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AGNPS Climate Data -- agGEM & preGEM

short url to this page: http://go.usa.gov/KL0

agGEM

If the location of interest is near enough to one of the 233 US stations for which statistical parameter files have already been developed, the user may employ agGEM to produce an AnnAGNPS climate file. The agGEM executable may be downloaded below. The link "already developed statistical parameter files" provides all 233 stations and any of those downloadable files are valid input files for agGEM.

Two output files are created by agGEM, standard GEM output and AnnAGNPS output. For the AnnAGNPS output, the user may choose to produce the file in either English units or SI units.  For these agGEM output options, the units are detailed below:

standard GEM output:

  • maximum air temperature of the day in degrees Fahrenheit
  • minimum air temperature of the day in degrees Fahrenheit
  • average dewpoint temperature of the day in degrees Fahrenheit
  • average wind speed of the day in meters per second
  • total solar radiation of the day in Langleys

AnnAGNPS-style output (English units):

  • maximum air temperature of the day in degrees Fahrenheit
  • minimum air temperature of the day in degrees Fahrenheit
  • average dewpoint temperature of the day in degrees Fahrenheit
  • average wind speed of the day in miles per hour
  • total solar radiation of the day in BTUs per hour per square foot

AnnAGNPS-style output (SI units):

  • maximum air temperature of the day in degrees Celsius
  • minimum air temperature of the day in degrees Celsius
  • average dewpoint temperature of the day in degrees Celsius
  • average wind speed of the day in meters per second
  • total solar radiation of the day in Watts per square meter

The agGEM output files (with the AnnAGNPS format option) can be used directly as climate data input files for AnnAGNPS.

preGEM

If the user wishes to develop input files to agGEM for a location other than these 233 stations, an adequate historical record of the six datatypes must exist. Although preGEM will work for any number of years of historic data, it is generally agreed by climate statisticians that for a record less than twenty years the output may be suspect. In addition, there must be no missing data. Whether the output remains statistically valid enough with the inclusion of estimated data is up to the user to decide. The units for the six datatypes required to run preGEM are as follows:

preGEM input data and units:

  • maximum air temperature of the day in degrees Fahrenheit
  • minimum air temperature of the day in degrees Fahrenheit
  • average dew point temperature of the day in degrees Fahrenheit
  • average wind speed of the day in meters per second
  • total solar radiation of the day in Langleys

Downloads:
                     agGEM & preGEM documentation file           AGNPS climate input data template       

                     preGEM            preGEM input template            Solar Radiation Unit Conversions

                     agGEM             already developed statistical parameter files

Details about preGEM

The program requires an input file of historical data in the following format. (Data columns are not of fixed format — simply separate by spaces.  This is a text file, which may have a .txt or any other extention.)

34  22.2022   159.4492   345   Princeville
Mon Day   Year Prec tmax tmin dewp   solar  wind
1    1    1949 0.00  81   69   68   320.95  4.11
1    2    1949 0.00  77   71   68   219.94  4.34
1    3    1949 1.54  77   64   64   343.97  5.64
1    4    1949 0.01  73   69   60   409.61  6.15
etc.


A template in Excel may be downloaded above. The first line data is:
number of years of data in the file, latitude, longitude, elevation, and sitename.

The program has no restriction on the number of years of data. However, a dataset of at least 20 years is preferred, as statistical reliability declines rapidly with fewer. In addition, there can be no missing data. Any missing data in the historic record must be estimated before running preGEM. The input data units are detailed above. The output file created is given the name of the input file with the extension .gm6 which is a valid input file for the program agGEM. (Output from preGEM is input to agGEM.)

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