Driving along California's freeways, it is hard to miss the electronic signs urging people to save water due to the serious drought. Rain, when it comes, is unevenly spread across the state, with the Central Valley — home to California's most productive agricultural areas — being especially vulnerable to this irregularity.

The drought is now so severe that in April Governor Jerry Brown called upon “all Californians, municipal water agencies, and anyone who uses water to do everything possible to...