26.5.1. φ-Index (Phi Index)

Try these first

1. 1.

   Define φ-Index in hydrology.

   Hint

   Think about how water movement into the soil is measured.

2. 2.

   Explain the application of the φ-Index in the Rational Method.

   Hint

   Consider how runoff relates to infiltration rates.

3. 3.

   What does the φ-Index measure?

   • Rate of evaporation
   • Average infiltration rate
   • Rate of irrigation

   Hint

   Think about its role in hydrology.

4. 4.

   True or False: The φ-Index is variable throughout a storm event.

   • True
   • False

   Hint

   Consider how we use it in calculations.

5. 5.

   A region experiences varying φ-Index values during different seasons (winter 10mm/hr, summer 25mm/hr). If a storm delivers 50mm of rain, calculate how much runoff occurs in both seasons if the rainfall intensity is 15mm/hr.

   Hint

   Calculate the difference between rainfall and φ-Index for both cases.

6. 6.

   During a storm, if the φ-Index is 12mm/hr and rainfall is 36mm/hr for three hours, determine the total runoff.

   Hint

   Keep track of time and the difference in rates.