(Bloomberg) - California’s worsening drought is raising the stakes for a $15 billion plan endorsed by Governor Jerry Brown to build two 30-mile (48-kilometer) water tunnels under an ecologically sensitive river delta east of San Francisco Bay.

The tunnels, each as wide as a two-lane interstate highway, would ship water more reliably from northern California to thirsty farms and cities in the south. They would also bolster the ecosystem of the Sacramento-San Joaquin River Delta, which is on the verge of collapse from feeding water to 25 million people and 750,000 acres (304,000 hectares) of farmland.

The drought, which officials say could be one of the worst in California’s history, is forcing farmers in the fertile central valley region to fallow thousands of acres of fields and has left 17 rural towns so low on drinking water that the state may need to start trucking in supplies. The tunnels are the biggest part of a $25 billion Bay Delta Conservation Plan.

Reservoirs are at about 60 percent of average, according to state water data, and falling as rainfall remains at record low levels. Mountain snowpack is about 12 percent of normal for this time of year. Brown is urging the state’s 38 million residents to conserve and warning that mandatory restrictions are possible.

“This isn’t a coming crisis,” Mark Cowin, director of the Water Resources Department, the state’s largest supplier, said last week. “This isn’t an evolving crisis. This is a current crisis.”

Biggest Rain
The San Francisco Bay area got the biggest rainfall of 2014 yesterday with 0.85 inch (2.2 centimeters) at San Francisco International Airport as of 4 p.m. local time, according to the National Weather Service. More precipitation is possible later in the week. Forecasters said it would take far more rain to end the drought.