Water Infiltration

Infiltration is distinct from the redistribution and drainage of water once it is inside the soil. It concerns specifically the passage of water across the surface into the topmost soil. The instantaneous infiltration rate declines over the course of a rainfall or irrigation event as the surface pores fill and the wetting front moves downward, eventually settling toward a near-steady rate.

Infiltration rate

The speed at which water enters the soil surface, usually declining during an event.

Wetting front

The boundary between wetted soil above and drier soil below as water moves down.

Runoff

Water that fails to infiltrate and flows across the surface, often carrying soil with it.

• Surface condition: crusting, sealing, and compaction sharply reduce infiltration, while residue cover and open structure raise it
• Texture: coarse, sandy surfaces generally infiltrate faster than fine, clayey ones
• Structure and macropores: stable aggregates, cracks, root channels, and earthworm burrows create fast pathways for water
• Initial moisture: dry soil infiltrates quickly at first, then slows as it wets
• Slope and rainfall intensity: steep slopes and intense rain increase the share of water lost to runoff

Infiltration governs how effectively rainfall and irrigation are captured. Where it is high, more water is stored in the root zone and less is lost to runoff and erosion. Where it is low, water ponds or runs off, crops receive less of the water applied, and soil and nutrients are carried away, degrading both the field and downstream water quality.

Management that protects the surface and builds structure improves infiltration. Keeping the surface covered with residue or growing plants shields it from crusting, while reduced tillage, organic matter, and stable aggregation maintain the macropores and channels that let water in.

• Maintain surface cover with residue, mulch, or cover crops to prevent crusting and slow runoff
• Reduce tillage and traffic to preserve macropores, aggregates, and biopores
• Build organic matter and stable aggregation to keep the surface open
• Use contour practices or roughened surfaces on slopes to give water more time to infiltrate