Once a decade, the Central Arizona Groundwater Replenishment District must decide how much water it must put back into the aquifer to offset the groundwater its members pump.

State law also requires this plan of operation to spell out how the district plans on acquiring that water, where it plans to store it underground and how it plans to finance all of this.

The stakes are high to get that foundational water number — called a “replenishment obligation” — right. Because the district is on the hook to replenish this pumping from existing development for the long haul.

If it overestimates, it could buy more supplies than needed — which isn’t cheap — jacking up costs for thousands of homeowners.

But if it underestimates — particularly given how difficult it is to find the renewable water sources required to replenish this pumping — it could easily fall behind on meeting its obligation.

And that could spell big trouble for communities on the outskirts of metro Phoenix, Pinal County and Tucson that rely on the district to “pay back” a major chunk of what they pump.

After months of work — and several pauses to see how proposed........

© Arizona Republic