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AGNPS Climate Generator GEM

factsheet and downloads

The climate generation model GEM (Generation of weather Elements for Multiple applications) was originally written for DOS in 1984, and published as Agricultural Research Service report ARS-8 "WGEN: A Model for Generating Daily Weather Variables", by Richardson, C.W., and Wright, D.A., August 1984. A more recent version of the program, named GEM6, incorporated the ability to generate datastreams of six climate variables, namely max, min, and dewpoint temperatures, precipitation, wind speed, and solar radiation. Neither the GEM or GEM6 models were adapted to "Windows" and ARS no longer provides support. However, the AGNPS development team has adapted the software into two programs (preGEM and agGEM) which can be used to generate climate data for AGNPS.

preGEM and agGEM

The AGNPS support team has adapted GEM6 to enable creation of synthetic climate data for use in AnnAGNPS. The GEM6 code can still be run, with statistical parameter files, previously generated by ARS for 233 stations in the US. These statistical parameter files were created by ARS, using two programs that are now available as preGEM, so that the user may obtain synthetic climate generation at any location for which an adequate historical record of the six datatypes exists. In addition, the program agGEM does everything that the original GEM6 program does, except that it generates a second output file, which is an AnnAGNPS formatted climate file. For further details and downloads, click here.

general information about the original GEM effort

For many applications, users need a continuous time stream of weather data, typically to be used as input to a computer model of some process of interest, such as water or wind erosion. A weather generator is a statistical model that can be used for this purpose. Presented here is brief information about one such stochastic weather simulation model, or weather generator technology known as the GEM (Generation of weather Elements for Multiple applications) model.

GEM delivers a time series (data stream) of daily weather data (precipitation, maximum and minimum temperature, solar radiation, dewpoint and wind speed). The model can generate as many years of simulated weather as desired, for many locations across the United States.

GEM provides easy access to simulated daily weather data for as many months or years as needed, for many locations within the 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.


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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