There is increasing recognition that the hydrology of the Greenland Ice Sheet plays an important role in the dynamics and therefore mass balance of the ice sheet. Understanding the hydrology of the ice sheet and being able to predict its future behaviour is therefore a key aspect of glaciological research. To date, the ice sheet’s hydrology has tended to be inferred from the analysis of surface velocity measurements, or modelled in a theoretical, idealised way. This study focuses on the development of a high spatial (100 m) and temporal (1 hour) resolution, physically based, time-dependent hydrological model which is applied to the ~2,300 km2 Paakitsoq region, West Greenland, and is driven, calibrated, and evaluated using measured data. The model consists of three components. First, net runoff is calculated across the ice sheet from a distributed, surface energy- balance melt model coupled to a subsurface model, which calculates changes in temperature, density and water content in the snow, firn and upper-ice layers, and hence refreezing. The model is calibrated by adjusting key parameter values to minimize the error between modelled output and surface height and albedo measurements from the three Greenland Climate Network (GC-Net) stations, JAR 1, JAR 2 and Swiss Camp. Model performance is evaluated in two ways by comparing: i) modelled snow and ice distribution with that derived from Landsat-7 ETM+ satellite imagery using Normalised Difference Snow Index (NDSI) classification and supervised image thresholding; and ii) modelled albedo with that retrieved from the Moderate- resolution Imaging Spectroradiometer (MODIS) sensor MOD10A1 product. Second, a surface routing / lake filling model takes the time-series of calculated net runoff over the ice sheet and calculates flow paths and water velocities over the snow / ice covered surface, routing the water into ‘open’ moulins or into topographic depressions which can fill to form supraglacial lakes. This model component is calibrated against field measurements of a filling lake in the study area made during June 2011. Supraglacial lakes are able to drain by a simulated hydrofracture mechanism if they reach a critical volume. Once water is at the ice / bed interface, discharge and hydraulic head within subglacial drainage pathways are modelled using the third model component. This consists of an adaptation of a component (EXTRAN) of the U.S. Environmental Protection Agency Storm Water Management Model (SWMM), modified to allow for enlargement and closure of ice-walled conduits. The model is used to identify how the subglacial hydrological system evolves in space and time in response to varying surface water inputs due to melt and lake drainage events, driven ultimately by climate data. A key output from the model is the spatially and temporally varying water pressures which are of interest in helping to explain patterns of surface velocity and uplift found by others, and will ultimately be of interest for driving ice dynamics models.

This thesis aims to better understand the relationships between basal water pressure, friction, and sliding mechanisms at ice sheet scales. In particular, it develops a new subglacial hydrology model (Hydro) to explicitly predict water pressures in response to basal water production and water injection from the surface. Recent research suggests that the Greenland ice sheet (gis) is losing a substantial volume of ice through dynamic thinning. This process must be modelled to accurately assess the contribution of the gis to sea-level rise in future warming scenarios. A key control on dynamic thinning is the presence of water at the ice-bed interface; Zwally et al. (2002) highlight the importance of supraglacial lakes' impact on basal ice dynamics, a process now con rmed by Das et al. (2008) and Shepherd et al. (2009). Many studies focus on the effects of surface meltwater reaching the bed of the gis but the underlying processes are often ignored. Geothermal, strain, and frictional melting, which evolves with basal hydrology, provide the background basal pressure profile that surface meltwater perturbates. Without understanding how these heat terms affect the background profile it is difficult to define basal boundary conditions in models and therefore difficult to model the dynamic response of the gis to surface melting. Hydro tracks subglacial water pressures and the evolution of efficient drainage networks. Coupled with the existing 3D thermomechanical ice sheet model Glimmer, model outputs include effective pressure N and the efficient hydraulic area. Defining frictional heat flux and basal traction as functions of N allow the modelling of seasonal dynamic response to randomly draining supraglacial lakes. Key results are that frictional heat flux, as a function of N, caps potential runaway feedback mechanisms and that water converges in topographic troughs under Greenland's outlet glaciers. This leads to a background profile with low N under outlet glaciers. Therefore, outlet glaciers show a muted dynamic speedup to the seasonal surface signal reaching the bed. Land-terminating ice does not tend to have subglacial troughs and so has higher background N and consequently a larger seasonal response. This, coupled with effects of ice rheology, can explain the hitherto puzzling lack of observed seasonal velocity change on Jakobshavn Isbræ and other outlet glaciers.

