Extreme drought conditions interact with urbanisation, affecting hydrological regimes and water quality in temperate lowland streams.

Since 2020, the Pampas ecoregion, a highly productive area of Argentina, has endured its most severe drought in the past century. Our understanding of the impact of droughts on physicochemical changes in lotic environments remains limited, and this knowledge gap becomes even more pronounced when considering the concurrent urbanisation and industrialisation that alter land-use patterns and pose threats to water quality. This study aims to analyse, on a short-term basis, how hydrodynamics and physicochemical characteristics of streams, exposed to different land uses, are affected by drought. During the winter of 2022, we conducted three samplings in streams within the Samborombón River basin. We selected three streams influenced by rural land use and three influenced by urban development. Our data collection encompassed meteorological information, the multivariate El Niño Southern Oscillation (ENSO) index and the Palmer drought severity index. We measured hydrological and physicochemical variables at each sampling site. Our analysis identified an extreme drought period coinciding with 'La Niña' conditions during the sampling period. The annual accumulated precipitation for this period was notably lower, at 622 mm, compared with the historical average (1958–2022) of 1006 mm for this region. Hydrological variables exhibited significant differences between urban and rural streams. Urban streams maintained a consistent water flow, while rural streams were predominantly characterised by disconnected pools. Additionally, urban streams exhibited significantly greater Secchi depth and higher nutrient concentrations, whereas rural streams displayed elevated pH, conductivity and turbidity levels. These findings underscore the crucial role of urban effluents in sustaining a base flow during extreme drought conditions. However, this base flow is associated with poorer water quality, disrupting the natural chemical and hydrological dynamics of the region during drought events. [ABSTRACT FROM AUTHOR]