Hydrology and soil physics – name of the property describing a soil’s ability to absorb incoming rainfall In surface-hydrology practice, what is the term used for the capacity of a soil surface to admit water during rainfall and thereby absorb moisture into the profile?

Introduction / Context:
The question tests the distinction between several soil–water terms used in hydrology and geotechnical engineering. When rain strikes the ground, how much of it enters the soil rather than running off depends on the soil’s ability to admit water at the surface—this is called infiltration, and the limiting rate is the infiltration capacity.

Given Data / Assumptions:

• We are concerned with rainfall reaching a soil surface.
• We want the soil property governing admission of water at the surface.
• Steady or time-varying rainfall may occur, but the definition is independent of a specific storm.

Concept / Approach:
Infiltration capacity = the maximum rate (or shorthand, capacity) at which a given soil, in a given condition, can absorb water through the surface. It depends on texture, structure, antecedent moisture, surface sealing, vegetation, and compaction. It is distinct from porosity (volume fraction of voids) and permeability (hydraulic conductivity governing flow within the saturated/unsaturated medium).

Step-by-Step Solution:
Identify the phenomenon at the soil surface during rainfall → infiltration.Select the specific term describing that limiting ability → infiltration capacity.Reject related but different bulk properties (porosity, permeability) or vague descriptors (perviousness).

Verification / Alternative check:
In infiltration models (e.g., Horton, Green–Ampt), the rate i(t) is capped by the infiltration capacity f(t). When rainfall intensity exceeds f(t), excess becomes surface runoff.

Why Other Options Are Wrong:

• Permeability: property controlling internal flow; not specifically the surface admission limit.
• Porosity: volume fraction of voids; does not directly set instantaneous intake rate.
• Perviousness: qualitative term; lacks the specific quantitative meaning.
• Sorptivity: early-time capillary-driven intake parameter, not the general capacity term sought here.

Common Pitfalls:
Using permeability and infiltration capacity interchangeably; a soil may be highly porous yet have low infiltration due to surface sealing.

Final Answer:
Infiltration capacity