Summary
Full appropriation of tributary streamflow during
summer, a growing population, and agricultural needs are
increasing the demand for groundwater in the Willamette
Basin. Greater groundwater use could diminish streamflow
and create seasonal and long-term declines in groundwater
levels. The U.S. Geological Survey (USGS) and the Oregon
Water Resources Department (OWRD) cooperated in a study
to develop a conceptual and quantitative understanding of
the groundwater-flow system of the Willamette Basin with an
emphasis on the Central Willamette subbasin. This final report
from the cooperative study describes numerical models of the
regional and local groundwater-flow systems and evaluates
the effects of pumping on groundwater and surface‑water
resources. The models described in this report can be used
to evaluate spatial and temporal effects of pumping on
groundwater, base flow, and stream capture.
The regional model covers about 6,700 square miles
of the 12,000-square mile Willamette and Sandy River
drainage basins in northwestern Oregon—referred to as
the Willamette Basin in this report. The Willamette Basin
is a topographic and structural trough that lies between the
Coast Range and the Cascade Range and is divided into five
sedimentary subbasins underlain and separated by basalts of
the Columbia River Basalt Group (Columbia River basalt)
that crop out as local uplands. From north to south, these five
subbasins are the Portland subbasin, the Tualatin subbasin,
the Central Willamette subbasin, the Stayton subbasin, and
the Southern Willamette subbasin. Recharge in the Willamette
Basin is primarily from precipitation in the uplands of
the Cascade Range, Coast Range, and western Cascades
areas. Groundwater moves downward and laterally through
sedimentary or basalt units until it discharges locally to wells,
evapotranspiration, or streams. Mean annual groundwater
withdrawal for water years 1995 and 1996 was about
400 cubic feet per second; irrigation withdrawals accounted
for about 80 percent of that total. The upper 180 feet of
productive aquifers in the Central Willamette and Southern
Willamette subbasins produced about 70 percent of the total
pumped volume.
In this study, the USGS constructed a three-dimensional
numerical finite-difference groundwater-flow model of the
Willamette Basin representing the six hydrogeologic units,
defined in previous investigations, as six model layers. From
youngest to oldest, and [generally] uppermost to lowermost
they are the: upper sedimentary unit, Willamette silt unit,
middle sedimentary unit, lower sedimentary unit, Columbia
River basalt unit, and basement confining unit. The high
Cascade unit is not included in the groundwater-flow model
because it is not present within the model boundaries.
Geographic boundaries are simulated as no-flow (no water
flowing in or out of the model), except where the Columbia
River is simulated as a constant hydraulic head boundary.
Streams are designated as head-dependent-flux boundaries, in
which the flux depends on the elevation of the stream surface.
Groundwater recharge from precipitation was estimated
using the Precipitation-Runoff Modeling System (PRMS), a
watershed model that accounts for evapotranspiration from the
unsaturated zone. Evapotranspiration from the saturated zone
was not considered an important component of groundwater
discharge. Well pumping was simulated as specified flux and
included public supply, irrigation, and industrial pumping.
Hydraulic conductivity values were estimated from previous
studies through aquifer slug and permeameter tests, specific
capacity data, core analysis, and modeling. Upper, middle
and lower sedimentary unit horizontal hydraulic conductivity
values were differentiated between the Portland subbasin and
the Tualatin, Central Willamette, and Southern Willamette
subbasins based on preliminary model results.
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First posted October 3, 2014
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