Question

Rate of flow of water through a porous medium under unit hydraulic gradients is known as

• A. Flux density
• B. Hydraulic conductivity
• C. Permeability
• D. Infiltration capacity

Hint:

Darcy's Law: \( v = Ki \).
If \( i = 1 \), then \( v = K \).
Think of \( K \) as the "conductivity" or "speed limit" of water in a particular soil.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks for the specific physical parameter defined by Darcy's Law when the driving force (hydraulic gradient) is equal to one.
Key Formula or Approach:
Darcy's Law is expressed as:
\[ v = K \times i \]
Where:
\( v \) = velocity of flow (discharge per unit area)
\( K \) = Hydraulic conductivity
\( i \) = hydraulic gradient (\( dh/dl \))

Step 2: Detailed Explanation:

• Definition of Hydraulic Conductivity (K):
If the hydraulic gradient (\( i \)) is set to 1 (unit gradient), the equation becomes \( v = K \).
Therefore, hydraulic conductivity is defined as the flux density (velocity) of water moving through a soil when the potential drop over a unit distance is equal to one unit.

• Factors Affecting K:
It depends on both the soil (pore size, connectivity) and the fluid (viscosity, density).

• Comparison with Intrinsic Permeability:
"Permeability" (\( k \)) is a property of the soil matrix alone and is independent of the fluid properties. Hydraulic conductivity (\( K \)) is the practical measure used in irrigation and drainage.

• Infiltration Capacity:
This refers specifically to the maximum rate water enters the surface of the soil. It is a specific case of hydraulic conductivity at the soil-atmosphere interface.

Step 3: Final Answer:
The rate of flow under a unit hydraulic gradient is the hydraulic conductivity.