Land use is a key factor affecting water balance, which in turn affects the water resource availability. Increasing urbanization has altered water balances in river catchments. Although urbanization impacts have been widely investigated, most studies focus on changes in land use area, and the percent imperviousness of urban (PIU) is typically downplayed or ignored. PIU is altered by permeable areas within urban regions being replaced by impervious ones and can influence the spatial flow pattern of water and the hydraulic efficiency of its flows in urban locales. In this study, we consider the increase in urban area PIU due to urbanization, and focus on the PIU’s impact on water balances. We utilize the Soil and Water Assessment Tool to describe the PIU’s impact on long-term (1970s ~ 2010s) water balances in a same catchment with different urbanization levels and quantify these characteristics on spatial and time scales with a combination of remote sensing and numerous hydrological and geographical data. We demonstrate that: (1) Increasing urban PIU has a significant influence on the balance of urban water resources, with an increase to 0.17 causing the urban surface runoff ratio to increase by 0.2; (2) the impact of PIU on surface runoff and baseflows is sometimes more important than area changes in a catchment, especially for those in settings characterized by rapid urbanization. Thus, we note that PIU changes directly affect the water balance and enhance the impacts of area changes. Fundamentally, in the study of water resource management in urban settings or catchments in these, the PIU is a highly important variable to consider.