Due to environmental concerns, most of persistent organic pollutants (POPs) have been eliminated or reduced in production and use; however, due to their great persistency, POPs are expected still to be found in the environment long after their use has ceased. Although, in recent years, POPs have rarely been detected in river water in the United Kingdom (UK), their concentrations in fish (biota) and sediment are expected to be notable due to their lipophilicity and bioaccumulation; however, there is a lack of information and data to understand the current contamination of POPs in catchments and evaluate their potential risk to the environment and ecosystem. This thesis describes the application of mathematical modelling approaches to (i) predict the current distribution and concentration of POPs in catchments, (ii) evaluate the influence of climate change and extreme weather conditions on the fate of POPs, and (iii) provide guidelines to inform decision-making on managing the potential risks of POPs in river basins. The modelling studies have mainly focused on polychlorinated biphenyls (PCBs). The River Thames catchment was chosen as the study area. The Fugacity level III model was initially used to describe the general distribution of PCBs between different compartments; it was predicted that the greatest mass of PCBs remain in the soil, but the fish and sediments represent compartments with the highest PCB concentrations. The contamination of PCBs in Thames fish was estimated to exceed the unrestricted consumption thresholds of 5.9 μg/kg for ∑PCBs set by the U.S. Environmental Protection Agency (EPA); no current EU Environmental Quality Standards (EQS) are available for PCBs in fish. It was indicated that the PCBs in fish could be linked to PCB contamination in sediment, which was predicted to be about three times higher than the fish concentrations, but insufficient observed data of PCBs in Thames fish and sediment are available to validate the results. In order to address this limitation in observed data, fish and sediment sampling and chemical analysis were carried out for the presence of POPs. In addition to PCBs, the measured results for hexachlorobenzene (HCB) and polybrominated diphenyl ethers (PBDEs) in Thames fish and sediment were assessed. Although the observed fish- and sediment concentrations of the chemicals appear quite variable, when normalised to organic carbon the levels in sediment, they were comparable to the fish lipid normalised concentrations. Using the temperature and rainfall data forecasts in the UK Climate Projections 2009 (UKCP09), climate change scenarios were established and assessed in the fugacity modelling. The modelling results suggested a modest influence of climate change on PCB fate over the next 80 years. The most significant result was a tendency, in the Thames catchment, for climate change to enhance the evaporation of PCBs from soil to air. While the fugacity model successfully simulated the distribution and fate of PCBs, we used greatly simplified representations of climate, hydrology and biogeochemical processes of the catchment: to have a deeper understanding, a newly developed dynamic hydrobiogeochemical transport model - the Integrated Catchment Contaminants model (INCAContaminants) was applied. Using additional information about weather, river flows and water chemistry, the INCA-Contaminants model provided new insights into the behaviour of contaminants in the catchment; this led to a better representation of PCB contamination in sediment. In addition, INCA demonstrated the important impact of short-term weather variation on PCB movement through the environment. It was shown that PCBs contamination in Thames sediment was greatly disturbed by the severe flooding that occurred in early 2014. This thesis presents the application of the INCA model to assess - in addition to POPs - the behaviour of metaldehyde in the River Thames catchment. Metaldehyde is a type of pesticide used mainly to kill snails and slugs. Its application in agricultural areas within the catchment area has in recent years caused severe problems with drinking water supply. The INCA model has proved to be an effective tool for simulating the transport of metaldehyde in the catchment, predicting observed metaldehyde concentrations at multiple locations in the River Thames; this is the first time that a dynamic modelling approach has been used to predict the behaviour of metaldehyde in river basins. Modelling results showed that high concentrations of metaldehyde in the river system are a direct consequence of excessive application rates. In this thesis, a simple decision-support tool was derived from modelling results, based on variable application rates and application areas. This decision-support tool is now being used by Thames Water to help control peak concentrations of metaldehyde at key water supply locations.

Forested watersheds play a dominant role in the global hydrological cycle. Very few experimental observatories especially in tropical forested regions of India have been undertaken. This study has been initiated for this reason and to gain insights into functioning of the hydrological system in such climatic conditions. This study involves experimental setup of a watershed, it’s monitoring till date, modelling of the hydrological processes observed and the challenges in modelling components of the water balance in this watershed. A Small Experimental Watershed of 4.3 Km2 was set up at Mule Hole, in South India along the Kerala-Karnataka State borders, and is situated inside the Bandipur National park. After an overview of watershed studies, review of literature related to forest watershed studies and processes in the first two chapters, Chapter 3 introduces the study area, Mule Hole Experimental Watershed and explains the methodology used to study this watershed. Model SWAT was used initially to simulate the water balance components. A brief description of the model, methodology adopted and discussion on the results obtained is presented in Chapter 4. The watershed initially modelled as an ungauged watershed using the default parameters in the model, simulated very high groundwater contribution to the runoff. The calibrated model although performed favourably for annual average values and monthly calibration, the daily calibration was unsatisfactory. An auxiliary study on quantification of actual and potential evapotranspiration (ET0) has been carried out in Chapter 5 . Ten methods including Penman-Montieth were compared and evaluated for efficacy of the methods. All methods except for Hargreaves method showed agreement with the Penman-Montieth for annual average values. Priestly-Taylor method was found be the best estimator in comparison with Penman-Montieth method, when used to estimate AET. Adjusted Hargreaves and FAO Blaney -Criddle method were found to be very useful when few or limited climatic data were available for estimation of Potential evapotranspiration. A multidisciplinary approach of estimating recharge consisting of chloride mass balance technique coupled with study of water table fluctuations and groundwater flow analytical modelling has been attempted in Chapter 6. Direct and localized recharge was estimated at 45 mm/yr and indirect recharge 30 mm/yr for the monitored years in the watershed. The low values of recharge rates implied an unexpected very high evapotranspiration rate. It may be inferred that in the absence of groundwater flow to the stream, the recharge joins groundwater flow as outflow of the hydrologic system. An integrated lumped model incorporating the regolith zone and the capability of the tree roots to access this store is presented in Chapter 7. The model was able to simulate the pattern of lag-time between water table rise was observed in shallow piezometers in comparison with hillslope piezometers. The patterns of water table variation among the different hillslope piezometers suggest that they are linked with local processes and not by a regional aquifer dynamics. This study shows that water uptake, combined with the spatial variability of regolith depth, can account for the variable lag time between drainage events and groundwater rise observed for the different piezometers. Chapter 8 discusses the results, conclusions derived from this study and possibility of further scope of studies.