Would it be distance of farmland from the city, or the quantity of farmland within reasonable proximity of the city that would dictate this? I'm assuming that there are ways that you can do calculations for a particular city using consumption statistics and GIS mapping, and agricultural yields that would give a sense of how locally sustainable a given region is. I have not been able to find such data however, either in map form or tabular form. Does anyone have some ideas about where such data could be found or further ideas on how it could be generated?