TOWARDS HOLISTIC BASIN SCALE INTEGRATED MODELLING: A CASE STUDY USING THE THAMES BASIN, UK

As pressures increase on water resources from greater demand and environmental change, so there is a need to understand, model and manage catchments on a holistic basis. In Britain, a greatly variable succession of both permeable and impermeable deposits from Lower Palaeozoic to Holocene can be found. The sedimentary rocks generally dip from the north-west to south-east of the country. The hydrology is characterised by a great number of relatively small basins, the largest being the Thames (9478 sq km to its tidal limit at Teddington Lock). Each river basin contains a number of different aquifer units and, due to the variability of lithologies, these aquifers do not connect laterally.

In order to understand the basin hydrology and the impacts of extreme events an holistic approach to modelling is necessary. The groundwater systems in the Thames Basin are made up of about 20 distinct aquifer units from Jurassic age Limestones in the west of the catchment through to Palaeogene sands and silts in the east. These are then sometimes overlain by variable thicknesses of Quaternary superficial deposits and Holocene alluvium. The majority of these units are not in hydraulic connection and the only way water is exchanged between these aquifer systems is via the River Thames and its tributaries. Also the Thames is a highly managed river and the main aquifer units are extensively exploited. This has significant implications for how the hydrologic system works, how different components respond to extremes events and how a reasonable simulation of groundwater flow can be achieved.

In a first attempt at a holistic approach, a series of independent groundwater models are being developed and linked using a river model and later to an estuary model. Thus, the modelling system consists of a number of independent models coupled using the model linking standard OpenMI. Given that knowledge of the different aquifers is highly variable, a range of groundwater models are being used from simple transfer function or “bucket” conceptual models through to distributed 3D time variant models. This linked system of models can then be used to describe the impacts of extreme events on the basin as a whole. Examples include the recent droughts and flooding events where the distribution of rainfall can have distinct consequences for water resources management.