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Understanding and predicting the impacts of runoff on nutrients under different climate conditions within an unsaturated zone of soils is a fundamental challenge in hydrological research. The aim of this study is to provide new understanding of surface and subsurface hydrological controls on nutrient fluxes within mineral soils at the hillslope and catchment scales, and how these are influenced by climate variability. The study covers three nested spatial scales: the Blind Beck catchment (9.2 km2), the sub-catchment (0.09 km2) and the plot scale (2 m2) within the hillslope located in the Upper Eden basin, Cumbria, UK. The methodology combines field experiments and sampling, laboratory analysis and modelling approaches. Runoff experiments were conducted on two hillslope runoff plots to identify runoff processes and to quantify nutrient fluxes, one under perturbed (i.e. increased rainfall) and another under control plot. The SHETRAN physically-based hydrological model was then used to simulate runoff and nutrient flux for climate scenarios. These represent current and future (intensified hydrological cycle) conditions and were generated using the UKCP09 weather generator. Analyses of flow for different climate conditions within the unsaturated zone suggested: i) overland flow varied from 14% in dry conditions (before treatment) to more than 64% in the enhanced rainfall conditions of the total measured overland flow, ii) lateral subsurface flow dominates hillslope runoff during the transition period, iii) overland flow occurs in the winter during periods of frozen soil as Hortonian flow and iv) nutrients were most concentrated in the topsoil. In extreme climate conditions, saturation excess overland flow is probably a major contributor to storm flow followed by the subsurface flow. The results have shown that sensitivity of different runoff processes to different types/size of storms can support analysis of impacts of enhanced climate variability. The enhanced rainfall treatment in the overland flow reduced the DOC concentration 1.7 times, while increasing the NO3 - concentration 2.5 times. Under the enhanced rainfall treatment at the perturbed plot, C losses are lower in the overland flow (9.7 kg/ha) compared with the subsurface flow (22 kg/ha). This indicates enhanced loss of the DOC by the subsurface leaching pathway relative to losses through the overland flow. Decreases in the mean rainfall between 0.6% and 2.6% for the 2020 and ii 2050 period of the A1B emission scenario are modelled to decrease annual runoff 0.4% and 3.4 %. For the A1FI emission scenario, a decrease in rainfall between 1.2% and 3.2% is modelled to decrease annual runoff by 3.4% for the 2020 period and by 4.8% for the 2050 period.

This thesis presents the first method for the imaging of multiple fluid phases at conditions representative of subsurface flow by the use of X-ray micro-CT, focussing on four principal applications: (1) Capillary Trapping; (2) Ganglion Snap-off and remobilization; (3) Contact angle measurement; and (4) Dynamic phenomena associated with CO2 drainage. Firstly the pore-scale arrangement of CO2 after drainage and imbibition was imaged in three carbonates and two sandstones. In each sample substantial amounts of CO2 were trapped, showing that residual trapping can be used to locally immobilise CO2. The size distributions of larger residual ganglia obey power law distributions with exponents broadly consistent with percolation theory, over two orders of magnitude. To examine snap-off in more detail residual CO2 was imaged at high resolution in a single carbonate. The capillary pressures of residual ganglia were found to be inversely proportional to the radius of the largest restriction surrounding each ganglion. The remobilization of residual ganglia was assessed using a reformulation of both the capillary and Bond numbers, finding the majority of ganglia in this system were remobilized at reformulated capillary numbers of around 1. Thirdly this thesis presents the first method for the measurement of in-situ contact angle at realistic conditions by the use of micro-CT, applied to a single carbonate sample at 50oC and 10 MPa. Contact angles ranging from 35o to 55o were observed, indicating that the CO2-brine-carbonate system is weakly water-wet. Finally, we use fast synchrotron-based X-ray micro-CT to examine drainage into a brine saturated carbonate. The equilibrium capillary pressure change associated with drainage events is not sufficient to explain the accompanying snap-off, showing that dynamic forces can have a persistent impact on the pattern and sequence of the drainage process.

A detailed study of the several methods for the determination of reaeration rates in streams has been made. Particular attention has been given to the floating soluble solids technique which appears to be a simple and reliable method for the measurement of reaeration rates in small streams. A comprehensive review of both conceptual and empirical mathematical models for the prediction of the reaeration rate coefficient is presented in an attempt to identify the practical outcome and application of these models to- natural stream conditions. A critical review and discussion of the three primary methods usually employed for the reaeration measurement in streams, namely the oxygen balance, disturbed equilibrium and tracer techniques, is presented to allow the understanding of the main principles, related assumptions, degree of applicability and the errors involved in each of these methods. The theoretical basis of a new method, the floating soluble solids technique is introduced, and previous work results presented. The physico-chemical para'meters effect upon the reaeration rate coefficient are discussed in the last part of the literaýure review. An extensive laboratory programme has been conducted, during which the individual and interaction effects of three important hydraulic variables (velocity, depth and roughness) upon atmospheric reaeration, propane gas desorption and solids dissolution processes have been investigated using statistical experimental design and analysis techniques. The assumption that the ratio between the reaeration and propane desorption rate coefficients js constant and independent of mixing and turbulent conditions has been herein confirmed using a different approach (surface response methodology) from the previously reported tests. A weaker depth dependence of both reaeration and propane desorption rate coefficients has been verified at low velocity levels, and which becomes stronger as average water velocity increases. Natural stram data and energy dissipation response to the same hydraulic factors variation appear to further substantiate the above evidence. The common parameters and main 'limitations controlling both reaeration and dissolution processes have been identified and dimensionless correlation equations were developed for the determination of the reaeration coefficient from' velocity of solids dissolution measurements. Field trials, involving the disturbed equilibrium, steady state propane gas tracer, and floating soluble solids methods, appears to confirm the evidence that reaeration coefficients might be successfully estimated from, velocity of solids dissolution measurements in small streams.

