CEEn 531

The ModClark model was developed to allow you to take advantage of spatial variations of different watershed parameters like rainfall and CN.  Rather than using a time area curve like the Clark model it subdivides the grid into a number of grid cells and lags the flow to the outlet.  If you were to assign a constant CN value to each cell, theoretically you should get the same result with Clark and ModClark, but the ModClark allows you to examine better the spatial variability of rainfall excess rather than averaging things together.

Part 1

Before applying ModClark to the American Fork watershed I want you to get a little experience and confidence running it within WMS.  Click here to download the tutorial data and complete it.  Run the tutorial three different times using resolutions of 10x10, 25x25, and 50x50 (the tutorial specifies 50x50 only but all other steps should be the same).  Show the results of each and include a brief discussion on your assignment web page.

Part 2

Now that you have some experience getting a spatially distributed model like the ModClark method in HMS I want you to compare it with the Clark method in HEC-1.  You should use the American Fork Watershed that we have done most of the other investigations into hydrologic methods in HEC-1 to run this comparison.  Do the following:

1. Run your American Fork watershed analysis with WMS/HEC-1 using the following methods/parameters
   1. NOAA Atlas 100 year 24 hour storm with the SCS Type II temporal distribution.
   2. A single basin with a CN computed from your land use and soils data and the table used in the earlier CN assignment .
   3. The Clark unit hydrograph method computing the time area curve from the DEM data and estimating Tc and R with the SCS lag equation.
   4. Use a six minute time step and be sure you run the simulation for at least 48 hours.
2. Run your American Fork Watershed with WMS/HMS using the following parameters
   1. NOAA Atlas 100 year 24 hour storm with the SCS Type II temporal distribution.
   2. Use a grid where every cell has the same CN value as you computed for the average in the HEC-1 model above.  You decide an appropriate resolution based on your findings from Part 1.  The easiest way to get a constant CN value may be to create a single polygon land use coverage where the CN value for hydrologic soil groups A, B, C, and D are all that value, then be sure to use this land use file when mapping the gridded parameters.
   3. The ModClark Unit hydrograph method using the SCS lag equation to estimate Tc and R (you may have to enter the computed average CN value if it is 0).
   4. Use a six minute time step for two days.
3. Run your American Fork Watershed the same as step 2, but this time use your actual land use and CN mapping table to compute a different CN value for each grid cell.

Compare the Clark and ModClark methods from runs 1 and 2, and then average and spatially varying CN values from runs 2 and 3.  Report your findings on your assignment web page.  I recommend that you export your hydrographs to Excel so that you can overlay and compare results better.