Without water no energy, significant trade-offs between carbon and water footprints important for global energy and water policy Winnie Gerbens-Leenes1*, Junguo Liu2 1 Integrated Research on Energy, Environment and Society (IREES), University of Groningen, Nijenborg 6, 9747 AG, Groningen, The Netherlands; [email protected] 2. Southern University of Science and Technology (SUSTech), Shenzhen, China; [email protected] Water and energy are strongly related. Water supply needs energy and energy supply needs water. The focus of the pre-2009 water for energy studies was mainly on the quantification of cooling water use in thermoelectric generation and on water use for transport fuel production. Most of the studies were based on grey literature using data from industry, often from the USA. Water footprint (WF) studies have made it possible to quantify water for bioenergy and hydropower, because the assessments were made based on publically available data, e.g. weather data. WF studies provided new information on the amount of water needed for specific renewable energy types. Energy that originates from photosynthesis (e.g. crops, trees or algae) has relatively large water footprints compared to fossil energy sources. Energy that originates from hydropower also has large average WFs, but variation is large. This paper gives an overview of the contribution of water footprint studies on water for energy relationships. It first explains why water is needed for energy, gives an overview of important water-energy studies until 2009, shows the contribution of WF studies, and indicates how this contribution has supported new research. Finally, it provides knowledge gaps that are relevant for future studies. Energy source categories are: 1. biofuels from sugar, starch and oil crops (food crops); 2. cellulosic feedstocks (residues and energy crops); 3. biofuels from algae; 4. firewood; 5. hydropower and 6. various sources of energy including electricity, heat and transport fuels. Especially category 1, 3, 4, 5 and to a lesser extent 2 have relatively large WFs. This is because the energy source derives from agriculture or forestry, which has a large water use (1,2,4), or has large water use due to evaporation from open water surfaces (3,5). WFs for these categories can be calculated using the WF tool. Category 6 includes fossil fuels and renewables, such as photovoltaics and wind energy and has relatively small WFs. However, information needs to be derived from industry. The policy to decrease carbondioxide emissions has consequences for water. Energy policies need to account for significant trade-offs between carbon, land and water footprints.