Question

In situ unsaturated hydraulic conductivity is measured by

• A. Tension infiltrometer
• B. Double-ring infiltrometer
• C. Tensiometer
• D. Neutron probe

Hint:

To measure "unsaturated" water flow, we must exclude flow through large cracks and wormholes (macropores). A "tension" infiltrometer does this by keeping the water under suction, ensuring it only moves through the unsaturated soil matrix.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Hydraulic conductivity measures the soil's ability to transmit water.
It is highly dependent on soil water content, being maximal at saturation and decreasing rapidly as the soil dries (unsaturated conditions).
Measuring unsaturated conductivity in-situ requires applying water under negative pressure (tension) to prevent flow through larger macropores.

Step 2: Detailed Explanation:
Let us analyze the functions of the listed instruments:
Tension Infiltrometer (A): Also known as a disc permeameter, this device applies water to the soil surface under a controlled negative pressure (tension).
Because the water is under tension, it cannot enter large macropores (like cracks, wormholes, or root channels) and flows only through smaller mesopores and micropores.
This makes it the standard instrument for measuring in-situ unsaturated hydraulic conductivity.
Double-ring Infiltrometer (B): This device ponds water on the soil surface, forcing it to infiltrate under positive pressure.
It is used to measure saturated infiltration rates and saturated hydraulic conductivity.
Tensiometer (C): This instrument measures soil matric suction (water potential), not hydraulic conductivity.
Neutron Probe (D): This device is used to measure soil volumetric water content, not conductivity directly.

Step 3: Final Answer:
In-situ unsaturated hydraulic conductivity is measured using a Tension infiltrometer, corresponding to option (A).