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.
How GEM Functions.
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.
Go to Water & Climate Center page to
download the original GEM code
References
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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