Environmental Engineering Reference
In-Depth Information
chemical, isotopic, and heat tracer; Darcy; and
modeling approaches are commonly applied.
Water-budget methods (Feth
et al
.,
1966
) and
environmental groundwater tracers (Manning
and Solomon,
2004
) have been used to estimate
subsurface mountain-block discharge to differ-
ent parts of the Salt Lake Valley.
Increasing populations in the western
United States underscore the need for sustain-
able groundwater supplies for the increasing
number of households in mountainous areas.
Groundwater in valley aquifers has histori-
cally been of high interest; recharge studies in
mountain blocks, however, have garnered little
attention (Wilson and Guan,
2004
). Recent stud-
ies have begun to address mountain recharge
directly, in spite of difficulties imposed by com-
plex geologies and large differences in eleva-
tions. Bossong
et al
. (
2003
) used a watershed
model and estimated base flow to be equiva-
lent to 19 mm/yr (3% of precipitation) across a
120 km
2
area of the Turkey Creek watershed in
Colorado. Elevations across the Turkey Creek
watershed vary by more than 1000 m.
Tucson. Recharge to these aquifers occurs as
mountain-front recharge as described in
Section
9.4.1
. Ephemeral streams that drain adjoining
mountains tend to lose their water to infiltra-
tion as they flow across valley floors. Susbsurface
flow from mountain blocks is also an important
source of water in parts of the region where
mountains consist of permeable carbonate
rock. Diffuse recharge from precipitation can
occur in the valleys, as shown by Stephens and
Knowlton (
1986
), but usually this is a small per-
centage of total recharge. In undisturbed water-
sheds, natural groundwater discharge is mostly
by evapotranspiration, often through playas
in the basin floors, or by groundwater flow to
neighboring basins. Historically, groundwater
discharged to streams in some areas, such as
the Santa Cruz River in southern Arizona, but
groundwater pumping has resulted in declining
groundwater levels and a substantial reduction
in groundwater discharge to streams in many
areas (Webb and Leake,
2006
).
Recharge processes for watersheds in the
coastal mountains of California are generally
represented by a slightly different conceptual
model. Most recharge originates as precipitation
in the mountains and either infiltrates or flows
over the surface to streams in the valley floors.
The streams are usually losing streams, pro-
viding water to underlying aquifers. However,
unlike systems in more arid settings, these val-
ley streams do not typically lose all of their flow
to infiltration. A substantial amount of stream-
flow can flow directly into the ocean rather
than becoming recharge, especially for stream-
flow generated by intense rainfall events.
Most methods for estimating recharge dis-
cussed in
Section 9.4.1
are also applicable in the
Alluvial Basins region. In particular, Wilson
(
1980
) and Wilson and Guan (
2004
) discussed a
number of approaches. Recharge to the Central
Valley aquifer system in California (the lar-
gest groundwater system in the Alluvial Basins
region) was estimated through application
of a highly detailed groundwater-flow model
(Faunt, 2009). Flint and Flint (
2007
) applied a
soil water-budget model (Basin Characterization
Model) to generate an average estimate of
drainage beneath the root zone of 20 mm/yr
9.4.2 Alluvial Basins
The Alluvial Basins region consists of alternat-
ing basins and mountain ranges and covers
much of the southwestern United States, as well
as parts of Washington and Oregon. The region
is dominated by broad valleys that are bordered
by mostly low and narrow mountain ranges. This
is in contrast to the Western Mountain Ranges
in which the valleys are narrow and the high
mountains are the dominant topographic fea-
ture. The coastal ranges in southern California
are an exception to this description; although
contained within the Alluvial Basins region,
they actually resemble the Western Mountain
Ranges in a topographic sense. Climate in this
region is mostly arid to semiarid. Precipitation
rates in the basins of Nevada and Arizona are
typically in the range of 100 to 400 mm/yr
(Heath,
1984
).
The basins consist primarily of erosional
debris and can store substantial amounts of
water. Basin aquifers are the primary source of
water supply in the region, serving even large
metropolitan areas such as Albuquerque and
