Linking downstream river water quality to urbanization signatures in subtropical climate.

Urbanization has several hydro-ecological effects on receiving waters. Hence, understanding how urbanization influences river water quality is essential for proper river management. However, an inappropriate approach that correlates urbanization signatures with water quality may result in spurious correlations. This study aimed to investigate the relationship of urbanization signatures with two key pollutants of stream flows: nutrients and pathogens. In contrast to the commonly used approaches that are based on economic or demographic metrics, our approach represents urbanization signatures using related anthropogenic activities and evaluates the effect of such activities on water quality parameters. The approach was also applied to evaluate the impacts of urbanization on nutrient and pathogen trends in the river waters of Hong Kong. The data were collected for the period of 1986-2020 from the Environmental Protection Department and monthly measurements were performed. Total nitrogen (TN), total phosphorus (TP), Escherichia. coli (E. coli), and fecal coliforms (FC) showed consistently decreasing trends. However, the long-term seasonality of nutrients differed from that of pathogens. TP and TN exhibited homogenous seasonality with an approximately sinusoidal relationship from January to December, whereas the seasonality of pathogens was more complex and not dependent on river flow dilution effects. Additionally, urbanization impacts on station nutrients and pathogen characteristics were found to be unevenly distributed; under high water temperatures, nutrient concentrations were found to be decreased because of the rainfall dilution effect on river flows. Both urban point and diffuse sources of pollution significantly contributed to nutrient pollution in rivers. Furthermore, the concentrations of FC were not highly influenced by suspended solids, and dissolved oxygen was negatively correlated with all pathogens. Furthermore, the river flow rate was found to improve the water quality in terms of both nutrients and pathogens; urban point source pollution and river temperature alteration were shown to mainly contribute to seasonal variations in both nutrients and pathogens.

Urban hydrological responses to climate change and urbanization in cold climates.

This study explores hydrological response of urban catchment in Southern Finland to climate change and urbanization. Process-based urban hydrological modelling and statistical analysis are applied to various urbanization and climate scenarios. Future changes in precipitation and temperature under Representative Concentration Pathways 4.5 and 8.5 (RCP4.5 and RCP8.5, respectively) clearly influence urban streamflow all year-round. We found snowpack shrinks during 2061 to 2090, snowmelt becomes earlier and the amount of melted runoff is reduced under both climate scenarios. The most significant runoff increase occurs in winter with the growth rates of 79% and 127%, respectively. It is also found that the dominant portion of urban streamflow shifts from summer to autumn in the future under both RCP4.5 and RCP8.5. Results indicate that urbanization has direct impact on hydrological response due to the change of imperviousness, but climate change will have more significant impact on seasonal distribution of urban streamflow. Additionally, urbanization impacts shrink monthly streamflow differences along with climate change.

Assessment of LID practices for restoring pre-development runoff regime in an urbanized catchment in southern Finland.

This study quantifies the effects of common stormwater management techniques on urban runoff generation. Simulated flow rates for different low impact development (LID) scenarios were compared with observed flow rates during different urban construction phases in a catchment (12.3 ha) that was developed from natural forest to a residential area over a monitoring period of 5 years. The Storm Water Management Model (SWMM) was calibrated and validated against the observed flow rates in the fully developed catchment conditions, and it was then applied to parameterize the LID measures and produce scenarios of their hydrological impacts. The results from the LID scenarios were compared with the observed flow rates in the pre-development and the partially developed catchment conditions. The results show that LID controls reduce urban runoff towards the flow conditions in the partially developed catchment, but the reduction effect diminishes during large rainfall events. The hydrographs with LID are still clearly different from the observed pre-development levels. Although the full restoration of pre-development flow conditions was not feasible, a combination of several measures controlling both volumes and retention times of storm runoff appeared to be effective for managing the stormwater runoff and mitigating the negative impacts of urban development.