Treasure Valley growth projections worry water managers

Sean Ellis/Capital Press Water flows through an irrigation ditch near Nampa in Idaho's Treasure Valley area last July. A new study projects water demand in the valley is poised to explode in the coming decades.

BOISE — A new analysis prepared for the Idaho Water Resource Board shows demand for water in the Treasure Valley is projected to increase substantially in the coming decades.

The projections are based on current population estimates for the valley in southwestern Idaho, the state’s main population center.

Brian Patton, chief of the IWRB‘s planning bureau, said the study is intended to help plan for that growth so it doesn’t impact senior water rights, which in this area means agriculture.

“We in no way want to impact the water supplies of those irrigation districts to accommodate the growth,” he told Capital Press. “We have to figure out how to bridge this gap without affecting those senior water rights.”

The study was commissioned by the board and prepared by SPF Water Engineering. Current estimates by cities and counties show the valley’s population growing from 600,000 people today to 1.57 million in 50 years.

Based on that growth, current domestic, commercial, municipal and industrial water demand is projected to increase from 110,000 acre-feet per year now to between 270,000 to 390,000 acre-feet. That’s a growth in water demand of between 245 and 357 percent.

Based on those projections, even looking out 10 or 15 years, water demand in the valley would grow by 50 to 70 percent, which is a concern to irrigation districts and water supply managers.

Those water demand growth projections don’t include agriculture, which is by far the biggest water user in the Treasure Valley.

Tim Page, manager of the Boise Project Board of Control, which provides water to five irrigation districts and 167,000 acres, said his main question is, “Where’s that water going to come from? Where do they intend to get it from?”

The study says it will be important to increase water supplies in the valley through a combination of measures, including conservation, increased groundwater pumping, aquifer recharge, more diversions from the Boise River, reusing treated water from sewage plants, increasing reservoir storage capacity and possibly diverting water from the Snake River.

Clinton Pline, a farmer and chairman of the Nampa and Meridian Irrigation District’s board of directors, said accommodating the new growth “will take conservation measures from all who live here. But, more storage on the Boise River system will be necessary if we are going to grow to the extent projected in the (study).”

Patton said additional water storage is one of the most promising options and there are parts of the valley that can withstand additional groundwater pumping.

“Conservation certainly has a part to play in this and there might be some things we haven’t thought of yet,” he said. “I think what will happen is there will be a mix and match of all of those things.”

He said the study will help the state get ahead of the challenge.

“We want to plan for this so the growth can occur without impacting senior water rights, which is agriculture by and large in the Treasure Valley,” Patton said.

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