Water is a scarce resource in most of America, especially outside of the Southeast and Great Lakes states. The USA faces two macroscale drivers of water scarcity in the coming century: a growing domestic population and the continuing depletion of fossil fresh water ground reserves. Between 2005 and 2050, the US Census Bureau predicts a population increase of greater than 40% [43] , increasing demands on water for domestic and commercial applications. At the same time, the US Geological Survey estimates that over 40% of agricultural fresh water and more than 30% of nonagricultural fresh water is drawn from deep aquifers that do not refresh over meaningful timeframes [ 26] . The magnitude of this resource, accumulated over geological time scales and currently being depleted faster than natural recharge rates, is poorly quantified nationwide.

Terrestrial energy crops grow as an open system, incurring water losses from irrigation inefficiencies [22], soil evaporation [27], and transpiration for leaf cool­ing and motive force for nutrient uptake; it is estimated that only 0.2-0.4% of water used in agriculture is fixed as plant matter [11, 17, 19] (Table 2). Agriculture is by far the largest source of water utilization in the USA, accounting for fully 80% of consumption from all sources [51]. Whether water is supplied by irrigation or natu­ral rainfall, agricultural water use limits availability for other applications [ 34] . Similarly, open-pond algal systems suffer from evaporative losses [44], although such losses are ameliorated through the use of closed systems or salt-tolerant spe­cies [9, 24].

Several advanced biofuel systems offer the opportunity to diminish concerns sur­rounding water withdrawals by creating closed bioreactors that obviate water losses from evaporation and transpiration [30, 31, 49]. Closed systems facilitate complete

nBOE barrel of oil equivalent on an energy basis bSupp Calc 1 cSupp Calc 2 dPETRO FOA targets

eSupp Calc 5 with El Paso, Texas for reference fSupp Calc 6 with North Dakota as reference gGGE gasoline gallon equivalent on an energy basis

hBiomass growth on saline or brackish water: 0 g/GGE [9, 24]; cellulosic ethanol from switchgrass (no irrigation): 1.9 g/GGE (thermochemical). 5.8-9.8 g/GGE (biochemical) [68]; open-pond batch algae systems could consume 223-1,421 g/GGE fresh water [44, 62]

‘Supp Calc 3. Upper limit includes water use from dry mill com processing Calculations: O-biocatalyst/woody biomass; 1.2-2.4-switchgrass; 58-algae [44]

Calculations: O-biocatalyst/woody biomass; 1-8-algae [44, 45]

Calculations: O-biocatalyst/woody biomass; 3-algae [45]

water recycle: in the limit, water is required during biofuel production only as the ultimate source of electrons during water oxidation, providing two electrons (as hydride) and molecular oxygen during CO2 reduction. This requirement amounts to roughly 1 gal of water/gal of fuel (Supp Calc 3), plus whatever water is consumed by growing cultures. While these values are somewhat imprecise at this early stage, they will almost certainly represent a vast savings over terrestrial plants and over at least some forms of photosynthetic organisms.