The USDA / NASS studies tracked harvested acres without differentiating between irrigated and non-irrigated acreage; it gathered data on planted vs. harvested acres for some crops but not others; it
did not account for systems in which «baby vegetable» crops (usually
organic) are grown in short rotations on the same plot (such as spinach,
lettuce, and carrots) and thus have lower yields; and it omitted some data that would have revealed too much information about individual farmers, in cases where very few growers produce a particular crop.