Analysing the impact of climate change and land-use change on the water quality of the River Thames

Dr Gianbattista Bussi, Postdoctoral Research Assistant, and Prof Simon Dadson, Associate Professor in Physical Geography, present the results of the POLLCurb project.

Human activities can cause changes in climate and land cover, which affect the water cycle and the water quality of rivers and lakes. These drivers of change, coupled with natural climate variability, lead to alterations in water quality that can have strong repercussions on water supply, ecosystems, navigation, conservation and recreational uses of water.
The River Thames, in Southern England, is a prime example of a river subject to strong human pressure. It is the main source of drinking water for around 14 million people and the recipient of the effluent for around 3 million people. It is also of very high conservation and recreational value for local communities.

In order to study the impact of land-use and land-cover changes over river water quality under a changing climate, the Natural Environment Research Council funded the POLLCurb project (Changes in Urbanisation and its Effects on Water Quantity and Quality from Local to Regional Scale), as part of the Changing Water Cycle programme. The project, which ended in September 2016, was led by the Centre for Ecology and Hydrology in Wallingford and was carried out jointly with the University of Oxford, the University of Bath and the University of Surrey.

As part of the project, Dr Gianbattista Bussi and Prof Simon Dadson, of Oxford University’s School of Geography and the Environment, analysed the joint impact of climate and land-use changes on the sediment transport and water quality at the scale of the River Thames catchment.

The research provided great insights into the sediment dynamic of the River Thames. Sediment dynamics of lowland rivers like the Thames are of vital importance in building resilient strategies to manage environmental change.

The project described the seasonal and inter-annual variability of suspended sediment concentration in Thames, by analysing long-term, intermittent records collected by the Environment Agency since 1974. In particular, the flushing effect, i.e. the increase in sediment load that takes place with the first floods after a dry period, was quantified. This is responsible for increasing the sediment concentration of the Thames by 1.5 – 2 times. A decrease in the flushing effect which began in the 1990s was also observed.

Furthermore, the joint impact of climate change and changes in the extension of arable land was analysed by using a mathematical model called INCA (INtegrated CAtchment model). The results showed that climate and land cover each exert an individual control on sediment transport. The suspended sediment yield of the River Thames is expected to decrease by 4% by the 2030s, although this figure will be strongly affected by the future variations of extreme precipitation and future agricultural practices.

Lastly, the impact of climate change, land management change and waste water treatment strategies on the nutrients and phytoplankton of the river Thames was also assessed. Phytoplankton pose a serious threat to the use of surface waters for water supply purposes. In this project, a river phytoplankton model was employed to evaluate the effects of climate alterations on flow, phosphorus concentration and phytoplankton concentration of the River Thames.

The model demonstrated that an increase in average phytoplankton concentration, especially potentially harmful Cyanobacteria, is highly likely to occur due to climate change. However, an optimal mitigation strategy, which combines reduction of fertiliser and phosphorus removal from wastewater, can help to reduce the phytoplankton concentration, and in some cases, compensate for the effect of rising temperature.

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