Model-based estimation of the isolated impacts of urban expansion on projected streamflow values under varied climate scenarios

The use of computationally intensive hydrologic models under future climate scenarios has become a common practice to project water resource concerns in the coming decades. Under this approach, hydrologic models are parameterized and run under various climate forcings. Although urban areas are expected to expand during the time frame of these simulations, potentially impacting watershed hydrology, the uncertainty of forecasted streamflow is usually estimated based on the ensemble of climate scenarios, with minimal (if any) attention given to the uncertainty introduced by land transformations. The objective of this study is to quantify the Isolated Impacts on Projected Streamflow (IIPS) caused by urban expansion as climate changes in a watershed in the midwestern United States. IIPS time series were estimated as the difference between projected streamflows under future climate scenarios with and without urban expansion and weighted by the historical (1980–2010) monthly average. Two gradual and two abrupt urbanization scenarios, having equivalent developed areas by the end of the 21st century, were implemented. Results indicate that gradual urbanization could result in both increased (up to 26 %) and decreased (up to 16 %) projected streamflows, suggesting the increase in variability of extremes, with potential impacts on human and natural systems. Yearly minimum and maximum IIPS for all scenarios were found to be more likely to occur in summer and fall months, respectively. Impacts of the abrupt urban expansion were mainly observed in the cumulative IIPS and the ensemble variability of extreme IIPS. These results provide insights into the uncertainty of future streamflow estimates attributable to urban expansion.