Hydrological modelling is an inexact science where we have incomplete knowledge and understanding of hydrological systems. This has important implications for how we evaluate and discriminate between competing hypotheses of hydrological behaviour as model evaluation is tempered by the information, uncertainty and error within the available data used for model evaluation. This thesis presents multiple studies that investigate the different choices that matter for hydrological model evaluation by developing a diagnostic framework for the evaluation of multiple hydrological model structures that incorporates a UK national assessment of discharge uncertainties. There is a particular emphasis on (l) testing multiple hypotheses of catchment behaviour, (2) developing novel frameworks to quantify the quality of river discharge data and (3) accounting for observational discharge uncertainties in model diagnostics. The first results chapter provides evidence that the value of diagnostics to discriminate between model structures is dependent on catchment characteristics. Furthermore, these results contribute to a better understanding of the links between model structure choice, model performance and catchment characteristics and dynamics. The second results chapter focuses on the development of a novel generalised framework to estimate discharge uncertainties at many gauging stations with a variety of different errors in the stage-discharge relationship. Significantly, it was shown that despite regional differences in the type of gauging station, the number of historical rating curves and stage-discharge measurements, discharge uncertainties are highly place specific. The third results chapter emphasises the importance of recognising data quality in model evaluation frameworks and demonstrates that the choice of discharge uncertainty estimate impacts model identification. Overall, this thesis presents results that are important for the development of model evaluation frameworks and provides guidance to the hydrological community on how to test and reject competing hypotheses of catchment behaviour.

This work introduces a new integrated flood modelling tool in urban areas by coupling a hydrodynamic model with a hydrological model in order to overcome the drawbacks of each individual modelling approach, i.e. high computational costs usually associated with hydrodynamic models and less detailed physical representations of the underlying flow processes corresponding to hydrological models. Crucial to the simulation process is to first divide the catchment hydraulic and hydrological zones where the corresponding model is then applied. In the hydrological zones that have more homogeneous land cover and relatively simple topography, a conceptual lumped model is applied to obtain the surface runoff, which is then routed by a group of pre-acquired ‘unit hydrographs’ to the zone border, for high-resolution flood routing in the hydraulic zones with complex topographic features, including roads, buildings, etc. In hydraulic zones, a full 2D hydrodynamic model is applied to provide more detailed flooding information e.g. water depth, flow velocity and arrival time. The new integrated flood modelling tool is validated in Morpeth, the North East of England by reproducing the September 2008 flood event during which the town was severely inundated following an intense rainfall event. Moreover, the coupled model is investigated and evaluated according to the effects from temporal and spatial resolutions, friction, rainfall, infiltration, buildings and coupling methods. In addition, the model is also employed to implement flood damage estimations with different scenarios of the upstream storage and flood defences in the town centre. Whilst producing similar accuracy, the new model is shown to be much more efficient compared with the hydrodynamic model depending on the hydrological zone percentage. These encouraging results indicate that the new modelling tool could be robust and efficient for practitioners to perform flood modelling, damage estimation, risk assessment and flood management in urban areas and large-scale catchments.

Hydrology is yet to fully understand the role that catchment characteristics have in determining a river’s response to precipitation variability. This thesis assesses the influence that catchment characteristics have on modulating a river’s response to changes in precipitation throughout the UK. Central to this aim is the concept of the precipitation- to-flow relationship (the transformation of precipitation into river flow), which is characterised using the Variogram, a way of indexing temporal dependence (i.e. the average relationship between river flow on a given day and river flow on the previous days). Firstly, 116 catchments were grouped into four clusters, based on the shape of their variogram, which significantly differed in their catchment characteristics demonstrating that catchment characteristics control how, on average, precipitation is transformed into river flow. Furthermore, over 70% of un-gauged catchments could be clustered correctly using information about their soil type, slope and the percentage of arable land. Secondly, a new method which identifies the changes in the variogram parameters over 5-year overlapping moving windows was developed to investigate temporal changes in the variogram parameters. This method was successfully demonstrated to detect changes in multiple aspects of artificially perturbed river flow time series (e.g. seasonality, linear changes and variability). On average >70% of the variability in the catchment variogram parameters was explained by the precipitation characteristics, although there was large variability between catchments. Finally, the influence that the catchment characteristics have on the temporal changes in the variogram parameters was analysed, demonstrating that rivers in relatively impermeable upland catchments have a relationship with precipitation which is closer to linear and less variable than lowland, permeable catchments. This thesis contributes significant new knowledge that can be used for both assessing how individual catchments are likely to respond to projected changes in precipitation and in informing data transfer to un-gauged catchments.

