Some places in the U.S. are already struggling with groundwater depletion, such as California, Arizona, Nebraska and other parts of the central Plains. … [Jonathan Winter, an associate professor of geography at Dartmouth College and an author on a new study on future U.S. irrigation costs and benefits] used a computer model to look at how heat and drought might affect crop production by the middle and end of this century, given multiple scenarios for the emissions of warming greenhouse gases. In places like California and Texas where “everyone is dropping their straw into the glass” of groundwater, as Winter put it, current levels of irrigation won’t be viable in the long term because there isn’t enough water. But use of irrigation may grow where groundwater supply isn’t presently an issue.