Water footprints of bioethanol cropping systems in Uruguay

Bioenergy is the most widely used type of renewable energy. However, an assessment of water consumption and pollution is necessary to determine the water demand of this energy source. The Uruguayan public policy to decarbonize energy sources highlighted the use of bioenergy. In this regard, we analyzed the water footprint (WF) of four bioethanol cropping systems: (1) maize-wheat-sorghum rotation without harvested crop residues (MWS), (2) maize-wheat-sorghum rotation with harvested crop residues (MWS-R), (3) continuous sweet sorghum (Ss), and (4) switchgrass (Sw). In order to assess the WF of bioethanol production, green (WFgreen) and gray (WFgray) components of crop production were calculated by considering the different volumes of water involved in evaporation, rainfall, and fertilizer pollution. Annual cropping systems (i.e., MWS, MWS-R, Ss) had the largest WFs (23.1–30.9 m 3 L e t h a n o l − 1 ). Switchgrass had the lowest values per hectare and per liter of ethanol (12,735 m3(ha yr)−1 and 3.8 m 3 L e t h a n o l − 1 , respectively). The volume required to assimilate phosphorous (P) and nitrogen (N) fertilizers played a significant role in bioethanol cropping systems. In annual systems, WFgray was the main fraction (87%) of total WF (WFT). Averaged across all cropping systems, WFgray related to P was 13 times larger than WFgray related to N.