The UK Acid Waters Monitoring Network (now the UK Upland Waters Monitoring Network) was set up to provide crucial chemical and biological data on the extent and degree of surface water acidification in the UK uplands, in particular to underpin the science linking acid deposition to water quality and to monitor the response of aquatic ecosystems to reductions in air pollution. The sites making up the network were selected on the basis of acid deposition inputs being the only major sources of pollution, i.e. with no point sources of pollution or direct catchment disturbances other than traditional upland land use practices such as sheep grazing or forestry.

The water chemistry and biological data provided by the UK UWMN provide the highest quality datasets for the development and application of Critical Loads models which are used on a national basis for the provision of data for freshwater ecosystems under the UN-ECE Gothenburg Protocol. Likewise, they have been widely used in the development of dynamic acidification models such as the MAGIC Model which provides information on the pre-industrial reference conditions of lakes and streams, and predicts future responses to current or planned levels of acid deposition in order to determine the likely effectiveness of measures to reduce pollutant emissions.

UK UWMN water chemistry and biological data provide the highest quality datasets for the development and application of Critical Loads models

Since its inception, the UK UWMN has been closely linked to the Defra research programme on the impacts of air pollutants on freshwater ecosystems, initially the Freshwaters sub-group of the Critical Loads of Acidity Group (CLAG), and subsequently the Critical Loads of Acidity and Metals (CLAM) and Freshwater Umbrella programmes.

The Network has evolved since its beginnings, when the primary focus was to assess responses to reductions in sulphur deposition. This was achieved through comparisons of high and low deposition sites across a gradient of sensitivity, encompassing rivers and lakes and comparing forested with moorland catchments. In 1995, following recognition of nitrogen (N) deposition as a key driver for surface water acidification, measurement of total dissolved nitrogen (TN) was added to the suite of chemical determinands. At the same time, toxic metal concentrations were also identified as a potential problem and one of the UK UWMN sites in north east Scotland, Lochnagar, was set up to monitor mercury (Hg) in atmospheric deposition, water and aquatic plants.

Data from the UK UWMN are central to ongoing research programmes on a range of drivers of change in upland catchments

Climate change was also recognised as potentially a major driver of change at these sites. Surface and deep water thermistors were installed at all lake sites in 1999, and a weather station was installed at Lochnagar in 2002. From 2013, chains of thermistor loggers have been deployed in all UWMN sites to provide a detailed picture of variations in water temperature over time and with depth. Thermistors had been installed at all stream sites by 2015.

In addition to monitoring recovery from acidification, data from the UK UWMN are central to ongoing research programmes on other drivers of change in upland catchments, including N deposition and eutrophication, land use change and climate change.

Further detailed analysis of UK UWMN science can be found in the interpretive reports and Network scientific publications.