Building a Groundwater-Flow Model for Idaho's Treasure Valley

The U.S. Geological Survey, working with the Idaho Department of Water Resources (IDWR) and the Idaho Water Resource Board, is building a numerical model of groundwater flow in the Treasure Valley and the area around it. Water managers will use it to test how human activity and climate could affect groundwater, for planning and managing supply. Along the way, the understanding of the aquifer system will be updated with two decades of information and new data gathered for the study.

The valley and its water

The U.S. Board on Geographic Names defines the Treasure Valley as "the agricultural area that stretches west from Boise into Oregon"; it is also commonly called the lower Boise River Basin. It holds Idaho's three largest cities and its sixth — Boise, Meridian, Nampa and Caldwell. In 2016 about 630,000 people lived there, about 37 percent of Idaho's population. Apart from the 30 percent of Boise's city supply taken from the Boise River, every home and town in the valley relies on groundwater.

A private engineering firm has projected that the valley's population will reach about 1.6 million by 2065, with domestic, commercial, municipal and industrial demand for water rising to match. In response, the Idaho Senate passed Concurrent Resolution 137, which asks, among other things, for a groundwater model for the valley along with the monitoring networks it needs.

The Treasure Valley, looking southwest from the Boise Front, November 19, 2016. Photograph by James R. Bartolino, USGS.

The aquifer system is generally taken to be bounded by the Snake River to the south and west, the Mountain Home plateau to the east, and the uplands dividing the Boise and Payette River basins to the north. The new model extends north to the Payette River valley and the towns of Emmett, New Plymouth and Fruitland, because the divide between the two river basins does not fully block groundwater flow — and because that area is being developed, with more large-scale housing proposed.

Towns, counties and the approximate model boundary. USGS map.

The geology beneath

The Treasure Valley lies in the Snake River Plain, a curving depression across southern Idaho. Its western half is mostly sedimentary rock and its eastern half mostly volcanic rock, and each half has its own large aquifer system; they are separated by a hydrological boundary near King Hill.

The western plain formed when volcanic activity led to faulting that dropped it relative to the mountains around it. The basin filled with river and lake sediments, interbedded in places with basalt, and those deposits make up the aquifer system under the valley. Geological activity and glacial cycles while they were laid down made them complex, so aquifer properties can change sharply over short distances in depth or place, dividing the system into many connected aquifers with different water levels, chemistry and yields. Once the basin stopped filling, the Snake, Boise and Payette Rivers carved today's landscape.

Groundwater is both unconfined and confined; where the pressure in a confined aquifer is higher than the land surface, some wells flow on their own. Depth to water ranges from at or above the surface to more than 800 feet. Broadly, the system has three parts: a shallow water-table aquifer; a complex, deeper confined aquifer beneath it; and at the bottom a confined geothermal aquifer. In the shallow aquifer, water generally flows from high ground toward rivers and drains.

Rivers, canals and wells

The main natural waterways are the Snake River to the south, the Boise River through the middle of the valley and the Payette River to the north. River water for farming was the main reason people settled here, and a complex network of reservoirs, canals, laterals, ditches and drains was built to irrigate.

Until the late 1940s, groundwater came only from very shallow wells or from wells that flowed under artesian pressure. From 1891, artesian wells near the old State Penitentiary supplied geothermal water for heating, and by 1897 artesian wells in Hulls Gulch were Boise's municipal supply. Pumping rose steadily once pumps able to lift water efficiently from greater depths arrived in the late 1940s.

Earlier water budgets found that the aquifer is recharged mostly by seepage from the canal system, including Lake Lowell, and by irrigation water soaking in. Most water leaves by seeping into rivers and drains, and next by pumping from wells. Those budgets agreed on the relative sizes of these flows but can be improved with new data, such as groundwater flowing into drains, and new techniques, such as estimating evapotranspiration from satellite images. None looked at changes from year to year or season to season. This study builds a monthly water budget for the 30 years from 1986 to 2015.

What a groundwater model does

A model is a simplified representation of how a real object or system looks or works. A groundwater-flow model tries to reproduce the workings of a real aquifer by solving a set of mathematical equations. The geology is usually represented as a grid of rectangular three-dimensional blocks, or cells, inside a hydrological boundary. The computer program works out how much water flows horizontally and vertically between the cells, and how the amount stored in each cell changes. Such models are often the best tool available for management decisions.

Part of a hypothetical basin-fill groundwater system: an unconfined aquifer over a confining unit and a confined aquifer, a stream gaining water from the ground, infiltration from irrigated fields and recharge along the mountain front. Arrows show the direction of flow. USGS diagram, adapted from Leake (1997) and Reilly and McAda (2002).

A useful model has to capture the parts of the real system that matter, and which ones matter depends partly on what the model is for. The aims also decide how far and how deep the model reaches, the size and shape of its cells and layers, how its boundaries are represented, and any special methods it needs.

Eight published groundwater models already cover some or all of the valley, built with different aims, detail and extent. The new USGS–IDWR model will use data and interpretations those lacked. As a transient model, one that changes through time, it is meant to improve basic understanding of the aquifer and ultimately to explore how changes in water use, recharge or discharge could affect groundwater and its connection with surface water. Building it will also show which parts of the system matter most to its forecasts, guiding what data to collect next.

The project

The USGS installed the valley's first streamgage in 1895, the same year Idaho created the Office of the State Engineer, a forerunner of the IDWR; the two have worked together, and with local governments and others, ever since.

The five-year study was to start in late 2016 and finish in 2021; new data collection, including flow measurements on drains, began in October 2016. The model is being built with a version of MODFLOW, a public-domain program. An initial report on the updated geology, water budget and conceptual model was scheduled for 2019. At the end, the USGS was to publish the documented model and a final report on how it was built, its limitations, and one or two simulations covering a range of human activity and water conditions, designed with stakeholders. The model is to be freely available through USGS and IDWR websites.

A technical advisory committee, with members sought by the IDWR from cities, irrigation organisations, water-user groups and other stakeholders, is to keep the work transparent and bring in stakeholders' views — as similar committees did for two earlier USGS–IDWR models, of the Wood River Valley and the Spokane Valley–Rathdrum Prairie.

A USGS hydrographer measures stream discharge in Sand Run Gulch near Parma, Idaho, March 14, 2017. Photograph by James R. Bartolino, USGS.

Words on this page from people and organisations outside the federal government are paraphrased; rewritten in hubnx's own words.

Sources

• James R. Bartolino (USGS) and Sean Vincent (Idaho Department of Water Resources), A Groundwater-Flow Model for the Treasure Valley and Surrounding Area, Southwestern Idaho, USGS Fact Sheet 2017–3027. https://doi.org/10.3133/fs20173027
• USGS Idaho Water Science Center: https://id.water.usgs.gov