Last week, the Idaho Department of Water Resources issued a curtailment of groundwater rights affecting 100,000 acres in the Bingham Groundwater District. The order also affects other counties and water districts throughout eastern Idaho.

Earlier in May, the Idaho Department of Water Resources, or IDWR, rejected nine eastern water districts’ mitigation plans as deficient, even though they adequately planned to replace the water they use from the aquifer.

IDWR Director Mathew Weaver reportedly decided to revert to an earlier water agreement and interpreted the language to require the local water districts to replace all water used within their location, even for those that are not members of the district. In short he is requiring the districts to pay their own water bill and that of their neighbors, according to the Bingham Groundwater District.

The water districts were following the most recent water agreement enacted in 2012, which requires each district to replace the ground and surface water used from the aquifer — to pay their own water bill.

Because the IDWR has decided that the Twin Falls Canal Co. won’t have sufficient water come August, it is trying to maximize the amount of recharge by the districts it has leverage with.

First, why is the Twin Falls Canal Co. so favored? Because it has senior water rights, meaning it applied for water rights before local farmers who have junior water rights. The rule of the water world is “First come, first served.”

Second, why does the IDWR think there will be a shortfall this year? A year with reservoirs currently full, an above average snowpack still melting, and so much water that the Bureau of Reclamation is having to flush out 600,000 acre-feet of water to prevent flooding. Because it changed the way to calculate the water forecast to make shortfalls larger and more common. From these calculations it needs to dry up 500,000 acres of private farmland in eastern Idaho. Bingham County is required to supply a fifth of those acres.

The question is why did it make that calculation change, which would favor the Twin Falls Canal Co.?

“The department has acted more like an advocacy group than an administrative agency,” said Bingham Groundwater District Board Chair Connie Christensen. That’s one opinion. Who it might be advocating for specifically is yet another question.

Idaho Groundwater Appropriators’ attorney Thomas J. Budge suggests that IDWR is curtailing water to the district as a whole because it doesn’t want to have to directly curtail water from anyone specifically.

Third question is why is there a concern about water? It is tempting to look at the Snake and Blackfoot rivers and assume there is enough water.

The reality is that groundwater and the surface water flowing in the rivers are largely fed by the massive 10,800 square mile Eastern Snake Plain Aquifer, which is a basalt and lava rock underground sponge that ranges about 4,000 feet below the surface that stores water.

The total storage capacity of the aquifer is 200 million to 300 million acre-feet or roughly equal to Lake Erie, according to Josh Johnson of the Idaho Conservation League. One acre-foot of water is about 325,000 gallons of water. This means the aquifer has between 65 million and 97.5 million gallons of water.

However, it is the top 300 to 500 feet of broken basalt that allows the water to flow in and out so easily. Between the size and the broken basalt top layers, the aquifer is one of the largest and most productive aquifers worldwide.

Again, why the concern? A very brief history sketch will help here.

Idaho was settled by farmers that built canals so they could flood irrigate their fields. This practice returned irrigation water to the aquifer through the groundwater system. Net effect on the aquifer — none.

Two things happened to change that. In the 1950s, technology allowed wells to pump deeper than 20 feet to 32 feet down. Now, the Earth’s core is the limit and wells have opened up more farmlands further east from the rivers that previously couldn’t be reached by canals.

Incidentally, these new eastern ag lands now being farmed have more sandy soils — perfect for the perfect potato. The world’s potato capital was born.

Then in the 1980s, farmers began using pivots instead of flood irrigating. With flood irrigating, the plants used only 30 percent to 60 percent of the water. However, with pivots the plants utilized 85 percent to 95 percent of the water. Pivots were less labor intensive and yielded “more crop per drop.” The unintended consequence, however, was far less water was returned to the aquifer.

Using well data and surface measurements, IDWR determined that on average the aquifer needs about 250,000 square acre-feet of water returned to it each year. This is done through aquifer recharge sites such as the 400 acre one at Egin Lakes in Fremont County. In the early spring, the Fremont-Madison Irrigation District diverts canal water into the recharge site and lets it seep down to replenish the sponge-storage underground — making sure all senior and junior water users have enough to last through harvest.

Why the focus on ag? What about the 300,000 people also using the aquifer? Reality is that 97 percent of all groundwater goes for farming and only 3 percent for domestic uses, including commercial, municipal and industrial uses.

As for Blackfoot, Mayor Marc Carroll said the city is part of the Coalition of Cities Water Mitigation Plan, which has been accepted by the IDWR. Blackfoot does not have to curtail water.

Even so, Blackfoot is planning to conserve water this summer with an odd and even day ordinance for domestic outdoor watering. Those Blackfoot residents who have an odd-numbered address will water on odd-numbered days and even numbers on even days.

Bingham County, however, will be greatly affected.

The Bingham Groundwater District estimated the curtailment could cost the local economy “hundreds of millions of dollars” to lose 100,000 acres — which will affect about 70 percent of district members.

With that kind of loss looming, the whole area — including cities — will be affected.

Accordingly the Bingham County Commissioners have invited Carroll to join their discussion of the curtailment order.

With so many questions about how the shortfall is figured and the legality of requiring payment for non-paying farmers contrary to the most recent agreements, this looks like it could end up in the courts.