In my reading and research of this question, I came up with two methods of calculating: the carbon footprint and the water footprint.
A carbon footprint is "the total set of greenhouse gas (GHG) emissions caused by an organization, event, product or person". Several organizations have calculated carbon footprints of products. The US Environmental Protection Agency has addressed paper, plastic (candy wrappers), glass, cans, computers, carpet and tires. Australia has addressed lumber and other building materials. Academics in Australia, Korea and the US have addressed paved roads. Companies, nonprofits and academics have addressed manufacture and operation of cars, buses, trains, airplanes, ships and pipelines. The US Postal Service has addressed mailing letters and packages.
The water footprint is an indicator of water use that includes both direct and indirect water use of a consumer or producer. The water footprint of an individual, community or business is defined as the total volume of freshwater that is used to produce the goods and services consumed by the individual or community or produced by the business. Water use is measured in water volume consumed (evaporated) and/or polluted per unit of time. A water footprint can be calculated for any well-defined group of consumers (e.g. an individual, family, village, city, province, state or nation) or producers (e.g. a public organization, private enterprise or economic sector). The water footprint is a geographically explicit indicator, not only showing volumes of water use and pollution, but also the locations.
I would have to say that I trust the figures obtained from the Carbon footprint more than I do the figures from the water footprint.
The water footprint does not provide information on how the embedded water is contributing to water stress or environmental impacts. Basically, the water footprint concept includes sums of water quantities without necessarily evaluating related impacts.
Additionally, I would add that the water necessary to produce crops in a humid area of the world would be less than that required to produce crops in an arid part of the world. Also, some countries have the technology to recycle industrial waste water back into the water cycle, whereas underdeveloped countries do not. This could further affect the computation of a worldwide figure.
The carbon footprint method overall is the more accurate method because
carbon emissions are not simply summarized but normalized by CO2 emissions, which are globally identical, to account for the environmental harm.