What's a "food desert"? Hint: It's not something you eat when you've finished your dinner.

Imagine, if you will, a life where shopping for groceries entails paying a visit to a gas station convenience store. For a significant number of Americans who live in "food deserts" - areas of the industrialized world where healthy, affordable food is difficult to obtain* - this is an unfortunate reality. I've never lived in an area without easy access to multiple supermarkets and grocery stores, and I didn't think anyone in America did. Certainly not in my home state of California!

This article put some things in perspective for me.

Nearly 1 million Californians live in "food deserts" where there is no nearby supermarket or large grocery store, according to data released this week by the U.S. Department of Agriculture.

Displayed in an interactive map, the data shows that nearly 13.5 million people – 46 percent of whom are classified as low-income – live in food deserts nationwide. Nearly 45 percent of the 976,467 Californians with low access to retail food outlets are low-income.

"The data is a key piece in how we get from the problem to the solution," said Judith Bell, president of PolicyLink, an economic and social equity research and advocacy group. "It will be a great resource for community leaders, for supermarket operators, for policymakers, to understand this incredibly significant and long-term problem."

Still, the information is limited because it's based on 2000 census data and a list of supermarkets and large grocery stores compiled in 2006, she said. The USDA plans to update its map with 2010 census data next year, she said.

"We know there are some places that are food deserts – that if you were actually walking through the blocks or walking through the community, you'd recognize it as a food desert," Bell said. "But that would not show up on the map."

To locate food deserts, the USDA evaluated population and income data in census tracts – relatively small areas within a county – and locations of supermarkets and large grocery stores.

In general, census tracts were identified as low-income if they had a poverty rate of at least 20 percent. People in those areas were considered low-income if their annual household income was 200 percent or less of the federal poverty level – $34,100 for a family of four in 2000. They had low access to a supermarket or large grocery store if they lived more than one mile away in an urban area or more than 10 miles away in a rural area.

Under these definitions, about 10 percent of the 65,000 census tracts in the U.S. contain food deserts.

People in these food deserts may travel outside their neighborhoods to find fresh and perishable foods, or they may end up paying more for the same products at smaller grocery or convenience stores, according to the USDA.

Food deserts can be found in 371 California census tracts, according to USDA data. The vast majority – 85 percent – are in urban areas. More than 1.9 million people live in these census tracts, and about half of them have low access to retail food stores.

Yet in 70 census tracts in the state, the entire population – more than 300,000 people all together – do not live near a supermarket or large grocery store.

"There are a lot of locations across the state of California where a grocery store would be viable," Bell said. "And then there are some places, in particular in rural communities, because this is a rural and an urban problem, where a full-scale grocery store could not be successful. In those communities ... we need to look for alternatives."

A bill by Assembly Speaker John Perez that would expand access to healthy foods in underserved communities calls for ending food deserts in seven years. AB 581, the California Healthy Food Financing Initiative, is pending.

The next time you find yourself looking for a parking space at your local supermarket, bemoaning the crowds that showed up to buy groceries at the exact same time you did, and cursing the shopper who got into the express lane with more items than the store allows, be thankful that you have the opportunity. It sounds like many people wish they could be in your shoes.

* Definition courtesy of Wikipedia