The Columbia Plateau Aquifer: Groundwater Falling Under Irrigated Farmland

The Columbia Plateau Regional Aquifer System study area. USGS.

The plateau

The Columbia Plateau is a broad basalt plateau between the Cascade Range and the Rocky Mountains, in Washington, Oregon and Idaho. Much of it is semiarid — 7 to 15 inches of precipitation a year in the center — yet it supports a $6 billion a year farm economy: apples, corn, grapes, hops, mint, potatoes, stone fruit and wheat.

• Irrigation from groundwater pumping and river diversions waters about 5 percent of the Nation's irrigated land.
• Groundwater is the main source of drinking water for about 1.3 million people on the plateau.

The study

The USGS runs large regional studies of groundwater availability. This one, of the Columbia Plateau Regional Aquifer System (CPRAS), set out to give citizens, communities and resource managers:

1. better knowledge of the state of the groundwater;
2. how it has changed over time;
3. tools to forecast how it may change.

The aquifer

The system covers about 44,000 square miles of the Columbia River's drainage. Its main aquifers are in the Columbia River Basalt Group and, in places, the sediments above it.

• The basalts are lava flows that poured from fissures and vents along the Washington–Oregon–Idaho border in the Miocene, 17 million to 6 million years ago.
• More than 350 flows have been identified, each from 10 to more than 300 feet thick, with sediment interbeds where soil and sediment built up between eruptions.

Basalt bluffs at Wallula Gap, Washington, 2005. Lyn Topinka, USGS.

The aquifer system in three dimensions, looking northeast (not to scale): surface in blues and greens, basalt units in browns, arrows for approximate groundwater flow. USGS.

What was measured

Water levels. In spring 2009, the USGS and 10 other federal, state, tribal and local agencies measured water levels in 1,752 wells, to map groundwater elevations and flow directions.

• In 470 wells also measured 25 years earlier, in spring 1984, levels had fallen in 83 percent, by more than 25 feet in 29 percent.
• Levels have fallen across more than 10,000 square miles — about 23 percent of the system — where pumping exceeds recharge. The worst areas rely heavily on groundwater for irrigation: the Odessa Subarea in the central north, parts of the Yakima River basin, the Pullman–Moscow area, and parts of Oregon's Umatilla River basin.
• By contrast, about 5 percent of the system has seen levels rise, where the Bureau of Reclamation's big irrigation projects apply river water to fields.

Long-term water levels in four wells, in feet below land surface. Well B, inside the Bureau of Reclamation's Columbia Basin Project, rose about 6.2 feet a year; the others fell 1.3 to 4.0 feet a year. USGS, modified from Snyder and Haynes (2010).

Measuring the water level in a well near Yakima. Karen Payne, USGS.

A 3-D geologic model. Records from 13,226 wells defined the tops and bottoms of the subsurface units. A web tool let users draw a "well log" anywhere or a cross section between sites.

Evapotranspiration. A new Simplified Surface Energy Balance (SSEB) method uses satellite data to estimate monthly evapotranspiration — never estimated here before, and large: up to 100 percent of annual precipitation in arid areas and 45–70 percent in the wetter uplands.

Irrigation and recharge. A soil-water balance model computed monthly irrigation needs and the surplus water that recharges groundwater, from which pumping, river diversions and irrigation recharge were estimated.

The groundwater-flow model

The main product is a numerical groundwater-flow model: a mathematical picture of how water moves through the aquifers, built from aquifer properties, recharge, discharge and water levels. It can test water supply and demand, management strategies, exchanges with rivers, and climate effects.

• Grid: 126 rows by 131 columns of 3-kilometer cells.
• Layers: five hydrogeologic units split into layers about 100 feet thick — 100 layers in all, to capture vertical flow.
• Uses: annual water budgets; flow between units; change since predevelopment; what happens if 2007 conditions continue to 2050; and the effect by 2050 of more irrigation demand in a warmer climate.

Findings

• Many thin layers better represent vertical gradients and links to streams.
• Pumping has grown substantially since the 1970s–1980s, lowering deep water levels and reducing stream base flows across much of the area.
• In dry to average years, pumping causes a net loss of stored groundwater: it now exceeds recharge in all but the wettest years.

Fuji apples east of Moses Lake, Washington, 2015. Sue Kahle, USGS.

The issues ahead

Demand from farms, economic growth and ecosystems competes for limited water. Issues for future groundwater availability:

• widespread water-level declines from pumping;
• less base flow in rivers, affecting water temperature and quality;
• little surface water left to allocate;
• pumping under junior water rights capturing river water held under senior rights;
• climate change raising pumping demand and altering recharge, base flow and sustainable yields.

Efforts to restore fisheries and find more water for farms, cities and homes may also be affected by groundwater withdrawals and by Endangered Species Act rules for several stocks of salmon and related fish.

Ice fishing on Moses Lake, Washington, 2011. Karen Payne, USGS.

Sources

Based on Sue C. Kahle and John J. Vaccaro, "Groundwater Resources of the Columbia Plateau Regional Aquifer System," U.S. Geological Survey fact sheet, summarizing USGS Professional Paper 1817 by J.J. Vaccaro and others; a work of the United States government in the public domain. The introduction, figures and USGS photographs are taken from the fact sheet's PDF; five photographs credited to individuals outside the USGS are left out.