Hydrology — Why Infiltration Rate Falls Early Then Levels Off During a rainstorm, the infiltration rate commonly declines rapidly at first and then approaches a near-constant (quasi-steady) value after several hours. Which of the following factors together explain this behavior? I. Filling of fine soil pores with water lowers capillary suction and reduces the initial infiltration capacity. II. Clay minerals swell as they moisten, narrowing pore throats and reducing permeability. III. Raindrop impact breaks aggregates and splashes fines that seal the surface, clogging pores.

Introduction / Context:
Understanding why infiltration rate declines during storms is a core concept in surface hydrology, infiltration modeling, and watershed management. Early-time infiltration is often high due to strong capillary suction and dry pore space. As rain continues, hydrologic and soil-structural processes act to reduce intake, eventually approaching a near-constant, lower value known as the steady or equilibrium infiltration rate (often approximated by the saturated hydraulic conductivity at field scale).

Given Data / Assumptions:

• The storm persists for several hours and the soil is initially unsaturated.
• Three candidate mechanisms are provided: loss of capillary suction as pores fill, clay swelling that reduces pore size, and surface sealing by raindrop impact and fine-particle migration.
• We evaluate whether all three contribute materially to the observed infiltration pattern.

Concept / Approach:

Physically based infiltration theories (e.g., Green–Ampt and Richards-equation reasoning) show early infiltration is driven by strong matric (capillary) potential gradients into a dry soil. As the wetting front advances and the profile wets, matric suction declines, so intake slows. Simultaneously, swelling clays can reduce pore radius and hydraulic conductivity. At the surface, raindrop impact disrupts aggregates and mobilizes silt and clay, forming seals and crusts that impede entry. Each factor pushes the rate downward toward a quasi-steady limit.

Step-by-Step Solution:

Identify early drivers: large capillary gradients + empty pore space → high initial infiltration.As pores fill, matric suction decreases → reduced driving force (supports I).Smectitic and other expansive clays swell upon wetting → smaller pore throats, lower K (supports II).Raindrop impact + kinetic energy → aggregate breakdown; fines plug pores and create crusts → additional resistance (supports III).All mechanisms act together → select the inclusive choice.

Verification / Alternative check:

Field observations show sharp declines in infiltration rate within minutes to hours, with especially strong crusting effects on bare soils and notable reductions in swelling clay terrains. Laboratory rainfall simulators replicate these outcomes, confirming the combined role of hydraulic and mechanical processes.

Why Other Options Are Wrong:

I, II or I, III or II, III: Each subset omits at least one real, documented mechanism, giving an incomplete explanation of the typical field behavior.

Common Pitfalls:

Assuming the steady rate equals a lab-saturated conductivity without considering surface sealing, macropore collapse, or structural change. Also, ignoring antecedent moisture and vegetation cover, both of which alter early-time dynamics.

Final Answer:

All of the above