Water security

Thames Barrier.
  • How will the effects of climate change impact on water security in the region?
  • What are the implications on society and the environment?

One of the major geological units beneath the Thames basin is the Cretaceous Chalk Group, which represents one of the largest aquifers within the UK and is therefore an essential water resource for the Thames region. This not only includes water resources for several essential services, including public drinking water, but also plays a major contribution to river flow and wetland habitats within the catchment.

The security of this water resource is under threat from a number of factors including changing climate, particularly sea-level rise and its effect on groundwater processes, as well as demand from an increasing population and urbanisation.

One of the major challenges in securing future water resources for the region is understanding how groundwater processes will respond to climate change and what the potential impacts of these changes may be. This knowledge will help devise suitable strategies for future development and enable policy-makers to make more informed decisions.

Whole-system approach

Considering the system as a whole, integrating processes that operate at all levels of the hydrosphere from the soil through to the regional geological framework will provide an opportunity to interface with, and attempt to improve the quality of, predictive models of the wider environment. Applying this understanding in the context of social models has the potential to improve policy on groundwater management at a range of levels.

More specific questions that we are helping to answer:

What is the resilience to drought within the Thames basin and how would water resources stand up to, for example, a 'three-winter drought' or similar?

We are collaborating with Imperial College, Reading University and University College London to improve the prediction and characterisation of extreme events in the Thames catchment under a changing climate. To forecast and mange extreme droughts properly, we require a good understanding of climate science, the availability and accessibility of water within the whole catchment and water management practices. This project brings together climatologists, hydrologists, hydrogeologists, water resource planners and mathematicians to help develop a new generation of water cycle models to better understand the impact of droughts in the Thames catchment.

As specialists in groundwater science, BGS's role on the project is to assess how groundwater resources might be affected during extreme drought. To do this a whole-system approach is taken whereby the separate aquifer systems and the surface water drainage networks are considered in combination using linked hydrological and groundwater models. The Thames hydrological models are being used evaluate water resources across the whole catchment and help inform water resource planners such as Thames Water and the Environment Agency.

Learn more about integrated surface-water groundwater modelling in the Thames catchment.

How can security of water supply be achieved under the combined pressures of population increase (i.e. human demand) and competition from the natural environment?

Recharge calculation studies tend to simulate the run-off flow component of river flow in a simplistic way, often as a fraction of rainfall over a particular period. The method outlined in this study aims to improve the calculation of recharge estimates in distributed recharge models and is not presented as an alternative to complex overland flow simulators. This method was applied to two sub-catchments in the Thames basin using seasonally varying coefficients to calculate run-off for specified hydrological classes or run-off zones previously used to model base flow index (BFI) variations across the basin, and employs a transfer functions model to represent catchment storage.

How can sustainable flows be maintained in rivers?

To effectively manage resources and protect sensitive environments it is essential to understand more about the processes controlling groundwater-surface water interactions. Hydrophysical and hydrochemical techniques have revealed a complex pattern of interactions between chalk groundwater, surface water and shallow gravel groundwater along a river bank and below the river. The river is broadly in hydraulic contact with the river bed and adjacent gravels and sands, but these sediments are mainly hydraulically separate from the underlying chalk at this site. The relationship between the river and underlying alluvium is variable, involving components of groundwater flow both parallel and transverse to the river and with both effluent and influent behaviour seen. The degree of groundwater-surface water interaction within the hyporheic zone at this site seems to be controlled by a number of factors including lithology, topography, and the local groundwater flow regime. While the gravel aquifer is significant in controlling groundwater-surface water interaction, its importance as a route for flow down the catchment is likely to be modest compared with river discharge.

What impact will long term environmental change (including climate change) have on water resources in the Basin?

Typically groundwater levels in chalk decline drastically in response to drought and then rebound again when the drought breaks. Research has been done into the possible water quality effects of this fluctuation in groundwater-fed chalk streams. This study monitored springs, boreholes and surface water in the Pand and Lambourn catchments in southern England during a major drought recovery in 2006–2008. Hydrochemistry, stable isotopes and age indicators were used to characterise the waters. This research is important because climate predictions for southern Britain include greater extremes in rainfall and temperature, and consequently greater amplitudes of water level changes. Springs showed little change in water quality as did boreholes and the River Lambourn. The buffering effect of the chalk aquifer appears to protect the quality of chalk springs and streams.

BGS is doing a four-year NERC funded project to investigate the effects of climatic extremes on groundwater systems in collaboration with Imperial College, the University of Reading and University College London. Three contrasting hydrogeological settings will be assessed. The project aims to exploit current climate science and statistical methods to improve and enhance projections of potential change in hydrological parameters over a time-scale of 10–60 years, in particular extremes of heavy precipitation and drought, and build on the analysis of historical data to improve scientific understanding and develop innovative methods for the modelling of extremes. These should help to improve the representation of hydrological processes in land-surface models and the enhanced modelling of unsaturated zone and groundwater processes on land-atmosphere feedbacks.

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