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Lettuce has 96% water content. Let us use this as a base line water requirement of hydroponically grown lettuce which means that it takes 96 g of water to produce 100 g of lettuce. Since hydroponic lettuce need 90% less water, this means that 96 g is just 10% of required for soil grown lettuce. Thus soil grown lettuce require at least 9.6 kg of water to produce 1 kg of lettuce. With the lettuce production of the entire US in 2003 was 4.5 million tons using traditional soil growing techniques, this therefore requires 4.5 million tons x 1000 kg per ton x 9.6 kg water per kg or 43.2 million tons of water.
However, growing these using Urban Farming hydroponic techniques, the water requirement is only 10% or 4.32 million tons for savings of 38.9 million tons. The US per capita water consumption is around 1,750 cubic meters. 1 cubic meter of water is 1000 liters or 1000 kilos. Thus 38.9 million tons or 38.9 million cubic meters / 1750 cu. m per American, results in water for 22,228 Americans.