The analysis used satellite data and cropland information from the U.S. Agriculture Department to map the locations of abandoned cropland and how long it had been out of use. The researchers conclude that during the study period about 12.3 million hectares — or 30.39 million acres — of cropland went unused in the contiguous United States.

The biggest changes took place around the Ogallala Aquifer, whose groundwater irrigates parts of Colorado, Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, Texas and Wyoming, and which has been drying out because of excessive pumping and droughts. Other abandonment hot spots were located around Mississippi, the Atlantic Coast, North Dakota, northern Montana and eastern Washington state.

The fate of the former cropland varied. About half (53 percent) changed to grassland and pasture, while 18.6 percent became shrub land and forest. Other abandoned cropland became wetlands (8.4 percent) and non-vegetated lands (4.6 percent), while some of the rest was recultivated or could not be classified. The land was abandoned at an average of 0.51 million hectares — 1.26 million acres — a year.

The study did not focus on the reasons farmers stopped using the cropland. But the researchers reported that less than 20 percent of the abandoned land was enrolled in the USDA Conservation Reserve Program, which pays farmers to take out of agricultural production environmentally sensitive land that’s at risk of soil erosion, habitat loss or reductions in water quality.

That surprised researchers. “A lot of the assumptions were that this former cropland had a lot of overlap with formal conservation programs,” Tyler Lark, an assistant scientist at the University of Wisconsin-Madison’s Center for Sustainability and the Global Environment who co-authored the study, said in a news release. “But we saw that they’re almost entirely distinct pools.”

Understanding where abandoned cropland lies will allow researchers to analyze how such lands might be better used, the researchers say. “The cropland abandonment we identify here is expected to persist,” they write.