Almonds, walnuts, and pistachios are making bank in the Golden State—but the groundwater supply is paying the price. California has entered the age of King Nut: The state produces more than 80 percent of the world’s almonds, and roughly 30 and 40 percent of the world’s pistachios and walnuts, respectively. Most of the production takes place in the Central Valley, a swath of farmland in California’s midsection. A single almond requires 3.8 litres of water to grow—bad news in the midst of…