Scientific American: Recently the United Nations warned that the world could suffer a 40 percent shortfall in water by 2030 unless countries dramatically cut consumption. Since 70 percent of the worlds fresh water goes to agriculture, this means changing the way people farm. The need is ubiquitous. In Californias Central Valley, farmers drilling for water are now tapping stores 30,000 years old. In Kenya, which is facing the worst drought since 2000, farmers are hand-digging wells to reach the receding water table,...