The increase in runoff from urbanization is well known; one extreme example comes from a 13 hectare residential neighborhood in southeast Arizona where runoff was 27 times greater than an adjacent grassland watershed over a forty‐month period from 2005 to 2008. Rainfall‐runoff modeling using the newly‐described KINEROS2 urban element and tension infiltrometer measurements indicate that 17±14 percent of this increase in runoff is due to a 53 percent decrease in the saturated hydraulic conductivity of constructed pervious areas, as compared to the undeveloped grassland. Directly connected impervious areas, primarily streets and driveways, cause 56 percent of the increase in runoff, and indirectly connected impervious areas, primarily rooftops and sidewalks, and a decrease in canopy interception account for the remaining 27 percent increase. Tension infiltrometer measurements show that saturated hydraulic conductivity (Ks) is about double in the grassland watershed than in the urban watershed, 6.2 ± 3.5mm/hr and 2.9 ± 1.6mm/hr, respectively. Ks in the urban watershed identified from calibrating the rainfall‐runoff model to measured runoff is 9.5 ± 2.8 mm/hr—higher than what was measured but much lower than the 26 mm/hr value indicated by a soil‐texture based KINEROS2 parameter look‐up table. A new component of the KINEROS2 modeling framework, the urban element, forms the basis for the model by simulating a contiguous row of houses and the adjoining street as a series of pervious and impervious overland flow planes. Tests using different levels of discretization found that watershed geometry can be represented in a simplified manner, although more detailed discretization led to better model performance.