It is the year 2000 and regulations concerning nonpoint source pollution from watersheds have been passed. Since you are one of nation's leading experts concerning nonpoint source pollution control, you have been hired to evaluate current nonpoint source pollution and to suggest potential solutions if a problem exists in an 813 acre watershed located near Lafayette, IN. It is suspected that the watershed does not currently meet regulations for nitrogen, phosphorus, and sediment delivered to its outlet.

The nonpoint source pollution regulations require that watersheds of this size meet guidelines for 2 rainfall events. The 2 rainfall events are an April 1 event of 1.8 inches with an erosivity index (USLE R) of 16 and an antecedent moisture condition of 2; and a July 1 rainfall of 3.5 inches with an erosivity index of 24 and an antecedent moisture condition of 2. The guidelines for each of these events require that less than 2 lbs of N per acre leave the watershed, less than 2 lbs of P per acre leave the watershed, and less than 0.125 tons of sediment per acre leave the watershed. It is also desirable to limit erosion within the watershed to less than 7.5 tons per acre for each of these events.

At the present time, the cropped area in the watershed is in a corn-soybean rotation (see ansi.crops71 in the agnps LOCATION for crops this year). The corn seedbed is prepared on May 1 using a tandem disk. The corn is planted on May 2, cultivated on June 5, and harvested on October 10. Fertilizer for the next year's soybeans is applied before the corn stalks are disked on October 25. The soybean seedbed is prepared by disking with a tandem disk on May 10. The soybeans are planted in rows on May 15, cultivated on June 15, and harvested on October 10. Fertilizer for the next year's corn crop is applied and the bean stubble disked on October 20. The fertilizer applied would be considered moderate for the next year's soybean crop and high for the next year's corn crop.

You have decided that the best way to determine if this watershed meets current regulations is to simulate its response to the storm events described above using the AGNPS model. If it does not meet regulations, you will propose changes in land uses and management practices and evaluate these changes with AGNPS. The data required to run the AGNPS model for the current watershed land uses and management practices are available in the GRASS GIS (select the agnps location after starting GRASS).

Refer to the WWW pages describing the AGNPS/GRASS interface as you begin to evaluate the initial watershed conditions and potential changes. You may modify the following GRASS GIS files if you determine that changes are required: ansi.C (contains the USLE C factors) (use RUSLE to obtain C values), ansi.luse (contains land uses - initially set to fallow for the areas to be cropped and will be changed to row crops for the July 1 storm), ansi.mach (describes the primary tillage machinery used), and ansi.nut (describes the amount of nutrients applied). Refer to the WWW pages for possible map category values for these data layers. The ansi.farm GIS layer may be useful in defining new ansi.C, ansi.luse, ansi.mach, or ansi.nut layers for proposed changes (use r.reclass). Note that files must be named in the manner described in the AGNPS/GRASS WWW pages and use the map category naming convention described there as well.

The ansi.C41 layer contains USLE C values for current crop rotations and crop conditions for April 1. The ansi.C71 layer contains USLE C values for current crop rotations and crop conditions for July 1. These C values were estimated using RUSLE for the crop rotations and practices described above. One of these files will need to be copied to the ansi.C layer (use g.copy) before running the AGNPS/GRASS interface.

Evaluate the current conditions for the April 1 event (the GIS files are currently set for April 1 conditions). Instructions for creating AGNPS files, running AGNPS, and viewing the results are provided below.

Evaluate the current conditions for the July 1 event (you must use g.copy to copy the ansi.C71 layer to ansi.C; make the hydrologic conditions good since crops are growing by using g.copy to copy temp_hy_cond.good to temp_hy_cond (you'll need to remove this file when evaluating April 1 conditions in the next steps); use r.reclass to create a new ansi.nut file with low nutrient values; use r.reclass to create a new ansi.luse file in which fallow is changed to row crops).

Assuming the results using the current conditions do not meet the regulations, evaluate the following scenarios to determine what action should be taken.

• Evaluate a change in the timing of nutrient application from fall to spring just prior to planting (the ansi.nut file should indicate low values for nutrients for both the April 1 and July 1 events).
• Evaluate a switch to no-till in the fields that are cropped for both the April 1 and July 1 events. Copy the hydrologic condition file to be used in generating curve numbers from the temp_hy_cond.good layer to the temp_hy_cond layer (g.copy temp_hy_cond.good,temp_hy_cond). This changes the hydrologic condition from fair to good. You will also need to modify the USLE C factors to reflect no-till conditions. Use RUSLE to obtain C values for the conditions described above for April 1 and July 1. You can use r.reclass to reclass the ansi.crops71 map to obtain appropriate C factor layers for each event (ansi.C). Evaluate for fertilizer application for fall and for spring just prior to planting.

Note that GRASS will allow you to also create a GIS layer with the same name that is used for layers in PERMANENT. For map layers named the same, the map layer that will be accessed is the one in the first mapset (note g.mapsets can be used to change order of mapsets).