This study examines historical variations in flood frequency and magnitude in the Yorkshire Ouse basin, northern England, over the last 900 years. The causes of temporal and spatial variations in flooding are evaluated through investigation o f climatic and land-use controls. Documentary evidence o f flooding and climate suggests that a series of large floods between 1263 and 1360 were associated with climatic deterioration from the Medieval Optimum. A shift to generally milder conditions between 1361 and 1549 resulted in no floods being documented in the Ouse basin The frequency o f large magnitude floods increased dramatically between 1550 and 1680, as a result o f low temperatures, increased surface wetness, more frequent snowfall and a southward shift of prevailing storm tracks over middle latitudes, associated with the onset of the "Little Ice Age’. In contrast, during a wanner phase of the Little Ice Age, between 1681 and 1763, the frequency of localised summer flooding increased in the Ouse basin due to more frequent high intensity, short duration convective storms. Extensive lowland flooding became more common between 1764 and 1799 due to an increase in heavy rainfall, followed by a 50-year period characterised by relatively moderate flood frequencies and magnitudes. The later half of the nineteenth century experienced high flood frequencies and magnitudes, particularly in the 1870s and early-1880s, coinciding with high rainfall totals and a high incidence of cyclonic flood generation. Gauged flood and climate data, and land-use records indicate that the period between 1900 and 1916 was characterised by very low flood frequencies and magnitudes, associated with low rainfall, warm temperatures, and an increase in westerly flood generation. Between 1916 and 1943 there were marked variations in flood magnitude between the rural northern rivers and southern industrialised rivers. Magnitudes generally increased on northern rivers, whilst on some southern tributaries of the Ouse, flood magnitudes declined as a result of widespread channel improvement and flood defence schemes. Around 1944 a marked and sustained increase in flood frequency on northern rivers was associated with an increase in the incidence of heavy daily rainfall, greater westerly flood generation and large-scale upland and lowland drainage. Very low flood frequencies and magnitudes between 1969'and 1977 resulted from extremely low rainfall totals. Whereas the most recent period, between 1978 and 1996 has experienced some of the highest flood frequencies and magnitudes on record, associated with an increase in the frequency of floods generated under cyclonic and south-westerly synoptic situations, and a number of land-use changes promoting more rapid runoff including, large increases in upland livestock numbers, an increase in the area under winter-cereals and the cumulative effects of moorland gripping.

Cardiff Bay is an artificial freshwater lake created by the impoundment of 100 ha of estuarine intertidal mudflats in the early 2000s. This dramatic environmental change is reflected in the sedimentary record as a sharp transition between lacustrine gyttja and estuarine clay sediments. This study utilises a combination of geochemical, molecular genetic and novel cultivation based approaches to explore how the methanogen community of Cardiff Bay sediments has responded to the transition from brackish to freshwater conditions caused by impoundment, and how they might respond to future climate change. Microbial methanogenesis is active in newly deposited Cardiff Bay lacustrine gyttja sediments. Porewater methane concentrations regularly exceed saturation limits, suggesting that ebullition may be a direct pathway for atmospheric methane emissions. Sediment slurry incubations suggest that methane production in Cardiff Bay sediments would be further enhanced by both increased substrate availability and climate relevant temperature increases. Hydrogenotrophic methanogenesis mediated by the genus Methanobacterium is the major pathway for methane production in Cardiff Bay, which is atypical for either estuarine or freshwater environments, and reflects the importance of recalcitrant terrestrial carbon inputs. In fact, cultivation based experiments provide tentative evidence that syntrophic acetate and methylamine utilizing consortia involving hydrogenotrophic methanogens were able to outcompete acetotrophic and methylotrophic methanogens under certain environmental conditions. The temperature and salinity ranges and characteristics, and substrate types, of Cardiff Bay methanogenic communities varied little between sediments deposited pre- and post-impoundment. Differences between the methanogen community composition of lacustrine gyttja and estuarine clay sediments were driven predominantly by a reduction in the quantity and availability of organic matter with increasing sediment depth. A phylogenetically and physiologically diverse range of methanogens were enriched from Cardiff Bay sediments, including thermophilic methanogens and selected marine methanogens. Methanogen strains isolated from Cardiff Bay were closely related to cultivated strains, yet displayed novel physiological characteristics. These included a strain of the thermophilic genus Methanothermobacter able to produce methane at 25 °C, and a strain of the marine genus Methanolobus which grew under freshwater conditions (0.01 M NaCl).

