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National Water and Climate Center


Weather Generator Technology (GEM)

Fact Sheet
Formation of the ARS-NRCS Weather Simulation Team (WST)


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 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.


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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