Here’s why water must be Arizona’s top public-policy debate:

Back in 1980, the Legislature finally responded to the rapid drop in groundwater levels caused by population growth and farming. It set a goal for the Phoenix, Tucson and Prescott areas to balance our groundwater withdrawals with natural and artificial recharge by 2025.

That last method — artificial recharge — is largely accomplished by importing Colorado River water 300 miles across the state as far as Tucson. Locally, various water users, led by Tucson Water, have recharged so much river water in the past few years that our area has reached the state goal. That’s great, but there’s every reason to worry it won’t last.

Tree-ring studies indicate that over a long history, the Colorado’s annual flow has been less than the amount promised to seven states with dibs on the water. Of immediate concern, this is the 15th year of a drought that has slowed the Colorado’s flow so much that Lake Powell and Lake Mead are below 50 percent capacity. If that storage keeps falling to the point that there isn’t enough water for everyone, Arizona is the big loser. By a legal agreement we struck long ago, in times of great shortage Central Arizona Project deliveries are cut before California loses any water.

It’s high time for Arizona to discuss a long-term plan that doesn’t just bank on the Colorado River and groundwater for our future water security.

That point goes doubly on both sides of the Santa Rita Mountains. On the west side, assume the Rosemont Mine pays to extend the Colorado River delivery pipe to Green Valley. That helps, until you ask, What happens if CAP deliveries are ever curtailed or stopped? Then we’ll have all of the present overdrafting users with their straws in the ground plus a new one, Rosemont.

On the east side of the mountains — where the mine would be built — groundwater use also continues without broad public discussion of the long-run implications. There were about 630 domestic and stock wells in the Cienega basin in 1980. By 2010 there were more than 1,800.

Beyond the topic of groundwater, there are also questions about Rosemont’s effect on surface-water flows from rain and snowmelt. The primary drainage near the mine is Barrel Canyon, which drains into Davidson Canyon, which discharges into Cienega Creek near Interstate 10. It’s one of few areas the state has labeled “outstanding waters” for their quality.

There is no doubt that the mine will reduce flow into Barrel and Davidson canyons – an estimated 17 percent a year. Hydrologists have done monitoring and modeling to come up with this estimate, but it’s exactly that. In a desert where most of our surface flows were lost long ago, we are down to precious few outstanding waters. The Rosemont plan should prompt Arizona to re-examine whether our protections — considering both quality and quantity together — are rigorous enough.

The Rosemont Mine may or may not have a significant impact on our water quantity and quality. What is certain is that if we continue to grant mines and certain other users a nearly guaranteed right to pump groundwater, the state will be worse off in the long run. And if we keep pretending that plenty of Colorado River water will always be available, Arizona is inviting a crisis.

We have hope that the state is ready to debate our water future. Just last week it released a report, “Arizona’s Next Century: A Strategic Vision for Water Supply Sustainability,” that clearly describes our most important public-policy challenge. It’s at www.azwater.gov.