It was an interesting question for a third date, and the first thing my (now ex-) boyfriend and I disagreed on.  I said “cheap oil” and he answered “water.”

Our difference of opinion was largely a product of our upbringings.  I was raised on thunderstorms and flooded basements back East, while he grew up amid droughts and “water wars” in California.

Months after we parted ways, I was reminded of that conversation as I drove through California’s Central Valley, past billboards plastered with water propaganda and tractors shadowed by dust clouds.

Few issues polarize the Great State more than water supply.  The most apparently wasteful demand is concentrated in the dry south, in hydrologic black holes like the Los Angeles basin, where 28 million people sprinkle lawns and fill pools in an area that, in isolation, could provide liquid sustenance for only about one million residents.  Still, two thirds of the state’s freshwater demand is agricultural; after consecutive years of drought, farmers have watched crops wither and orchards die.  This year, even with the Sierra snowpack at 136 percent of normal, the state anticipates meeting only 60 percent of the growing demand.  Supplements will doubtless come from belowground reservoirs, pumped to the surface by hundreds of unregulated wells that mine a precious resource faster than it is being replenished.

Yes, despite the apparent abundance of rain that broke our string of sunny January days on Sunday, our freshwater supply is, at least in part, a nonrenewable resource.  This is true across a disturbingly large fraction of the country — and the globe — where freshwater supplies are being consumed (and contaminated) far faster than the planet’s natural hydrological cycles can restore them.  And with climate change shifting precipitation patterns and converting ice caps and snowpacks into rising sea levels, supplying a growing world population will only become more challenging.

How much water does one person need?

Anyone who actually drinks the recommended two liters per day consumes only 730 liters (less than 200 gallons) of water directly each year.  But the average American’s water footprint is 2.5 million liters per year — about the volume of an Olympic swimming pool.  By comparison, the global average is 1.2 million liters, but one third of humanity survives under “scarce” conditions — less than one million liters per year. That’s because water goes into the production of almost everything: from growing food crops to cotton for clothes, from cooling manufacturing equipment to hosing down thirsty livestock.  The jeans you’re wearing consumed nearly 1,100 liters in production, the equivalent of nine cups of coffee at 140 liters each, but less than half the volume of a 2,400 liter hamburger.  As much as cheap oil provides hidden subsidies for everything we do, an invisible weight of water is folded into the very clothes we wear.

These water subsidies translate into dramatic trade fluxes.  When the United States exports a ton of wheat, it also transfers 1.3 million liters of freshwater used to grow and process the crop.  Globally, 1.6 quadrillion (that’s 10¹⁵!) liters cross international boundaries each year, a staggering volume (equivalent to 10 Lake Tahoes) that decouples local populations from their resource bases.

Trading this “virtual water” allows us to economize on water use: why turn desert into cropland when one can import food grown more efficiently elsewhere?  But it also results in a dangerous disconnect between supply and demand, letting wealthy countries export their water footprints and attendant environmental degradation.  It’s hard enough to worry about sustainability and environmental degradation in our own backyards. Here in California, where only 10 percent of our original wetlands remain, we continue to divert more and more water for human consumption. Are we willing to pay a premium and reduce demand to preserve the resources of a country half a world away?

If your answer is “yes,” there are a few ways you can reduce your water footprint.  Low-flow taps and showerheads — especially ones that provide continuous feedback on water usage — have proven consumption-reduction records.  And although most of us don’t grow our own food, we can patronize growers who use drip irrigation (which minimizes wasteful evaporation) and couple crop choices to local hydrology.  And, unsurprisingly, at 5,000 liters per day, an omnivorous diet uses more than double the water volume of a vegetarian lifestyle.  Plus, choosing low-water options tends to reduce your carbon footprint, too.  Because, in all likelihood, the next World War will have more than one cause.

(This column was originally printed in the Stanford Daily on Thursday, February 3, 2011.)