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National Water and Climate Center


Weather Generator Technology (GEM)

Fact Sheet
Introducing the GEM Weather Simulation Model (ARS GEM Page)


Weather and climate data are often the most critical pieces of information needed for strategic environmental planning. Agricultural, natural resource and engineering management decisions require a variety of climatic information, dependent on the specific application. Unfortunately, very often only rough estimates of weather are used because historical records for the location of interest are either unavailable or of insufficient duration. To address the need for readily-available climate data for any location, a new stochastic simulation model is being developed which delivers accurate time series of daily or sub-daily weather elements.

The model is known as GEM (Generation of weather Elements for Multiple applications), and is being developed by researchers within the USDA-Natural Resources Conservation Service (NRCS), Agricultural Research Service (ARS), and collaborating universities. Overall project leadership is being undertaken by the NRCS National Water and Climate Center in Portland, Oregon. Building upon the strength of stochastic weather modeling work by several ARS researchers over the past 20 years, GEM retains the basic internal structure of the USCLIMATE (Hanson et al. 1994) and WGEN (Richardson and Wright 1984) models but includes several significant improvements.

What GEM 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 (Johnson et al. 1996). 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 from GEM, including additional elements such as dewpoint temperature and wind speed, as well as higher time resolution data (such as hourly precipitation), and a spatial version of the model for realistic weather simulation over a small region, such as a watershed or small basin.

Current Enhancements and Improvements to GEM

Distribution of GEM Parameters for Spatial Modeling

A method of spatially distributing the necessary parameters for GEM using the PRISM modeling system (Daly et al. 1998) 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, WNTSC, Portland, Oregon, (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.

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 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.

Applications of GEM

GEM is being used by numerous agencies, research institutions and private companies. The model has been linked to other computer models, including those used to manage crops, predict yields, and determine runoff and erosion, as well as to investigate the impact of potential climate change on climate variability and shifts in agricultural production. It is in use in several countries in addition to the United States.

For Information about GEM

For more information please contact ARS-NRCS Weather Simulation Team Leader Dr. Greg Johnson, USDA-NRCS, WNTSC, Portland, Oregon, (503) 273-2424, email: Greg.Johnson@por.usda.gov.


Daly, C., W.P. Gibson, G.H. Taylor, G.L. Johnson and P.A. Pasteris. 1998. New methods for mapping climate in complex regions. J. Appl. Meteor. (In Review)

Hanson, C.L., K.A. Cumming, D.A. Woolhiser and C.W. Richardson. 1994. Microcomputer program for daily weather simulation. U.S. Dept. Agric., Agric. Res. Svc. Pub. No. ARS-114, 38 pp.

Johnson, G.L., C. Daly, G.H. Taylor and C.L. Hanson. 2000. Spatial variability and interpolation of stochastic weather simulation model parameters. J. Appl. Meteor., 39, 778-796.

Johnson, G.L., C.L. Hanson, S.P. Hardegree and E.B. Ballard. 1996. Stochastic weather simulation: Overview and analysis of two commonly used models. J. Appl. Meteor., 35, 1878-1896.

Richardson, C.W. and D.A. Wright. 1984. WGEN: A model for generating daily weather variables. U.S. Dept. Agric., Agric. Res. Svc. Pub. No. ARS-8, 83 pp.

Woolhiser, D.A., T.O. Keefer and K.T. Redmond. 1993. Southern oscillation effects on daily precipitation in the southwestern U.S. Water Resources Research, 29, 1287-1295.

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