In Fresno, Trump said he'd spent 30 minutes before his rally meeting with more than 50 farmers who complained to him about their struggles.
"They don't understand — nobody understands it," he said, declaring at one point: "There is no drought. They turn the water out into the ocean."
Trump appeared to be referring to disputes over water that runs from the Sacramento River to the San Francisco Bay and then to the ocean. Some farmers want more of that flow captured and diverted to them.
Politically influential rural water districts and well-off corporate farmers in and around California's Central Valley have been pushing back against longstanding federal laws protecting endangered fish and other species, saying federal efforts to make sure endangered native fish have enough water is short-changing farmers of the water they want and need for crops.
Water authorities say they can't do it because of the water rights of those upstream of the farmers, and because of the minimum-water allowances needed by endangered species in the bay and by wildlife in general.
The three-inch Delta smelt is a native California fish on the brink of extinction. The smelt has become an emblem in the state's battles over environmental laws and water distribution.
The farm lobby, a heavyweight player in California's water wars, also is seeking federal and state approval for billions of dollars in new water tunnels, dams and other projects.
Trump promised that, if he's elected, he would put their interests first. "If I win, believe me, we're going to start opening up the water so that you can have your farmers survive," he said.
California is the country's No. 1 agriculture producer. The state's drought is raising the stakes in water disputes among farmers, cities and towns, and environmental interests.
A count by The Associated Press found Thursday that Trump has reached the required number of delegates to officially clinch the Republican nomination.