Much of the discussion in this section relies on Chapter 2.1 of the 2006 edition of the Agricultural Resources and Environmental Indicators report prepared by USDA (2006), authored by Noel Gollehon and William Quinby. The cost per hectare for groundwater varies also with the volume of groundwater used during the season. These variables give rise to substantial variation in both the cost per m3 and the cost per hectare in different regions of the United States. As noted by Gollehon and Quinby (2006), nearly half of the farms in the country, representing more than 13 million hectares, use some groundwater.