This study focuses upon chemical variation in fluvial discharge over a 15 month period (May 2007 – July 2008) in a sub-tropical, monsoonal climatic regime in western India. Here, highly seasonal rivers rising at the Western Ghats escarpment discharge into the Arabian Sea. The Ghats present a topographical barrier to the SW monsoon, and thus generate one of the world’s highest orographic gradients. Two river basins were selected for this study, the Zuari and the Chapora, both characterised by high seasonal precipitation and run-off, with c. 85% occurring during the monsoon months (June – September). The rivers flow steeply down the Ghats then across the low-lying Konkan - Kanara coastal plateaux much of which is heavily weathered and covered by laterite. The water samples (n = 13 per month) were collected from seven sites along the Zuari River and six sites along the Chapora River. The samples were analysed using ICP-MS for cations and ion chromatography for anions. Major and trace element concentrations were found to be very low throughout both basins, although a marked increase was observed for various elements (e.g., Ca, K, Mg, Na, Rb, Sr, V, Cr, Mn, Fe, Co, HCO3-) shortly after the onset of the monsoon. We interpret this as a ‘rinse-out effect’. Despite absolute concentrations decreasing with increasing river run-off, total element fluxes increase during the monsoon. Additionally, due to the low elevation of the coastal plain, tidal effects are observed c. 40 km inland becoming forced seaward during the monsoon by high river flows. Silicate weathering is of primary importance in the long term global climate due to associated CO2 sequestration, and continental weathering is controlled by numerous factors, including lithology, climate, vegetation and anthropogenic effects; it is highest in the humid tropics due to high temperatures and precipitation. However, this study also identifies additional controls, these being the degree of weathering and the extent of weathering residuum, which are major limiting factors for elemental fluxes in tropical catchments.

Evaluating recovery of acidified lakes towards a target based on their pre-acidification status is a difficult task as there are very few biological records for these remote upland systems. The analogue matching approach can be used to identify modern analogues for the pre-acidification status of lakes using the sub-fossil remains of diatoms and Cladocera, which can be used as recovery targets. An 83-lake modern training set was created, with full diatom, cladoceran, hydrochemistry and catchment data for each lake. Fossil samples from 10 lakes of the UK Acid Waters Monitoring Network (UKAWMN) were chosen to represent pre-acidification conditions in those lakes. The closest analogues in the modern training set were identified for each fossil sample by means of the squared chord distance measure. The distributions of the Cladocera in the training set were analysed using a range of multivariate statistical technique. Individual cladoceran species response curves for selected environmental determinands were calculated. A 163-lake diatom training set has been compiled from existing data holdings and the patterns in the data were explored using direct ordination methods. A comparison with the diatom training set from the Surface Waters Acidification Project is also made. Close modern analogues were identified for 8 UKAWMN lakes. The majority of these modern analogues are located in North and Northwest Scotland; areas of low sulphur and nitrogen deposition. Comparison of the hydrochemical characteristics of the UKAWMN lakes to those of the modern analogues showed that the modern analogues had higher lake water pH and alkalinity levels and lower aluminium concentrations. Ionic strength and calcium concentrations in the analogue lakes were similar to observed values in the UKAWMN lakes. These results indicate that the analogue matching approach using diatom and cladoceran remains is a simple, robust and reliable method of identifying modern analogues for acidified lakes in upland areas of the UK.

Environmental history can be defined as the study of past relationships between people and natural environments. Descriptions of ecosystems prior to their anthropogenic degradation (reference conditions) and their past degradation are increasingly used to set conservation goals. To a much lesser extent environmental history has been used to evaluate past environmental management. This thesis evaluates the potential of historical information to describe an ecosystem’s reference conditions, degradation and recovery and the interrelated social, economic, technological and legislative factors which drove these changes. It also assesses the potential for the dissemination of historical information to influence public perceptions of an ecosystem’s present conditions and their predictions for its future. It uses the River Don as a case study of a system that has been both highly valued and severely degraded by local people over centuries and has seen substantial restoration over recent decades. The key findings are: 1) historical biological records are of limited value in describing historical community composition due to recording biases; 2) historical newspaper articles provide much information on past relationships between people and the river but neglect some important environmental degradation issues; 3) local people’s historical knowledge of the river influence their perceptions in ways which may foster support for its restoration; 4) reading historical rather than current information on the River Don under experimental conditions leads participants to hold more negative views about the river’s current environmental state but does not affect their expectations regarding the river’s future or their intentions to visit it. This research provides further evidence that historical information is valuable for planning environmental restoration but is often limited by sparsity and bias. It indicates that historical knowledge has the potential to foster support for conservation but further research is needed to better understand the relationship between historical knowledge and support for conservation.