Determine Duration of UHG
The duration of the derived unit hydrograph is found by examining the precipitation for the event and determining that precipitation which is in excess.
This is generally accomplished by plotting the precipitation in hyetograph form and drawing a horizontal line such that the precipitation above this line is equal to the depth of excess precipitation as previously determined.
This horizontal line is generally referred to as the F-index and is based on the assumption of a constant or uniform infiltration rate.
The uniform infiltration necessary to cause 1.65 inches of excess precipitation was determined to be approximately 0.2 inches per hour.