Water and Energy: Transportation by Automobile

I am still on "vacation" from blogging, but I felt compelled to post the following.

Water and energy are strongly linked. Energy production takes water, a great deal of water. Transportation fuels are to water as food is to water. Water is the basic requirement for life and water is a basic requirement for fuel production for transportation. Water and energy are linked and this fact is inescapable.

But, how much water is required for various forms of fuel for transportation?

First, we define some nomenclature and units used in the calculation. One way is to determine how many gallons of water are required to produce a gallon of transportation fuel (fuel hereafter). Because the use of fuel varies by the type of vehicles being used and fuel efficiency in miles per gallon of fuel varies widely, we will use a term to relate water and fuel efficiency directly. This term is gallons of water required per vehicle mile travelled. This term, gallons of water per mile travelled (abbreviated as gal H2O/mile) then spans the various fuels and the various vehicle fuel efficiency if we use an average fuel efficiency in miles/gal. Much of this material is from a paper by King and Webber (2008). "Water Intensity of Transportation", Environmental Science and Technology, Vol. 42, No. 21, pp. 7866-7872. source.

Let's Consider Some Numbers

If we assume an average fuel efficiency of a mix of cars, light trucks, and SUV's with an average efficiency of 20.5 miles/gallon, then we can compare the water use per mile (gal H2O/mile) of various fuels. The following are averages from the paper cited above.
1. Petroleum based gasoline and diesel fuels: 0.105 gal H2O/mile for gasoline and 0.08 gal H2O/mile for diesel.
2. Plug-in electric cars using the power grid: 0.37 gal H2O/mile (about 3.5 times as much as using gasoline).
3. Ethanol E85 (85% ethanol and 15% gasoline) from irrigated crops: 28 gal H2O/mile (about 267 times as much as gasoline).
4. Diesel fuel derived from irrigated soybeans: 8 gal H2O/mile (about 100 times as much as from petroleum based diesel).

These statistics mean that plug-in electric vehicles use about 3.5 times as much water per mile driven as conventional gasoline fueled vehicles. In addition, remember, electric power from the grid is highly dependent on electricity generated by coal power plants and nuclear power plants. Biofuels (ethanol (E85) and diesel) use about 100 to 300 times as much water per mile driven, as do conventionally powered gasoline vehicles.

Significance of Higher Water Use with Alternative Transportation Energy Sources

Supplies of traditional gasoline and diesel fuel can be obtained by expanded domestic exploration and drilling. Obtaining huge increases in available water (to use alternative transportation fuels) is much more difficult (or impossible) and it is expensive.

Quite simply, we can get more energy (oil, coal, nuclear power) for transportation, we know how, we know where, and it is economically viable. Water is another matter. Fresh water is a finite, and tightly limited resource. Water conservation will help, but we cannot conserve more that we use. It is not a matter of just pumping more groundwater (supplies are generally over used and groundwater levels are declining) or building more dams. Surface waters are already over allocated (more people have claims to the water than the amount that exists) and some dams are being removed based on questionable environmental reasoning. Desalination requires huge amounts of energy and where will we get the additional energy? Reclaimed sewer water can be used but most of it is already seen as a resource and supplies are limited. All potential sources of additional water are limited, infeasible, or impossible when we are considering increased water demand for transportation of approximately 3 to 300 fold.

Is it Right?

Every acre of cropland converted from food crops to biofuel production results in a reduction of food for people. While you as an individual may not be hungry, there are millions of Americans who are and billions more around the world.

Is shifting agriculture from food production to biofuel production wise? Is using food for transportation just? Is this a waste of food? In addition, is it a sin to waste food?