31.3. Common Infiltration Indices

Try these first

1. 1.

   What does the φ-index represent?

   Hint

   Think about how rainfall exceeds what can be absorbed.

2. 2.

   Name a factor that the W-index considers that the φ-index does not.

   Hint

   Consider how much water gets intercepted before infiltrating.

3. 3.

   What does the φ-index fail to account for?

   • Initial abstraction
   • Cumulative infiltration
   • Runoff volume

   Hint

   Think about what happens before water begins to infiltrate.

4. 4.

   True or False: The W-index gives a more accurate representation of infiltration by including initial losses.

   • True
   • False

   Hint

   Recall the definition of the W-index.

5. 5.

   During a heavy storm, you total 100 mm of rainfall over 5 hours, with an observed runoff of 40 mm and initial abstraction of 15 mm. Calculate the φ-index and W-index.

   Hint

   Use the relevant formulas for each index.

6. 6.

   Consider a scenario where you have a test catchment with a varying rainfall and runoff pattern. How would you analyze which infiltration index to use, based on historical data?

   Hint

   Identify the data characteristics and storm behavior.