Integrated surface-water groundwater modelling of the Thames catchment

The effects of climate change and the human exploitation of our environment pose a serious threat to the sustainability of our water resources, both for our natural environment and for human consumption. To manage water resources sustainably and to make useful projections of the future impact of these pressures they must be considered in combination. This necessitates the simulation of the complex interactions between different parts of the water cycle, and the development of integrated environmental models. This is particularly true for the heavily populated catchment of the River Thames.

From its headwaters to Teddington Lock the Thames flows for 235 km and covers an area of approximately 9950 km2. The western parts of the Thames basin are predominantly rural, whereas, the highly urbanised area of Greater London is located in the central and eastern part of the basin and is home to about 14 million people. Approximately 6 million m3/d of water is provided for public supply, which is split 60 /40 per cent between surface water and groundwater sources. Pressures on the resource are varied as are adaptation issues. The catchment has abstraction, pollution and land use management issues, suffers from both surface water and groundwater flooding, and has experienced severe drought events in the relatively recent past.

Thames catchment

Thames basin geology

To develop our understanding of water resources within the Thames, and to help manage them effectively, now and in the future, we use models. We develop these models both in collaboration with, and for, our stakeholders. Once calibrated, and their performance tested against past observations, such as river flows and groundwater levels, they can then be used in a number of scientific roles e.g. to test hypotheses about how a surface-water or groundwater system behaves or to make projections about potential future changes.

Modelling the water resources of the Thames catchment is challenging because it is composed of a series of distinct aquifers (water bearing rocks) that are separated by impermeable clays. The aquifers are connected only by the surface river network. For example, groundwater in the Jurassic limestone aquifer discharges to rivers of the Cotswolds, which then flow across the impermeable Oxford Clay and onto the Chalk – the major aquifer of south-east England. During past droughts the rivers of the Cotswolds, fed by groundwater, have maintained flows in the Thames some distance downstream.

To simulate this complex system we are developing an integrated model composed of a series of connected sub-models. The sub-models are constructed using different modelling codes, which are appropriate to the degree of complexity of the part of the system being considered. Currently there are four sub-models in the integrated composition:

  • a gridded ZOODRM model that simulates run-off and recharge across the catchment
  • a gridded ZOOMQ3D groundwater model of the Chalk aquifer
  • a semi-distributed model of the Cotswolds limestone aquifer
  • a Muskingum-Cunge river channel flow routing model

Rainfall recharge to the water table is simulated by the ZOODRM model and used to drive the groundwater models of the limestone and chalk aquifers. This model also provides the river flow routing model with simulated rates of surface run-off.  The two groundwater models are coupled to the river model using an implementation of the Open Modelling Interface (OpenMI), which allows them to pass information between each-other as they run. Rates of river leakage to and from the groundwater models are passed to the river model, which are combined with surface runoff from ZOODRM.  These flows are then routed along surface water channels by the river model.

Structure and coupling of the models

Structure and coupling of the Thames basin models.

This integrated modelling platform is being developed to enable large-scale water resources questions to be addressed within the context of environmental change.

Some of these science questions include:

  • How important is groundwater in supporting river flows during drought periods, and how do spatial variations in groundwater storage affect severity of drought experienced across the catchment?
  • How will sustainable yields of groundwater abstraction boreholes change in the future as the climate changes?
  • When might tipping points occur in the supply of water both for public supply and for the environment in the future?
  • What is the role of the unconsolidated geological deposits that cover the Chalk aquifer is sustaining river flows during summer?
  • What is the uncertainty in recharge estimates due to the parameterisation of the land surface?

Contact

Contact Dr Christopher Jackson for more information