Urban water systems are highly engineered. However, hydrology and ecology are still closely linked in semi-arid urban ecosystems in which surface characteristics, vegetation, and water flows are all highly transformed. Although these systems are human-dominated, there are many uncertainties in the water budgets of semi-arid cities, because evapotranspiration, runoff, groundwater recharge, and leakage are poorly constrained. Decision-making, governance, and socioeconomic factors play important roles in determining urban hydrologic budgets. We offer a framework to integrate these factors in studies that combine biophysical and social dimensions of the urban water system using the example of western US cities, which are facing critical issues in water supply and demand, and which can benefit from a more comprehensive understanding of the factors that determine water consumption, distribution and availability. Because of the severity of the water crisis in the western US, and the biophysical, institutional, and cultural barriers to developing and implementing new water management practices, this region provides useful lessons for addressing water challenges in other regions. Copyright © 2011 John Wiley & Sons, Ltd.