Climate Change and Urban Waterways: Alterations in the Flow Regime Under Multiple Ensembles

Adverse effects of urbanization on urban waterways have been established, however little is known about the anticipated impacts of climate change on hydrological flow regime and aquatic ecosystem. The present study focuses on the investigation of climate change impacts on the flow regime in the Lucas Creek catchment located in the Auckland region. Statistically and dynamically downscaled climatic variables from seven Global Climate Models (GCMs) are adopted under three Representative Concentration Pathway (RCP 2.6, RCP 4.5 and RCP 8.5) scenarios. Personal Computer Stormwater Management Model (PCSWMM) was calibrated and validated using the observed streamflow data. The performance of PCSWMM during calibration and validation was assessed using the Nash-Sutcliffe (NS) coefficient, the Root Mean Square Error (RMSE), and the coefficient of determination (R2). Additionally, low flow (Q90) and high flow (Q10) indices were compared during calibration and validation using Percentage BIAS (PBIAS) criteria to verify the trends in discharge simulations. The model showed a good match between the observed and the simulated data indicating a good calibration. Following this, the model was used to simulate flow time series under the climate change scenarios. Alterations in the flow regime were assessed through flow duration curves and indicators of hydrological alteration. The results show a significant rise in peak flow in the 2090s (2081-2100) in comparison to baseline (1985-2005) however, low flow mainly decreases under RCP 2.6. Monthly streamflow increases over the annual cycle, but minimal changes are observed in January, March and November. The extreme minimum conditions observe higher positive changes under RCP 2.6 and RCP 4.5 compared to RCP 8.5. Similarly, the magnitudes of maximum flow conditions have observed a diverse pattern. The base flow index has shown variations ranging from -18% to 18% for most of the GCMs. Annual extremes’ timings, duration of high pulses, rise rate and low pulse count observe a decreasing pattern. However, the duration of low pulses, fall rate, number of high pulse count and number of reversals follow a rising trend. The changes in the flow regime could have some advantages nevertheless, the aquatic ecosystem would observe severe adverse effects in the end.