Urban Runoff in the U.S. Southwest: Importance of Impervious Surfaces for Small-Storm Hydrology

Abstract

Impervious surfaces have long been recognized as one of the critical factors influencing the rainfall-runoff relationship in urban areas. Published guidance on how to treat urban imperviousness in hydrologic simulations varies, and few studies exist for the southwestern United States. In recent years, small-storm hydrology has become increasingly important to support water quality evaluations. Methods developed for large, infrequent storms are often applied to simulate runoff from smaller rain events, with questionable results. To test the impact of imperviousness on small-storm runoff, impervious cover in a 1.5-km2 urban basin in central New Mexico was mapped and differentiated into directly connected impervious areas (DCIAs) and unconnected impervious areas (UIAs) that are separated from the drainage system by a pervious buffer. Measured runoff from 25 small-magnitude storms was compared to model scenarios with varying treatment of imperviousness. Simulation results revealed that during small rain events, all runoff originated from DCIAs, while UIAs contributed no flow. For this particular basin with 28% DCIAs and 16% UIAs, losses from UIAs approached a constant value of approximately 4 mm. Findings were applied to a 142-km2 watershed to assess model performance for larger storms. The impact of losses from UIAs on the runoff response decreased with increasing storm magnitude. This study presents a modeling approach for DCIAs and UIAs that increased accuracy for small runoff events in urban areas without negatively affecting model performance for larger storms. Use of this method can improve model predictions, particularly when hydrologic simulations built with flood control in mind are used to assess the impact of more frequent storm events.