Water quality is deteriorating worldwide due to the combined pressures of increasing urbanization and more frequent and severe extreme events. This thesis looks specifically at water temperature and dissolved organic matter (DOM), which despite being master variables of river water quality are not well understood in urban rivers. This thesis aims to increase understanding of how extreme events and urbanization combine to change the dominant processes for water temperature and DOM dynamics. Resultantly research was conducted in a range of headwater streams within Birmingham, UK from June 2016 to September 2018.
Research gaps on the effects of urbanization and extreme events on water temperature and DOM were identified and four research themes were described. Firstly, the effects of precipitation on water temperature surges at 11 sites in an urban catchment were investigated, and the choice of precipitation dataset on the results evaluated. Secondly, the effects of extreme high and low flows on river temperature were analyzed for 27 sites in 3 catchments and the influence of land use evaluated. Thirdly, the impacts of shading and water temperature on photodegradation and biodegradation rates of urban DOM were studied. Fourthly, in-situ a fluorometer was used to investigate DOM response to storm events, and the influence of hydrometeorological and land use predictors were investigated. The primary findings were 1) High intensity precipitation events cause water temperature surges in urban catchments, while high temporal and spatial resolution datasets are required to capture this effect, 2) Water temperature anomalies are highest during extreme low flows, while urbanization is related to lower water temperature anomalies during extreme low flows. 3) Shading changes the composition of urban DOM by preventing photodegradation of the humic pool, however temperature had minimal effect. 4) Urban DOM is source-limited and exhibits exhaustion and dilution effects, with the main predictors of urban DOM during storms being water temperature and antecedent rainfall.
The results indicate new understanding of how a range of extreme events alter water temperature and DOM processes within headwater, urban rivers. The need to change urban land use practices to mitigate the impacts of extreme events on urban water quality is highlighted
