University of Central Florida Civil and Environmental Engineering CWR4101 Hydrology Laboratory 5B Infiltration Using NRCS Curve Number

OBJECTIVE

• A. To understand the use of a spreadsheet in Hydrology
• B. To determine the relationship between NRCS curve number, rainfall excess, and rainfall.

EQUIPMENT

Computer with spreadsheet software.

PROCEDURE

• A. Using the equations from the text (page 84) plot rainfall excess vs. precipitation for curve numbers of 30, 50, 70, 80, 90, and 100. Precipitation (on the x axis) should vary from 0.0 to 10.0 inches.
• B. For a 6 hr storm with constant rainfall intensity, calculate cumulative infiltration vs. time using half hour increments. Use a spreadsheet to perform the calculations of rainfall excess and infiltration. The total storm precipitation is 4.5". Perform the calculation for the curve numbers of 30, 50, 70, 80, 90, and 100. Calculate both the incremental infiltration and the total infiltration. Plot the cumulative infiltration for all six curve numbers on the same plot (cumulative infiltration as y vs. time as x).
• C. For the problem in part B calculate an infiltration rate ( Change in Infiltration/Change in Time) for the storm duration. Plot this infiltration rate vs. time for the six curve numbers. What does the area under this curve represent? How are the curves in part B and part C related. How does the plot of cumulative infiltration relate to a plot of cumulative runoff? How does the plot of infiltration rate relate to a plot of runoff rate? (Hint: remember the rainfall intensity or rainfall rate is constant).