ORDBEND, Calif. — In America’s fruit and nut basket, water is now the most precious crop of all.

It explains why, amid a historic drought parching much of the American West, a grower of premium sushi rice has concluded that it makes better business sense to sell the water he would have used to grow rice than to actually grow rice. Or why a melon farmer has left a third of his fields fallow. Or why a large landholder farther south is thinking of planting a solar array on his fields rather than the thirsty almonds that delivered steady profit for years.

“You want to sit there and say, ‘We want to monetize the water?’ No, we don’t,” said Seth Fiack, a rice grower here in Ordbend, on the banks of the Sacramento River, who this year sowed virtually no rice and instead sold his unused water for desperate farmers farther south. “It’s not what we prefer to do, but it’s what we kind of need to, have to.”

These are among the signs of a huge transformation up and down California’s Central Valley, the country’s most lucrative agricultural belt, as it confronts both an exceptional drought and the consequences of years of pumping far too much water out of its aquifers. Across the state, reservoir levels are dropping and electric grids are at risk if hydroelectric dams don’t get enough water to produce power.

Climate change is supercharging the scarcity. Rising temperatures dry out the soil, which in turn can worsen heat waves. This week, temperatures in parts of California and the Pacific Northwest have been shattering records.

By 2040, the San Joaquin Valley is projected to lose at least 535,000 acres of agricultural production. That’s more than a tenth of the area farmed.

And if the drought perseveres and no new water can be found, nearly double that amount of land is projected to go idle, with potentially dire consequences for the nation’s food supply. California’s $50 billion agricultural sector supplies two-thirds of the country’s fruits and nuts and more than a third of America’s vegetables — the tomatoes, pistachios, grapes and strawberries that line grocery store shelves from coast to coast.

Glimpses of that future are evident now. Vast stretches of land are fallow because there’s no water. New calculations are being made about what crops to grow, how much, where. Millions of dollars are being spent on replenishing the aquifer that has been depleted for so long.

“Each time we have a drought you’re seeing a little glimpse into what will happen more frequently in our climate future,” said Morgan Levy, a professor specializing in water science and policy at the University of California, San Diego.

For Rice Farmers, a Tricky Decision

California’s fertile Central Valley begins in the north, where the water begins. In normal times, winter rain and spring snowmelt swell the Sacramento River, nourishing one of the country’s most important rice belts. On an average year, growers around the Sacramento River produce 500,000 acres of sticky, medium-grain rice vital to sushi. Some 40 percent is exported to Asia.

But these are not normal times. There’s less snowpack, and, this year, much less water in the reservoirs and rivers that ultimately irrigate fields, provide spawning places for fish and supply drinking water for 39 million Californians.

That crisis presents rice farmers in the Sacramento Valley, which forms the northern part of the Central Valley, with a tricky choice: Should they plant rice with what water they have, or save themselves the toil and stress and sell their water instead?

Mr. Fiack, a second-generation rice farmer, chose to sell almost all of it.