During a winter of storms in a small UK catchment, hydrology and water quality responses follow a clear rural-urban gradient

This paper presents the hydrological and water quality response from a series of extreme storm events that passed across the UK during the winter of 2013/2014, in an experimental catchment with a strong rural-urban gradient across four nested sub-catchment areas. The Ray catchment in the upper Thames basin, UK, was extensively monitored using in-situ, high-resolution (15 min) flow and water quality instrumentation. Dissolved oxygen, ammonium, turbidity and specific conductivity are used to characterise the water quality dynamics. The impact of the Swindon sewage treatment works (SSTW) on water chemistry at the catchment outlet is considerable. Hydrological and water-quality response varies considerably during the events, with the rural catchments exhibiting a much slower hydrological response compared to urban areas. A simple hydrological model (TETIS) was developed to provide insight into water sources in nested subcatchments, highlighting the disparity of the hydrological dynamics across contrasting land-uses during events. The variation in stormwater runoff sources impacts water quality signals with urban sites contributing to dilution dynamics in ammonium, whereas the more rural site experiences a peak in ammonium during the same event. Dissolved oxygen concentrations vary on a rural-urban gradient and experience a notable sag at the Water Eaton outlet (4.4 mg/I) during the events, that would have resulted in significant ecological harm had they occurred during the summer in warmer temperatures. The water-quality legacy of these storms in the wider context of the hydrological year is somewhat negligible, with markedly poorer water quality signals being observed during the summer months of 2014. Although ammonium concentrations during the events are elevated (above the 'good' status threshold under the WFD), higher values are observed during spring and summer months. The high flows actually appear to flush contaminants out of the Ray and its subcatchments, though the urban sites demonstrate a resupply dynamic during interim dry periods. Data suggest winter storms following dry spells in urban catchments cause some short-lived and spatially extensive deteriorations in water quality. More chronic effects, although prolonged, are only seen downstream of SSTW. These are indicative of capacity of infrastructure being reached, and from the data do not appear to be severe enough to cause ecological harm. (C) 2016 Elsevier B.V. All rights reserved.