# 7.3 Problem Set

1. Use the curve number (CN) method to estimate the total runoff (mm) and the percent of annual precipitation partitioned to runoff (%) during the year 2013 for a field near Stillwater, OK, under two different scenarios:
• Fallow – bare soil with poor hydrologic condition, hydrologic group C, and CN = 91.
• Small grain – contour farmed with adequate crop residue, soil with poor hydrologic condition, hydrologic group C, and CN = 81.

An Excel spreadsheet with 2013 daily rainfall data from the Oklahoma Mesonet is available online at: http://soilphysics.okstate.edu/teaching/soil-4683/homework/

Work using rainfall in inches, and convert the final result to mm. Perform daily runoff calculations, then compute the annual sum. The “IF” function in Excel may be helpful.

1. Use the Online GIS Interface for the WEPP model (link) to estimate runoff and erosion for a watershed of your own choosing following these steps:
1. Zoom in to an area in the United States covering ~1 square mile.
2. Build the channel network.
3. Select the watershed outlet point of interest.
4. Build the subcatchments.
5. Review the watershed summary, recording:
• outlet coordinates to three decimal places
• two largest land uses and their percentages
• two largest soil types and their percentages
6.  Close the watershed summary window.
7. If necessary, change land use associations.
8. Setup the WEPP model.
9. Record the distance to the selected climate station.
10. Specify watershed and flow path simulation for a 10 year period.
11. Instruct the model to determine land use and soil type for each grid cell.
12. Run the WEPP model. This may take several minutes.
13. After the simulations are completed, view the erosion maps.
14. View the simulation summary, recording:
• the watershed area (ha)
• the annual precipitation (mm yr-1)
• the annual discharge (mm yr-1)
• the sediment yield (t ha-1 yr-1)
15.  If desired, save your project for future reference.