Many of the world’s major groundwater aquifers are rapidly depleting due to unsustainable groundwater pumping, while demand for food production – and therefore demand for irrigation water – is increasing.  While it is clear that groundwater users will be impacted by the future’s inevitable reduction in groundwater availability, there is a major gap in our understanding of potential impacts downstream of pumping sites.  Due to inefficiencies in irrigation systems, significant amounts of abstracted groundwater become runoff, entering surface waters and flowing downstream to be re-abstracted and used again.  No previous studies have included unsustainably-pumped groundwater in a global-scale analysis of downstream irrigation water reuse.  In this study, we use a gridded water balance model to calculate the amount of unsustainably pumped groundwater that enters surface water systems by way of irrigation runoff, and quantify the additional irrigation water supplied by the re-use of this water.  We assess the global budget of unsustainable groundwater sources and sinks, including downstream re-use, groundwater recharge, and transport to the oceans. We find that globally 80% of unsustainable groundwater is re-abstracted for irrigation either downstream or locally from groundwater recharge.  This re-abstracted water supports over 200,000 km2 of irrigated land globally.  Including irrigation runoff reuse in an assessment of irrigation efficiency, we see that the traditional concept of irrigation efficiency (net irrigation/gross irrigation) significantly overestimates water “waste”.  We define a basin efficiency for unsustainable groundwater use that includes re-use, and see that while global irrigation efficiency is often estimated at 50%, global average unsustainable water use efficiency is > 60%.  This result shows that increasing irrigation efficiency does not alleviate unsustainable groundwater demands.