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As projections of future climate raise concerns over water availability and extreme hydrological events, global hydrology models are increasingly being employed to better understand our global water resources and how they may be affected by climate change. Being a relatively recent development in hydrological science, global hydrology modelling has not yet undergone the same level of assessment and evaluation as catchment scale hydrology modelling. Until now, global hydrology models have presented just one deterministic model output for use in scientific research. Recently, multi-model ensembles have compared these outputs for different global models, but this has been done prematurely as the uncertainties within individual models have yet to be understood. This study demonstrates a rigorous uncertainty investigation of the 123 parameters within the Mac-PDM global hydrology model over 21 global river catchments. Mac-PDM was selected for its relative simplicity amongst global hydrology models, and its suitability for application using high performance computer clusters. A new version of the model, Mac-PDM.14 is provided, with updated soil and vegetation classifications. This model is then subjected to a 100,000 parameter realisation Generalised Likelihood Uncertainty Estimation (GLUE) experiment, requiring 40 days of high performance computing time, and outputting over 2Tb of data. The top performing model parameterisation from this experiment provides an annual average error of 47% when compared to observed records, a 45% improvement over the previous version of the model, Mac-PDM.09. Given the computational expense of such an experiment, smaller sample sizes of parameter realisations are explored. Whilst the top performing parameterisation in a sample size as small as 1,000 can perform almost as well as that from 100,000 parameterisations, the number of good parameterisations is fewer, and the range of model uncertainty may therefore be significantly underestimated. Mac-PDM.14 is shown to have a lower mean absolute relative error than all models involved in both the Water and Global Change (WATCH) project and the Inter-Sectoral Impacts Model Intercomparison Project (ISI-MIP). Parameter uncertainty is compared to model uncertainty, and the uncertainty range between the models within the WATCH and ISI-MIP projects is comparable to the parameter uncertainty within Mac-PDM.14. Catchment specific calibrations of the global hydrology model are explored, and it is demonstrated that the model performance is improved by 22 to 92%, for the Niger and the Yangtze respectively, with catchment specific parameter values over a global calibration. Approximate Bayesian Rejection is applied to explore the catchment specific parameter values that result in good parameter performance. Few trends can be identified from this analysis, which suggests that Mac-PDM may be over-parameterised. Catchment specific calibrations in both high latitude and arid to semi-arid regions show significant improvement over global calibration, which indicate a deficiency in model structure; the addition of a glacier component to Mac-PDM is recommended. Model calibrations are validated using the ISI-MIP forcing dataset, and the best model performance gives an error of 44%. This is a betterment on the performance with the WATCH forcing data used in calibration, and so implies that models not need to be recalibrated every time new forcing datasets are employed. This research highlights that the performance of global hydrology models can be significantly improved by running a parameter uncertainty assessment, and that in catchment scale studies, catchment specific calibration should be carefully considered. Furthermore, the uncertainty within individual global hydrology models is an important consideration that should not be overlooked as these models are increasingly included in ensembles and interdisciplinary studies.

