USDA-ARS
Walnut Gulch Experimental Watershed
The Walnut Gulch Experimental
Watershed operated by the Southwest Watershed Research
Center of USDA-Agricultural Research Service in Tucson,
Arizona is a premiere semi-arid experimental watershed.
The watershed (roughly an 80 minute drive from Tucson)
drains 150 square kilometers in southeastern Arizona
and is representative of the approximately 60 million
hectares of grass- and brush-covered rangeland found
throughout the semi-arid southwest and northern Mexico.
It lies in a transition zone between the Chihuahuan
and Sonoran Deserts. The scientific instrumentation
and research infrastructure at Walnut Gulch are unparalleled.
Detailed experiments and long-term observations are
conducted to improve understanding of semi-arid rangeland
hydrology and erosion. No comparable semi-arid hydrologic
database exists in the world (see http://www.tucson.ars.ag.gov
).
The Walnut Gulch facility consists of 29 nested watersheds
that range in drainage area from 0.002 to 150 square
kilometers. Rainfall and runoff instrumentation (including
85 recording rain gauges) has been in place since
1964. Eleven of the nested watersheds are gauged for
runoff with concrete supercritical flumes that are
specially designed to give very accurate estimates
of runoff (notoriously difficult to obtain in semi-arid
regions). Extensive monitoring of erosion and sediment
transport is conducted on eight of the smaller sub-watersheds.
Hydro-meteorological instrumentation at two locations,
one grass dominated and the other brush dominated,
provide measurements of the energy balance, soil temperature,
soil moisture and CO2 fluxes. Biotic characterization
has been ongoing. A high-resolution GIS database for
the watershed has been created. All of the recording
instrumentation is currently undergoing conversion
to digital systems with telemetry for remote data
transfer. The NEXRAD radar system installed by the
National Weather Service at Tucson provides radar
coverage. The ARS Walnut Gulch headquarters facilities
outside Tombstone include soils and sediment laboratories,
and workshops for electronics, machine and welding/fabrication.
The facilities and instrumentation are maintained
by four full-time support staff, and on-site lodging
is available for up to six visiting scientists.
The Walnut Gulch watershed and the containing San
Pedro basin continue to be a venue for highly instrumented
large-scale multidisciplinary research and watershed
characterization conducted by a variety of agencies,
universities, and members of the Upper San Pedro Partnership
to more accurately estimate the semi-arid water balance
and understand the water needs of the first Congressionally
designated Riparian National Conservation Area. Ongoing
ground, aircraft and satellite data remote sensing
collections continue as these watersheds serve as
the primary semi-arid validation site for NASA's Earth
Observing System. SPOT and LANDSAT images are being
routinely archived.
Arizona District of the United States Geological Survey
The U.S. Geological Survey conducts
extensive research on the groundwater resources of the
southwestern U.S. Many of the projects conducted by
the Arizona district programs have great relevance to
SAHRA's effort. In addition, scientists in the USGS
National Research Program provide research tools and
models that are standard to groundwater analysis and
research in the United States.
The Arizona District of the USGS located in Tucson
is leading a multidisciplinary groundwater project
called the Southwestern Groundwater Project that is
highly relevant to SAHRA. The project involves research
and evaluation of the spatial and temporal variability
of groundwater recharge and outflow in shallow aquifer
systems, and provides several opportunities for collaboration
SAHRA, including studies of ephemeral wash recharge
and groundwater outflow in the middle Rio Grande basin
near Albuquerque, New Mexico, and the San Pedro River
National Conservation Area in southern Arizona. These
investigations provide important research opportunities
for students in the hydrology and ecology programs.
Additionally, the USGS Arizona District office is
conducting a detailed investigation of groundwater
resources of the upper San Pedro River Basin. That
project and SAHRA have continuing collaboration on
data collection and analysis relating to the groundwater
flow system.
USGS operates the most extensive set of streamflow
gauging stations in the U.S., providing an essential
long-term database from which to evaluate outflow
from shallow groundwater systems such as the San Pedro
River. The USGS also has geophysical tools for assessing
characteristics of aquifer systems. A drill rig is
maintained in Menlo Park, CA, and is available to
the Arizona District for drilling high-quality monitoring
wells. A national database system maintains information
on wells and groundwater geochemistry.
Eddy Covariance Sites
The following sites are being used by SAHRA researchers:
Agricultural Research Service:
2 eddy covariance sites:
one is at the Charleston Mesquite site in the San Pedro
Riparian National Conservation Area (SPRNCA), 100 km from
Tucson near Charleston, Arizona; the second is being set
up in a mesquite bosque in spring 2003.
Los Alamos National Laboratory:
5 mobile eddy covariance stations
University of Arizona - HWR:
SAHRA staff members based at the University of Arizona
maintain a series of eddy covariance sites in diverse
bioregions.
- The Mt. Bigelow Field Site
is located in a subalpine Douglas fir forest in the
Santa Catalina Mountains north of Tucson. It includes
a 30-m tall eddy covariance tower, with a supporting
spatial network of three micrometeorological stations.
Instrumentation includes the eddy covariance tower
system with 4-way radiometer and three (elevational)
levels of sensors, soil heat flux, net radiometers,
infrared surface temperature, soil moisture probes,
and soil temperature probes. A portable leaf gas exchange
analyzer is also available for use at Mt. Bigelow
or at the San Pedro sites (discussed below).
- San Pedro - Two eddy covariance
sites are also set up adjacent to one another at lower
elevations near the San Pedro River, 100 km SE of
Tucson, in a sacaton grassland and a mixed mesquite/grassland.
Tower data will be integrated with that from flux
towers at complementary regions.
Sevilleta
LTER
This Long Term Ecological Research
site (LTER) in New Mexico is used for watershed-scale
and riparian research. Major advantages to conducting
research on the LTER include: a) security, (it is fenced
and patrolled), b) existing instrumentation networks
for measuring rainfall, solar radiation, soil temperature
and moisture, wind speed and direction, and other climate
variables, c) ongoing hydrologic studies, and d) available
satellite data. Eric Small (University of Colorado)
and Will Pockman (UNM) have obtained additional funding
from DOE for water addition experiments to parallel
their SAHRA-sponsored drought experiments there.
The Sevilleta LTER Program is located
primarily on the Sevilleta National Wildlife Refuge
in Socorro County of central New Mexico, and includes
biomes ranging from Great Plains grassland, Great Basin
shrub-steppe, Chihuahuan desert, to montane coniferous
forest. The area has a complex topography (1,350 to
2,797 m in elevation) and varied geology and soils,
providing an ideal opportunity for researchers to study
biome transition zones.
The major goal of Thrust Area
1 is to measure, estimate and model the spatial and
temporal components of the basin-scale water balance
(snow accumulation, distribution, and melt, rainfall,
evapotranspiration/sublimation, runoff, and infiltration)
by intensive and continuous field observations, coupled
with modeling, in areas above the mountain front and
a variety of ecological locations on the desert floor.
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