7.3 Problem Set
- 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.
- 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:
- Zoom in to an area in the United States covering ~1 square mile.
- Build the channel network.
- Select the watershed outlet point of interest.
- Build the subcatchments.
- 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
- Close the watershed summary window.
- If necessary, change land use associations.
- Setup the WEPP model.
- Record the distance to the selected climate station.
- Specify watershed and flow path simulation for a 10 year period.
- Instruct the model to determine land use and soil type for each grid cell.
- Run the WEPP model. This may take several minutes.
- After the simulations are completed, view the erosion maps.
- 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)
- If desired, save your project for future reference.