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Weather Generator Technology (GEM)
Fact Sheet
Formation of the ARS-NRCS Weather Simulation Team (WST)
Overview
A new ARS/NRCS team has been developed in response to ARS high
priority stakeholder needs identified at the Watershed Processes
National Program Workshop, and by the ARS-NRCS Partnership
Management Team (PMT). This new team is composed of ARS and NRCS
scientists involved in stochastic weather generation research and
has been named the Weather Simulation Team (WST).
The Need
The generation of long records of weather variables is needed for
evaluating different agricultural management scenarios in natural
resource models. However, long measured records are not available at
all locations in the US. Consequently, records must be generated
that have the same statistical characteristics as naturally occur
for a given location. Natural resource models such as AnnAGNPS
(watershed water-quality model) and WEPS (wind-erosion model) were
developed by ARS and NRCS and are used nationwide by the NRCS.
Weather inputs required by these models include daily maximum and
minimum air temperatures, precipitation, solar radiation, wind, and
dewpoint. The GEM (Generation of weather Elements for Multiple
applications) weather generator model, developed by ARS and NRCS, is
being used with AnnAGNPS, and soon will be integrated into WEPS and
other models. GEM needs further parameterization and development for
full implementation in these models, and for support of other water
quality, crop growth and wind erosion models.
What the GEM Model Provides
GEM provides easy access to simulated daily weather data for as
many months or years as needed, for any location within the
contiguous United States. The time series which is produced is
statistically representative of the weather that can be expected at
that location over a period of time. A recent study has shown that
data generated by GEM closely mimics nearly all aspects of the true
climate of a location. At present, GEM delivers a daily time series
of maximum and minimum temperature, precipitation amount and solar
radiation. Planned enhancements to the model will result in a more
complete suite of products. Additional elements such as dewpoint
temperature and wind speed will be included in GEM. Higher time
resolution precipitation data, such as hourly output, will be
provided, and a spatial version of the model will be included for
realistic weather simulation over a small region, such as a
watershed or small basin.
Stakeholders
Stakeholders at the ARS Workshop on Watershed Processes held in
1998 voted the weather generator as one of the highest research
priorities for ARS. The Partnership Management Team (PMT) rated the
weather generator as one of the highest national priority needs of
the NRCS. It was decided by members of a group of ARS and NRCS
scientists to further coordinate efforts so that the GEM weather
generator will satisfy a wide variety of user needs.
Action
Representatives from ARS locations in Boise ID, Coshocton OH,
Temple TX, Tifton GA and Tucson, AZ met with representatives of the
NRCS National Water and Climate Center in Portland, OR on June 30 to
July 1, 1999 to discuss a strategy for advancing the GEM weather
generator model to make it useful for current NRCS needs and for a
wide variety of other customers. The ARS/NRCS Weather Simulation
Team was formed. An action plan was developed for the next two years
to develop and test routines for generating daily dewpoint and wind;
parameterize the model across the U.S.; incorporate large scale
atmospheric forcings into the model such as El Nino and La Nina; and
develop and incorporate a storm generator that removes the daily
precipitation constraint and replicates true storm characteristics.
The team will investigate model requirements for many natural
resource models, and integrate GEM into these models. Progress on
these tasks will be dependent upon available funding. The WST will
develop a web page to facilitate communication and progress.
GEM Projects now Underway or Planned
Methods of Generating Sub-Daily Time Steps
Methods of generating weather at sub-daily time steps (hour,
minute) are being developed and will be incorporated into GEM.
Included in this is a method of generating within-storm
precipitation intensities with a resolution of the order of minutes.
Storm-occurrence and within-storm statistical characteristics for
any given location will be maintained. The short-time interval
precipitation outputs will enable hydrologic and natural resource
modelers to utilize more advanced water-movement process algorithms,
where previously a lack of widespread, appropriate precipitation
data limited their utility. Better synthesis of time series of
sediment yields, peak flows, runoff volumes and chemical loads will
result from these improvements to GEM. Lead Scientist: Dr. Jim
Bonta, USDA-ARS, Coshocton, Ohio (740) 545-6349; email:
jim.bonta@ars.usda.gov
Distribution of GEM Parameters for Spatial Modeling
A method of spatially distributing the necessary parameters for
GEM using the PRISM modeling system at Oregon State University has
been developed. This means representative weather scenarios can be
developed for any location, even in regions where no long-term
climatic data exist. Presently, this methodology has been used and
tested in a region of significant climatic diversity over portions
of Idaho and Oregon. Time series of daily precipitation and maximum
and minimum temperature can be generated for any 4-km grid point in
the region using a point-and-click, map-oriented user interface. It
is anticipated that this technology will be available for the entire
United States in the coming few years. Lead Scientists:
Dr. Greg Johnson, USDA-NRCS, 1201 NE Lloyd Blvd, Suite 1000,
(503) 273-2424, email: Greg.Johnson@por.usda.gov; and Dr. Chris Daly, Oregon State
University, Corvallis, Oregon, (503)754-5705, email: daly@fsl.orst.edu.
For Information about GEM
For more information please contact ARS-NRCS Weather Simulation
Team Leader Dr. Greg Johnson, USDA-NRCS, 1201 NE Lloyd Blvd, Suite
1000, (503) 273-2424, email: Greg.Johnson@por.usda.gov; or
Dr. Jim Bonta, Research Hydraulic Engineer, USDA-ARS, North
Appalachian Experimental Watershed, Box 488, Coshocton, Ohio 43812,
(740) 545-6349; email: jim.bonta@ars.usda.gov.
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