A recent article in the New York Times about declining water levels in deep aquifers in the western US got me thinking about the demand for corn to produce ethanol, and the resultant impacts on irrigation water. The Times piece (http://www.nytimes.com/2013/05/20/us/high-plains-aquifer-dwindles-hurting-farmers.html?ref=science) describes the impact of declining water levels on farmers who use center sweep irrigation to grow corn, wheat, milo, and other crops. The recent drought has increased the demand for groundwater, which in the High Plains comes from deep-seated aquifers that recharge very slowly. In essence, the water is mined, because rates of withdrawal well exceed the rates of recharge.
Have increased prices for corn, driven by expansion of the ethanol requirements for US gasoline, caused increases in corn planting on marginal land that requires irrigation? Data on this is limited, although it is clear that demand for corn from the non-ethanol market has remained flat, and there has been a 10% increase in corn acreage planted over the last decade due to ethanol expansion (http://www.agri-pulse.com/ERS_cropland_8192011.asp). Global demand for corn from the ethanol market is expected to grow by 40% through 2022 (http://ethanolproducer.com/articles/9546/usda-global-ethanol-production-to-grow-40-percent-through-2022).
Growing one bushel of corn with total irrigation requires 2500 gallons of water though the growing season (http://www.ars.usda.gov/is/AR/archive/aug11/water0811.htm). Now, there are very few areas where all the water to grow corn comes from irrigation - natural rainfall and soil moisture provides all the water in ~85% of corn grown. However, 2500 gallons per bushel is a good end-member point for our calculations. One bushel of corn produces ~2.8 gallons of ethanol. To compare with natural gas, 1000 cubic feet of natural gas (1 MCF) is the energy equivalent of 12 gallons of ethanol.
Now, to put this in another frame, one hydraulically fractured Marcellus shale gas well can be expected to produce between 1.4 and 2.1 BCF (billion cubic feet) of natural gas (http://marcelluscoalition.org/wp-content/uploads/2013/04/es305162w.pdf). Taking the median at 1.8 BCF, or 1.8 million MCF, a single average Marcellus well will produce energy equivalent to 21.6 million gallons of ethanol. Using the end-member circumstance for irrigation, the corn that would produce 21.6 million gallons of ethanol would require 19 billion gallons of water. A single Marcellus hydraulically fractured well requires ~5 million gallons of water (http://www.chk.com/media/educational-library/fact-sheets/marcellus/marcellus_water_use_fact_sheet.pdf) to produce that energy equivalent.
Obviously this is a simple back-of-the-envelope calculation that leaves out all sorts of other elements in both energy cycles - for the corn - gas for the tractors to till the fields, energy used by irrigation pumps, etc, and for the natural gas - the energy used by the drill rig, trucks, etc. There are other caveats to consider, as well. The water used for corn irrigation is returned to the atmosphere by evaporation, but leaves behind salt in the soil, of course. On the other hand, most water used in hydraulic fracturing remains in the subsurface, and what does come back is salt-laden.
However, the take-away point is that water use for irrigation of corn to produce ethanol is ill-spent. As long as ethanol use in motor fuel is mandated, demand for corn will increase, as will the need for irrigation. The water will not be found in the High Plains Aquifer.
(image from the NY Times article referenced above)