The quantity of water that the sub-soil retains against the pull of gravity is called what?

Introduction:
Aquifer storage concepts distinguish between water that can drain freely under gravity and water that remains held within the pore structure by capillary and adsorptive forces. Correct terminology is crucial in groundwater resource evaluation.

Given Data / Assumptions:

• We are asked for the term describing water retained against gravity.
• Terms offered include porosity, specific yield, and specific retention.

Concept / Approach:
Porosity is the fraction of voids; it does not indicate recoverability. Specific yield (Sy) is the fraction of water that will drain due to gravity. Specific retention (Sr) is the fraction that remains attached/held and does not drain freely. Sy + Sr ≈ n (effective relationship for many granular media).

Step-by-Step Solution:
Step 1: Define porosity (n) and note it includes all void water.Step 2: Define specific yield (Sy) as drainable water.Step 3: Define specific retention (Sr) as the retained water against gravity.Step 4: Match the description to Sr.

Verification / Alternative check:
Field pumping tests and lab drainage curves show that fine-grained soils have higher Sr (more water retained), while coarse sands have higher Sy.

Why Other Options Are Wrong:

• Yield: Generic term; not a precise storage parameter.
• Porosity: Includes both drainable and retained water; not specific to retention.
• Specific yield: Opposite of what is asked (the drainable fraction).
• None of these: Incorrect because Sr is a standard term.

Common Pitfalls:

• Assuming porosity equals recoverable water.
• Ignoring capillary/adsorptive forces in fine soils.

Final Answer:
Specific retention.