California is having the wrong debate about the future of one of its most valuable assets, the Sacramento-San Joaquin River Delta, which produces water for much of the state and about half of Silicon Valley.

The battle for the better part of the last two years has been about how big a new conveyance system — probably tunnels — should be, how much it should cost, and who should foot the bill. The result has been a political fight of the worst kind, pitting Northern Californians against Southern Californians and agriculture interests against environmentalists in a battle royal. At its worst, this could be one of the biggest water grabs in state history. And for California, that’s saying something.

The focus instead should be the operating conditions for the Delta, particularly the amount of water that needs to flow through it annually to maintain the health of the estuary. Once that standard has been established, then everything else will fall into place for the two coequal goals of providing a more reliable water supply and protecting, restoring and improving the Delta ecosystem. Before deciding on tunnels, helicopters, whatever, state water officials need to determine how much water can be expected to be delivered. That will drive how it’s allocated and how it should be conveyed.

Why has the debate focused on the conveyance system instead of this basic question? Follow the money.

Central Valley agriculture interests and Southern California water districts are footing the bill for the Bay Delta Conservation Plan, basically the tunnel plan that is backed by Gov. Jerry Brown. The farmers and water users would pay for most of the $25 billion project, so they want to guarantee as large a reliable water supply as possible for their customers. The bigger the conveyance system, the more water it can carry. Hence the call for two 30-mile tunnels capable of supplying nearly 10 million homes with water, even though we don’t yet know what taking that much water away will do to the Delta.

The plan’s backers aren’t interested in lesser alternatives, including the single-tunnel “Portfolio Alternative” backed by several members of Northern California’s congressional delegation, including Reps. Ami Bera, D-Elk Grove; John Garamendi, D-Walnut Grove; Doris Matsui, D-Sacramento; Jerry McNerney, D-Stockton; George Miller, D-Martinez and Mike Thompson, D-Napa. Their solution would carry about a third as much water as the twin-tunnel plan.

The state Department of Water Resources suffered a major embarrassment in the water world on Nov. 12 when it admitted that as part of the BDCP process, it had overestimated the cost of that single-tunnel project by a whopping $3 billion. This cut the estimate by a quarter to $8.6 billion, a much greater saving from the $14.5 billion estimated for the two huge tunnels.

The admission calls into question the accuracy of the entire BDCP proposal, which has raised serious questions from economists and experienced California water experts about the cost benefits every step of the way. The people working on the BDCP are advocates of massive diversions of water. Have other figures been misstated to make this plan look best?

It’s common for successful water projects to have the rules of operation in place first, so everyone understands how much water will be available, who will receive it, and who will be in charge of enforcing the rules. For all the numbers being tossed around with the Bay Delta plan, none of that is clear, and nobody seems inclined to face up to it.

That is why the vast majority of Northern Californians are scratching their heads instead of lining up in support. And unless answers are forthcoming soon, activists will need to stop bothering to ask the questions and start a head-on fight to stop what increasingly feels like the wrong plan for the Delta.