In Arizona, activists and the federal government are fighting plans to tap groundwater used by a vast housing development — a move that would reduce the water level of a protected river. Kansas accuses Colorado and Nebraska of allowing their farmers to divert Kansas’ share of the Republican River, which flows through all three states. A similar dispute between New Mexico and Texas is before the United States Supreme Court.

California, in the midst of a major drought, so far has witnessed but a few local skirmishes. In January, environmentalists and sport fishermen sued to halt the drilling of hundreds of new groundwater wells sought by Central Valley farmers, saying more pumping would lower stream levels.

That may not last long, said Stuart Somach, a Sacramento water-rights lawyer. California farmers have long grumbled about big-city designs on their water; Northern California has long grumbled about being the spigot that supplies most of the water to the dry south.

“We’re very close to the time that people are going to start staking out rights. We’re right at the cusp,” Mr. Somach said. “If this drought persists, depending on how state and federal agencies react, you’re going to get some real conflicts going.”

Image Pipes usually used for watering. Credit... Ilana Panich-Linsman for The New York Times

Actually, the laws that govern most of the West’s water seem tailor-made for fighting.

In many places, the rules for owning or using groundwater are still in flux: In Texas, landowners own the groundwater beneath their property, but a neighbor pumping groundwater from the same aquifer can siphon it away without penalty. The Arizona court battle over a proposed housing development hinges on the still-murky question of whether the state can allow the builder to pump groundwater that sustains a river that is under federal control.