The problem with whatever crop is getting exported is how much water is in it. So a Californian farmer grows a watermelon that when ripe contains, say, 3 liter

The problem with whatever crop is getting exported is how much water is in it. So a Californian farmer grows a watermelon that when ripe contains, say, 3 liters of water in it. If that watermelon is then sold to China, 3 liters of water have been removed from California. If it's instead sold in Fresno, then that water remains in the California system.