Agricultural non point source of pollution (AGNPS) model input data
tldr;

On the official support page titled AGNPS Climate Data -- agGEM &amp; preGEM you find a download link to a climate data template file. Your data files need to have this format. All meteorological variables are written into this file. The template file contains daily data. It is not clear if either the data has to have daily resolution or the data can be finer (hourly) or coarser (monthly) resolved.

detailed answer

Everything I write down below is described on one the official support pages for AGPNS: http://go.usa.gov/KL0 (This is the official short link provided by the United States Department of Agriculture (USDA))

Remarks


The meteorological input data files for AGNPS model (Agricultural non-point Source Pollution Model) are actually called climate input data, although the files contain short term meteorological data.
All the provided programs are Windows executables.


generate climate input data files for known locations

Use the program agGEM. This program uses an existing statistical parameter file (maybe: == climatological data?) of meteorological data to generate a climate input data file for AGNPS. There are 233 stations in the US, for which such statistical parameter files exist. There is no information on the support page whether the data are measurement or model data. The program can be downloaded without registration from the support page linked above. The existing statistical parameter files can be also downloaded on that page. 

generate your own statistical parameter file

The USDA provides a program called preGEM to generated statistical parameter files. preGEM needs an input data file with a long time series of meteorological data. What long means is to decide by the use.

The format of this particular input data file is not clear. According to the agGEM &amp; preGEM documentation file one can choose between two types of text files: csv and fixed format (see the manual what the latter means) during the execution of preGEMs. However, a preGEM input template is provided on the support page, which is a xls file. Maybe one can also choose xls as format for the input data file.

Please be aware that there are a lot of options when you save a xls as csv via the spreadsheet software of your choice: delimiters for strings, delimiter between columns (comma, tab, semicolon, ...) etc.

Build your own climate input data file

There is a climate input data template provided on the given support page. You can generate your own file without using arGEM and preGEM. There is a file header which nicely describes how the file has to be structured. The file looks as follows:

*************************************************
*** a few words of explanation:
*** The Climate input data for AGNPS has the following fixed format.
*** The first line is required and must contain the current version ID
*** The second line may contain any info the user wishes.
***
*** The third line contains site latitude, longitude, elevation, 
*** temperature lapse rate with elevation, 
*** percentage N in the precip, and unit code.
*** The temperature lapse rate units are degrees F per foot or degrees C
*** per meter of elevation and can be left at zero.
*** The nitrogen percentage in the precip is currently not used in the
*** program, but units are percentage by weight.
*** The unit code is 1 for English and 0 for metric.
***
*** The fourth line contains the start date for the data.
*** The fifth line contains the end date for the data.
***
*** The sixth through eighth lines contain data column headings
*** 
******* climate file starts below this line:
Climate File Input Version ID:          5.00
   56  Princeville       
   22.0708  159.1234    345.00   -0.0036    0.0000         1
01/01/0001
12/31/0100
     Daily   -Air Temperature-   Precip-     Dew        Sky      ----- Wind -----   Solar     Storm    Potent-
    Date     Maximum   Minimum   itation     Point     Cover     Speed  Direction Radiation  Type ID    ial ET
mm/dd/yyyy     [ºF]      [ºF]      [in]      [ºF]       [%]      [mph]       [º] [BTU/h/ft^2]    [-]      [in]
01/01/0001     73.08     67.10      0.00     59.46      75.4      3.67               87.30
01/02/0001     77.12     60.72      0.08     61.19      39.2      5.61              123.27
01/03/0001     78.22     58.41      0.28     60.80      80.9      3.85               79.23
01/04/0001     79.22     60.80      0.24     60.81      34.7      2.43              127.03
01/05/0001     80.62     69.15      1.84     67.06      79.8      6.96               79.44
01/06/0001     82.05     66.88      0.25     64.65      30.0      4.29              128.66
01/07/0001     80.89     62.94      0.07     60.28      52.7      3.59              114.29
01/08/0001     77.59     63.79      0.00     60.51      33.0      4.14              129.61
01/09/0001     75.78     61.29      0.00     57.00      53.0      3.69              115.94
01/10/0001     74.65     57.61      0.11     54.61      46.9      4.04              122.56
01/11/0001     78.17     60.47      0.52     61.41      38.5      5.44              126.81
01/12/0001     74.93     64.24      1.18     62.55      67.1      5.47               99.94
01/13/0001     74.97     62.35      0.63     66.70      84.6      3.82               73.72
******* This line and below are not in the input file.
*** In this template the data ends on 01/13/0001, but the actual file
*** must obviously continue to 12/31/0100 because of the specified
*** end date on line 5.