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Introduction
The Dry Creek Experimental Watershed is located in the semi-arid
southwestern region of Idaho, USA, and is approximately 16 km northeast
of the city of Boise, Idaho. The surrounding area is defined as the
Boise Front and includes mountainous and foothills topography. The
experimental watershed is delineated as the 28 km2 northeastward
trending Dry Creek drainage from the 1,000 m elevation at the junction
of Dry Creek with Bogus Basin Road to the headwaters of Dry Creek near
Bogus Basin. The headwaters of Dry Creek are located at an elevation of
2,100 meters in the granitic region of the Boise Front. The perennial
creek flows south to southwest from its origin to its confluence with
the Boise River west of the city of Boise, Idaho. The upper 11 km of Dry
Creek are within the DCEW. Shingle creek is the only perennial tributary
draining into Dry Creek. Numerous unnamed intermittent tributaries flow
into Dry Creek within and beyond the DCEW boundary. Multiple sites in
the watershed are instrumented for ongoing investigations into
geochemistry, groundwater recharge, infiltration, basin precipitation
processing, soil water distribution, streamflow generation, and runoff
over multiple scales. Current collaboration in the watershed brings
together researchers from the fields of hydrology, geochemistry,
engineering, and mathematics.
Geology and Soils
The DCEW is located over the Atlanta Lobe of the Cretaceous aged
Idaho Batholith. The Idaho Batholith is a granitic intrusion and is associated
with the Mesozoic subduction zone along the western margin of North
America. The Atlanta Lobe is 75 to 85 million years old and is
approximately 275 km long and 130 km wide (Johnson, Lewis, Bennett, and
Kiilsgaard, 1988). The dominant rock unit in the DCEW is biotite
granodiorite (Lewis, Kiilsgaard, Bennett, and Hall, 1987). The biotite
granodiorite consists of medium- to coarse-grained rocks, is light gray
in color, and is composed of plagioclase, quartz, potassium feldspar,
and 2 to 8% biotite (Johnson et al., 1988). The landscape in the area is
typified by moderately steep slopes, and strongly dissected by streams.
The local soils are derived from weathering of the Idaho Batholith and
are divided into three general soil taxonomies – Argixerolls,
Haploxerolls, and Haplocambids (USDA, 1997). The soils range from loam
to sandy loam in texture and have high surface erosion potential. The
Natural Resource Conservation’s soil survey of the Boise Front (USDA,
1997) provides a more detailed description of the soils underlying the
Boise Front region.
References
Johnson, K.M., Lewis, R.S., Bennett, E.H., and
Kiilsgaard, T.H. 1988. Cretaceous and Tertiary
intrusive rocks of
south-central Idaho. In: Link,
P.K. and Hackett, W.R. (editors), Guidebook to
the Geology of Central and Southern Idaho.
Idaho Geologic
Survey, Bulletin 27, pp. 55-86.
Lewis, R.S., Kiilsgaard, T.H., Bennett, E.H., and Hall,
W.H. 1987.
Lithologic and chemical
characteristics of the central and southeastern
part of the southern lobe of the Idaho Batholith.
In: Vallier, T.L. and
Brooks, H.C. (editors),
Geology of the Blue Mountains region of Oregon,
Idaho, and Washington – the Idaho Batholith and
its border zone: U.S.
Geologic Survey,
Professional Paper 1436, pp. 171-196.
USDA. 1997. Soil survey of the Boise Front:
Interim and supplemental
report, Natural
Resource Conservation Service, Boise, Idaho.
Williams, C.J. 2005. Characterization of the spatial
and temporal
controls on soil moisture and
streamflow generation in a semi-arid
headwater
catchment. Masters Thesis, Boise State
University, Boise,
Idaho, United States.
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DRY CREEK
EXPERIMENTAL WATERSHED AS DESCRIBED IN WILLIAMS (2005)
Climate
Climate in the region surrounding the DCEW results from the opposing
Aleutian Low and Pacific High weather systems. The Aleutian Low controls
winter weather and delivers cool, moist air from the west and northwest.
Winters in this region are moderately-cold to cold and produce abundant
precipitation in the form of snow in the highlands and rain in the
lowlands. The spring season begins with cool rainy months followed by
drier warming trends approaching the summer months. The Pacific High
system dominates the summer period and brings dry air from the Pacific
Ocean to the Boise Front. Summers are typically hot and dry with
occasional thunderstorm events. The autumn season is typified by clear
and warm months immediately following summer and cold and moist months
immediately prior to the early winter season.
There are two meteorological stations located in the DCEW: 1)
Lower
Weather Site (LWR), 1,100 m elevation and 2)
Treeline Site (formerly the
Upper Dry Creek Experimental Watershed), 1,650 m elevation. Both
stations have periods of record from 1998 to present. The average annual
precipitation at the LWR and Treeline sites are 37.25 cm and 57 cm.
Another meteorological station is located just north of the DCEW
boundary at the
Bogus Basin Snotel Site, monitored by
ARS. The Bogus
Basin site is located at an elevation of 1,930 m and has an average
annual precipitation 100 cm (1999 – present). Precipitation is greatest
December through February and the average monthly temperatures are
greatest in July and lowest in January. Evapotranspiration exceeds
precipitation during most of the year.
Vegetation
Vegetation along the Boise Front varies with elevation, geology,
microclimate, soil type, and topography. At lower elevations,
grass and shrublands, ponderosa pine (Pinus ponderosa), and
Douglas-fir (Pseudotsuga Menziesii) occupy south and west
aspects and Douglas-fir forests dominate north and east aspects.
Upper elevations are dominated by ponderosa pine and Douglas-fir
forest communities with patches of lodgepole pine (Pinus
contorta) and aspen (Populus tremuloides). Middle elevations
range from grass and shrublands to open forest communities of
ponderosa pine and Douglas-fir and function as an ecotone
between the sagebrush (Artemisia spp.) and grass dominated
lowlands and more densely forested uplands.
Land Ownership
Land ownership in the DCEW is divided among multiple
governmental entities (Boise National Forest – 42%, State of
Idaho – 3%, and Bureau of Land Management – 1%) and private
parties (54%). Land use in the basin includes timber harvest,
cattle and sheep grazing, and recreation activities (downhill
and cross country skiing, hunting and fishing, hiking, mountain
biking, motorcycling, snowmobiling, ATV riding, and natural and
cultural study).
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