AMARGOSA VALLEY, Nev. — In a rural corner of Nevada reeling from the recession, a bit of salvation seemed to arrive last year. A German developer, Solar Millennium, announced plans to build two large solar farms here that would harness the sun to generate electricity, creating hundreds of jobs. But then things got messy. The company revealed that its preferred method of cooling the power plants would consume 1.3 billion gallons of water a year, about 20 percent of this desert valley’s available water. Now Solar Millennium finds itself in the midst of a new-age version of a Western water...