The water physico-chemistry, hydrology and plant communities in the Montgomeryshire Canal were examined for evidence of successional phenomena over a five year period and comparisons were made with previous studies. Plant communities changed in a manner which suggested that classical successional ideas were applicable to canals. The changes were readily distinguishable when a period of twenty years had passed but as the examined period was decreased the complexity of the sequence became more apparent. In a single year there were changes in the plant communities. A later stage of succession was indicated as the number of changes increased and the available energy and habitat niches were filled. Succession did not take place in a slow orderly manner but progressed as a series of jumps when the plants were released from inhibitory factors. These sudden changes were promoted by changes in the habitat due to climatic variation, removal of grazers or alterations in the water level. Where the most important inhibitory factors were unchanging, the succession proceeded to a climax community decided by those factors. The physico-chemical factors affecting the canal changed during the study period. The canal became more acidic due to the effects of a cold wet spring. The underwater light climate in un-navigated lengths was determined more by the amount of shade upon the water surface than by any other factor. Levels of suspended solids were relatively unimportant. Established plant communities showed a great resistance to change, despite being subjected to traumatic perturbations. Skim dredging of a site with a well established community structure did not change the course of the succession, but removed inhibiting factors such as anoxic material or grazing and promoted a more rapid change to reedswamp. In the .long term falling water levels had the same effect as raising of the bottom due to infill, but in the two routes towards a less aquatic system were different. Infill led to a moving band of depth sensitive plants whilst loss of water encouraged rapid creation of a reedswamp. The study confirmed that limited navigation is not detrimental to the species composition or the productivity. Eutrophication was a limiting factor creating an unique climax community.

Research was performed at Crag Cave, Castleisland, southwest Ireland, and P8 Cave, Castleton, Derbyshire, in order to determine the main factors responsible for modifying rainwater geochemistry during flow through soil and karstic aquifer zones. Monitoring was performed on a daily basis in summer and winter at Crag Cave, and on a monthly basis over one year at P8 Cave. At both sites, biannual peaks in karst system Ca2+concentrations occurred due to: (i) promotion of microbial C02 production by increased summer temperatures, and (ii) retardation of gaseous exchange by ponding of elevated winter rainfall input leading to an unseasonable build up in soil zone C02. Therefore, speleothems at both sites may form biannual bands in hydrological years subject to elevated winter rainfall input. In addition to variations in carbonate weathering due to fluctuations in C02 levels, cation yields in Crag Cave matrix soil water were controlled by dolomite dissolution (Mg2+), plant uptake (K+), and evapotranspiration balanced by enhanced winter marine aerosol input (Na+). Strontium isotope analysis indicates that S~+ was derived from a 50:50 silicate/carbonate mixture, whilst the relatively light 313C signal was related to direct evolution of C02 into the aqueous phase in waterlogged pores. Within the Crag Cave aquifer variations in karst water geochemistry were controlled by dilution, flow switching, prior precipitation of calcite and dolomite dissolution along the flow path. Strontium isotope analysis indicates that dissolution in the aquifer dominated, with S~+ being sourced from a 25:75 silicate/carbonate mixture. Light karst water 313C values were constrained by the supply of light soil gas to the aquifer. Elevation in the Mg/Ca and Sr/Ca ratios in the Crag Cave speleothem record compared to present day analogues indicates that the former Holocene climate was drier, whilst heavier 87Sr/86Srratios and 813C values suggest variation in soil hydrology over time.

This thesis l concerned with the mathematical modelling of open channel flows governed by the Saint-Venant equations, which are used as a prediction or identification tools. A survey of the literature in these fields identified the problems in need of Immediate research. Numerical test runs were then devised which led to projecting a clear picture as follows. The performance of twn widely used Implicit finite difference schemes, the 4-point box and 6-point staggered schemes were compared In a wide range of circumstances. it is concluded that both schemes produce 'very close results, but the staggered scheme is prone to convergence problems In some extreme cases. It was also noted that a sharp change in geometric configuration of compound channels produced discontinuous features on the aim ulated depth and discharge hydrographs. The inability of the staggered scheme In handling a head-discharge relationship as a downstream boundary condition was tackled by proposing and implementing a scheme of second order accuracy. As model data are generally corrupted withh errors and noise, their effects together with that of other factors on the Identified friction parameters we Investigated. The results demonstte the paramount Importance of the effect of a choice of objective function on the Identified parameters. While the individual values of the identified M2nning n may vary from one flood event to another, their mean is shown both numerically and rigorously to be dependent upon the choice of objective function. It is shown that an objective function formulated by using absolute errors performs ideally and produces reliable results even in the presence of autocorrelated Gaucian noise samples. The mean of the Identified parameters is also found to be adversely affected if the observation station is affected by localized disturbances. Sensitivity of objective functions to the variation In the value of the friction parameter Is also found to be an Important factor, as Insensitivity leads to ill-conditioning.

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No description available.

No description available.