Why is the outer ring kept at the same water level as the inner ring?

To prevent lateral spreading from the inner ring

How is infiltration capacity computed?

f = dV/(A_inner dt), final steady value is f_c

What defines the phi-index?

Constant rate above which rainfall volume equals runoff volume

Average rate while intensity exceeds capacity, excluding storage

How does the W-index differ from phi?

It subtracts depression and interception storage, so is slightly smaller and more accurate

2076 Chaitra · 5+3 marks

Double ring infiltrometer, phi-index versus W-index

Short answer

A double-ring infiltrometer uses an outer buffer ring to ensure vertical infiltration; capacity is the steady rate from inner-ring water added over time. The φ-index is a constant loss rate yielding rainfall excess equal to runoff, while the W-index subtracts initial losses and applies only when intensity exceeds capacity.

Explain with neat sketch to determine the infiltration capacity of soil by using double ring infiltrometer. How do you differentiate $\phi$ index from $\omega$ -index?

Double ring infiltrometer

It consists of two concentric metal rings: an inner ring (about 30 cm diameter) and an outer ring (about 60 cm diameter), each 25 to 30 cm tall.

   ___________________________
  |   outer   |inner|  outer  |
  |   ring    |ring |  ring   |
  |  ~~~~~~~~~|~~~~~|~~~~~~~~ | <- same water level
  |           |     |         |
 =|===========|=====|=========|= ground
              |  |  |
              v  v  v  vertical flow

Procedure:

Drive both rings about 15 cm into the ground, keeping them level and concentric.
Fill both rings with water to the same depth (about 5 cm) and keep this depth constant by adding water.
The water in the outer ring prevents lateral spreading from the inner ring.
Record the volume added to the inner ring at fixed intervals (e.g. 5, 10, 15, 30, 60 min) until the rate becomes steady.
Infiltration capacity $f = \dfrac{\Delta V}{A_{inner}\,\Delta t}$ is plotted against time; the final steady value is $f_c$ .

Difference between $\phi$ -index and W-index ( $\omega$ -index)

Point	$\phi$ -index	W-index
Definition	Constant infiltration rate above which the rainfall volume equals the runoff volume	Average infiltration rate during the time that rain intensity exceeds infiltration capacity
Formula	$\phi = (P-R)/t_e$ , where $t_e$ is the duration of rainfall excess	$W = (P-R-I_a)/t_r$ , where $I_a$ is the initial loss (interception and depression storage) and $t_r$ is the time when rain intensity exceeds infiltration capacity
Initial loss	Included in the loss	Subtracted separately (interception and depression storage are removed first)
Period used	Periods with rain below $\phi$ are not given any excess	Only the time when intensity exceeds capacity is counted
Value	Usually slightly larger	Usually slightly smaller
Accuracy	Rough	Better, as it accounts for storage

✓ Answer checked by Claude Sonnet 5.5

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