wales. Using multiple climate and pollution scenarios, modelling experiments revealed that variations in the predominance of three key weather types modified the mean annual wet-deposited acid load by ±20% and the mean surface-water acidities by up to ±15%. Under the most extreme scenario the frequency of daily flows of less than pH 4.5 was increased by +90%. Whilst recognising the simplicity of the hydrologic ally-driven soil model, a feature common to all of the catchments was the exaggeration of existing seasonal discharge and acidity regimes. The sensitivity of individual catchments to a given climate change was found to be highly variable, depending upon the complex interaction of hydrology and catchment characteristics. It was concluded, therefore, that the effect of reduced emissions of acidifying substances on catchment recovery can be modified by climate change. Furthermore, acid-sensitive species occupying marginal sites, could be affected by the changes envisaged under the proposed BASE scenario. This underlines the need for a definition of the term 'critical load' that embraces desirable ecological responses with the required acidic deposition rates, for a given climatic context.

Hydrological pathways and episodic stream acidification were investigated by monitoring water quality in the Coalbum catchment during the later stage of canopy closure. Coalburn is a long-running experimental site, almost completely covered by a commercial coniferous plantation (part of the Kielder Forest in northern England). Regular sampling, event sampling and continuous monitoring were used to establish an extensive water quality data-base between 1993 and 1997. Cloud mist and precipitation at Coalburn are slightly acidic and the catchment can receive moderate levels of dry and wet acid deposition when the winds have tracked from an easterly direction. Coalburn streamwater chemistry displays well-buffered base-flows and acid episodes during high flows. Duration-frequency-magnitude analysis of the Coalburn acid episodes reveals they were less frequent than expected but had a longer duration. The acid events exceeded environment quality standards for freshwaters: pH and aluminium concentrations suggest a toxic impact on freshwater biota. However, the elevated calcium concentrations and presence of humic substances reduce the biological impact. The chemical signatures and variations in solutes were identified and used to postulate the causal mechanisms of the hydrochemical response and to produce the conceptual Coalburn runoff model. Mixing modelling was then applied to introduce a robust analysis into the process and produce the simplified Coalburn Model of acidification. Broadly, the Coalburn hydrochemical response can be explained by pre-event water sources and 'normal' catchment processes. However, during extreme events catchment conditioning and event-water can also influence the response. In improving knowledge of the likely water quality effects of coniferous plantation afforestation, the Coalburn study has contributed to calibrating the temporal and spatial variability of headwater acidification (and hence risks). This allows policy makers and environment managers to make informed decisions about land use/management and to apply the precautionary principle; the research findings were used to suggest a basic policy framework and protocols for catchment assessments to manage acidification issues through the cropping cycle within a headwater catchment.

There is a need to predict the river flow conditions necessary to support instream (and river corridor) biota. Many rivers have their flow regulated in some way and the field of 'habitat hydraulics' (or 'ecohydraulics') has grown up to encourage classification and characterisation of lotic physical environments. This thesis, based on field measurements at eleven sites in Northeast England (which are nationally representative), presents the data nonnally collected for the calibration of the PHABSIM model in alternative ways, to identify and characterise 'physical biotopcs'. These are segregated on the basis of subjectively (visually)-defined flow types. Statistical validation of biotopes as hydraulically discrete units shows them to be defined by a characteristic range of hydraulic variables, especially the Froude number. An investigation of the spatial and temporal variability of biotopes in different channel types and at different flows was carried out (1993-1995), which included the most extreme floods and droughts on record at some sites. The 'biotope approach' is shown to have potential as a reconnaissance approach to the assessment of habitat quality. Despite the success of the biotope approach e.g. in River Habitat Surveys, methodological improvement is still required, notably in the area of hydraulic characterisation by velocity measurements. Hydraulically rough channels and those with substantial macrophyte growth present problems; a logarithmic velocity profile cannot be assumed. A relationship between biotope diversity and stream ecosystem health is suggested; biological surveys are required to determine the direct relationship. This thesis represents a shift from the geomorphological riffle-pool theory to a broader classification of instream hydraulic units. It provides a framework for testing the debate in ecological theory regarding the role of patch dynamics within the continuum of the river environment. The study indicates that an 'environmentally acceptable flow regime' is one which creates a range of biotopes and maintains 'critical biotopes' at periods coinciding with key lifestage events. Flood events are beneficial for their role in 'resetting' the system.

No description available.

This study examines the sedimentary and morphological flood record of the River Tyne, northern England, over the last 400 years. Temporal trends in flooding and spatial variations in the timing and nature of river response to floods (flood "effectiveness") are investigated at 3 reaches representative of upland, piedmont and lowland locations in the Tyne catchment. Deposition of coarse flood sediments in headwater tributaries of the River Tyne appears to be strongly related to flood water:sediment ratios that is primarily controlled by sediment supply. Cobble - boulder bars, sheets and splays are deposited under Newtonian flow conditions from flows with relatively low sediment concentrations. Boulder berms and lobes result from floods with high sediment loads, associated with slope-channel coupling. Flood sedimentation in lower reaches of the Tyne comprises vertical accretion of fine-grained, well-bedded sediments at overbank and within-channel locations. Larger floods can be identified on the basis of textural discontinuities that reverse the overall fining-upward sequence. Estimation and interpretation of flow magnitude in each setting is complicated by progressive river bed incision over the period of investigation and significant changes in sediment supply which have affected the nature of flood sedimentation. Trace metal dating of fine-grained, and lichenometric dating of coarsegrained, flood units has enabled detailed and temporally well-defined records of channel and floodplain response to flooding to be established within the Tyne catchment. Since 1600 A.D. fluvial activity at all 3 study reaches has been characterised by alternating periods of river bed incision and stability. Episodic changes in vertical channel stability appear to coincide with non-random decadalscale fluctuatiions in flood frequency: 3 periods with distinct clustering of large floods (1740-1790, 1860-1899, 1940-1965) were associated with vertical instability (channel trenching) throughout the Tyne basin, while intervening periods with relatively few exceptional floods were associated with channel accretion and floodplain rebuilding in general, but with significant upstream/downstream variation. Distinct clustering in the timing of large floods correspond with shortterm hydroclimate changes associated with fluctuations in the configuration of the upper atmosphere. These findings have significant implications for longer-term fluvial development and interpretaion of Holocene alluvial sequences.