The major objective of the AGNPS-GRASS input interface is to minimize the user interaction in preparing the input data for the AGNPS model and to minimize the number of user supplied/developed GIS database layers. Of the 22 different data required by the model for each cell (appendix 2), the interface will prepare the input data with only 8 (appendix 2) basic GIS database layers supplied by the user and with minimal user interaction. There are 5 (appendix 2) parameters needed for the whole watershed which will be obtained from the user.
Name of the AGNPS-GRASS Input Interface Program: r.agnps50.input
Procedures to follow while executing the Input Interface:
First start GRASS 5.x from a shelltool window with proper mapset where the data resides
Execute r.agnps50.input program from the shelltool window where GRASS is running
A Watershed Input menu will appear on the shelltool window requesting the following data/information from the user:
Watershed Description: Enter a description about the watershed. This information optional and will appear in both input and output of the AGNPS model run.
Rainfall Amount in inches: Enter the total amount of rainfall in inches.
Erosivity Index Value: Enter the erosivity index value for that particular storm/rainfall event.
Enter the cell size in meters: The length of the side of a cell in meters is entered. The square of the cell size will be the area of each cell.
Enter a file name to save in AGNPS format: Enter a file name with .dat as the extension to save the AGNPS model input file created by the AGNPS-GRASS input interface tool. This will be stored in the current working directory.
Enter the watershed name: The name of the watershed should be entered here. All the input layers should have the same watershed name with the proper extension (as X in appendix 1). Please refer to appendix 1 for the valid extensions and valid category labels/values that are allowed for each layer.
Then hit the Esc key to continue.
Error Messages: If any of the category labels/values for the input layers is not valid, an error message is printed and the program quits. Run support on the layer where the category label is wrong to correct it. The program support helps to modify the supporting files of a map, which include header, category, color, and history. The wrong/misspelled category labels can be corrected by running support on the layer to be corrected and choosing the category option. If there is any problem with the flow direction layer, error messages are printed for those cell numbers that had problems and the nature of those problems (like circularity, sinks or holes, or flow directions pointing at each other). Use Dedit on the aspect map to correct the problems. The Dedit program helps to display and edit the aspect map with arrows on it, which should enable one to correct the flow direction on a cell by cell basis.
For each run of this input interface tool, a new temp_cell_num layer is created and stored in the current mapset. In this map, the cells are numbered from top to bottom and left to right. This is a useful layer for editing and interpreting the error messages due to conflicting flow direction for any particular cell. While using the Dedit program to correct the errors, this map allows one to locate the cell number where conflicts occur.
Once the input interface completes its run without errors, it saves the data in the AGNPS format. The AGNPS model can be executed with this data. The name of the executable file program to run the AGNPS model is agrun. Type agrun from the shelltool window where GRASS is running. The program will ask for the input file name. Enter the name of the file that was saved by running the AGNPS-GRASS input interface, with the .dat extension. The model will output the results of the run with a .nps extension. Both input and output ASCII files are needed to execute the AGNPS-GRASS output interface tool (the Visualization tool).
Appendix 1
---------------------------------------------------------- |Level of | Assumed fertilization (lb/acre) | | |fertilization | N P | Input | |--------------------------------------------------------| |None 0 0 | 0 | |Low 50 20 | 1 | |Medium 100 40 | 2 | |High 200 80 | 3 | ----------------------------------------------------------
-------------------------------------------------------------- Tillage practice Fertilizer availability factor (%) -------------------------------------------------------------- Large offset disk 40 Moldboard plow 10 Lister 20 Chisel plow 67 Disk 50 Field cultivator 70 Row cultivator 50 Anhydrous applicator 85 Rod weeder 95 Planter 85 Smooth or no till 100 --------------------------------------------------------------Appendix 2: AGNPS cell input parameters
box; l. 1. Cell number 2. Number of cell into which it drains 3. SCS curve number 4. Average slope % 5. Slope shape factor 6. Average field slope length 7. Average channel slope 8. Slope length factor 9. Mannings roughness coefficient for channel 10. Soil erodibility factor (K) from USLE 11. Cropping factor (C) from USLE 12. Practice factor (P) from USLE 13. Surface condition constant (based on land use) 14. Aspect (one of 8 directions indicating drainage from cell) 15. Soil texture (sand, silt, clay, peat) 16. Fertilization level (zero, low, medium, high) 17. Incorporation factor (% fertilizer left in top 1 cm of soil) 18. Point source indicator (indicates existence of a point source input within a cell) 19. Gully source level (estimate of amount, tons, or gully erosion in a cell) 20. Chemical oxygen demand factor 21. Impoundment factor (indicating presence of an impoundment terrace system within the cell) 22. Channel indicator (indicating existence of a defined channel within a cell)The 8 basic input GIS layers required for extracting the data by the AGNPS-GRASS input interface tool are:
For each watershed element, AGNPS requires the following 22 input data values (its distributed parameter information):
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