Rapid urbanization on streamflows may directly affect or be restricted by the sustainability of local water resources. This is particularly true for arid/semiarid areas such as the Wulanmulun River watershed in the rapidly-developing Ordos region of north central China. From 1997 to 2012, the gross domestic product (GDP) of the region increased fifty-fold, while the urban area grew by a factor of ten. This study fused multiple-source data on land use, hydrometeorology, and socioeconomics to examine temporal variations in the runoff due to climate change and urbanization. The results revealed that for the Wulanmulun River watershed, the runoff decreased consistently over the study period, with an inflection point around 2005. The average runoff from 2006 to 2012 was much smaller than that from 1997 to 2005, regardless of time scale; although the precipitation also fluctuated from 1997 to 2012, it exhibited no significant trend. From 1997–2005 to 2006–2012, both the urbanized area and GDP increased eight-fold while the population increased by 20%. Thus, urbanization rather than climate change is likely the major reason for the decrease in runoff after 2005. For the study watershed, low impact development practices (e.g. rain barrels) may need to be implemented during urbanization to achieve sustainable management of water resources.