SACRAMENNTO (AP) — California’s now 4-year-old drought will cost state agriculture $1.84 billion in 2015, researchers estimated in a study Tuesday from the University of California at Davis.

The biggest chunk of that cost will come from the fallowing of 542,000 acres that lack water for irrigation, the study said. That’s about one-fifth more land than drought forced out of production last year, researchers noted.

Agriculture, water and economic experts at the university stressed the extent to which farmers in California — the country’s leading agriculture state — are relying on groundwater pumping to make up for dwindling stores of water in state rivers, creeks, reservoirs and snowpack.

Overall in 2015, farmers have nearly 9 million fewer acre feet of surface water for irrigation, out of the 28 million acre feet that state water officials say California agriculture uses in an average year. An acre foot is the amount an average California household uses in a year, and it is one of the standard units of measurement for water.

To make up for that, farmers and ranchers are pumping an additional 6 million acre feet of water for irrigation out of the state’s underground water aquifers this year, Tuesday’s study said. The study adds to findings — from sources ranging from overbooked drillers of water wells to groundwater studies by NASA scientists — that California, in drought, is pumping up its groundwater at an alarming rate.

The study calls the rate of pumping of groundwater in the drought unprecedented. While California lawmakers in 2014 passed the state’s first legislation to try to protect key aquifers from getting pumped dry of useable water, the state’s 27-year timeline for bringing groundwater pumping under regulation is likely too long, the University of California at Davis researchers said.

The drought will hit farm workers as well as farm owners in 2015, costing 10,100 seasonal farm jobs, the study said. Agriculture overall employs more than 400,000 workers in California.

The study noted one area of agriculture that is booming despite the drought. The state’s acreage of almonds and walnuts has grown by 200,000 since 2010, despite constraints on water, the study said. Economists say growing demand from consumers in China for nuts as snack food is driving the almond-orchard boom here.

Agriculture consumes about 80 percent of all water from rivers, lakes and other sources that Californians use, and it accounts for about 2 percent of the state’s economy.