4.1 Soil properties and their influence on infiltration

Soil Properties and Infiltration

Soil properties control infiltration, the process by which water at the surface enters the ground. Texture, structure, porosity, and moisture content together determine how fast water can move into and through the soil profile. Getting a handle on these properties is essential for predicting runoff, groundwater recharge, and soil water storage.

Key Soil Properties for Infiltration

Three physical properties of soil have the most direct influence on infiltration: texture, structure, and porosity. They're closely related, but each describes something different.

Soil texture refers to the relative proportions of sand, silt, and clay particles. Texture controls the size of pore spaces between particles, which in turn controls how easily water flows through.

• Sandy soils have large pore spaces, so water infiltrates quickly. Think of water poured onto beach sand: it disappears almost immediately.
• Clayey soils have much smaller pore spaces, so water infiltrates slowly. Water tends to pond on the surface of a clay-rich soil before gradually soaking in.
• Silty soils fall in between. The USDA soil textural triangle classifies soils by their sand-silt-clay percentages, and you can use it to estimate relative infiltration behavior.

Soil structure describes how individual particles clump together into aggregates (also called peds). Structure determines the arrangement and connectivity of pore spaces beyond what texture alone would predict.

• Well-structured soils have stable, crumbly aggregates with large inter-aggregate pores that act as pathways for water. Healthy topsoil under perennial vegetation is a good example.
• Poorly structured soils have weak or no aggregation, so pore spaces are smaller and less connected. Compacted subsoil or heavily tilled ground often falls into this category.

Soil porosity is the fraction of total soil volume occupied by pore space (air + water), typically expressed as a percentage. A sandy soil might have a porosity around 35–40%, while a well-aggregated clay soil can reach 50–60% because of its mix of micro- and macropores.

• Higher porosity means more room for water to enter and be stored, generally supporting faster infiltration.
• Lower porosity restricts water entry. Hardpan layers or cemented horizons in the soil profile can have very low porosity and act as barriers.

Soil Moisture and Hydraulic Conductivity

Soil moisture content is the amount of water already present in the pore spaces. It has a strong effect on how fast additional water can infiltrate.

• When soil is dry, two forces pull water in: gravity and capillary suction (the tendency of water to be drawn into small, dry pores). Together, these produce high initial infiltration rates. A desert soil after a long dry spell will absorb the first rain very quickly.
• As pores fill with water, capillary suction decreases because fewer dry pores remain. Infiltration rate drops and eventually approaches a steady, lower value once the soil is near saturation.

Hydraulic conductivity (often symbolized $K$) measures a soil's ability to transmit water through its pore network. It depends on texture, structure, and pore size distribution.

• Coarse-textured soils like gravel or coarse sand have high $K$ values, sometimes exceeding 100 mm/hr.
• Fine-textured soils like clay may have $K$ values below 1 mm/hr.
• Hydraulic conductivity is not fixed for a given soil; it changes with moisture content. Saturated hydraulic conductivity ($K_{sat}$) is the maximum value, measured when all pores are filled with water. Unsaturated $K$ is always lower because air-filled pores don't transmit water.

Soil Characteristics vs. Infiltration Capacity

Infiltration capacity is the maximum rate at which a soil surface can absorb water under a given set of conditions. If rainfall intensity exceeds infiltration capacity, the excess becomes surface runoff.

Infiltration capacity depends on the same properties discussed above, but it also changes over time during a storm event:

1. At the start of rainfall on dry soil, infiltration capacity is high because capillary suction is strong and pore space is available.
2. As the soil wets up, capillary suction weakens and pores fill, so infiltration capacity decreases.
3. Eventually, the rate levels off at a roughly constant value close to $K_{sat}$ for the surface soil. This steady-state rate is sometimes called the final infiltration rate.

The main factors that set a soil's infiltration capacity:

• Texture: Sandy soils generally have higher infiltration capacities than clayey soils. A sand dune surface might accept water at 50+ mm/hr, while a clay plain might accept less than 5 mm/hr.
• Structure: Well-aggregated soils (e.g., healthy grassland) infiltrate faster than degraded soils with poor structure (e.g., overgrazed pasture), even if their textures are similar.
• Porosity: Higher effective porosity supports greater infiltration capacity. Volcanic ash deposits, for instance, are extremely porous and infiltrate rapidly.
• Antecedent moisture: Soil that has been dry for weeks will have a much higher initial infiltration capacity than the same soil after recent rain.

Impact of Compaction on Infiltration

Two surface-level processes can dramatically reduce infiltration, even in soils that would otherwise drain well: compaction and surface sealing.

Soil compaction is the reduction of pore space caused by external pressure. It crushes aggregates and closes macropores, lowering both porosity and hydraulic conductivity.

• Common causes: heavy machinery (construction sites, farm equipment), livestock trampling (feedlots, overgrazed pastures), and repeated foot traffic (hiking trails, sports fields).
• Compacted soil can have infiltration rates several times lower than the same soil in an undisturbed state.

Surface sealing (also called crusting) is the formation of a thin, low-permeability layer at the soil surface.

• It happens when raindrop impact breaks apart surface aggregates and fine particles wash into pore openings, plugging them. Bare agricultural fields are especially vulnerable.
• Even a crust just a few millimeters thick can cut infiltration rates substantially because all water must pass through it to enter the soil below.

Management practices to reduce compaction and sealing:

• Avoid driving heavy equipment or allowing livestock on soil when it's wet, since wet soil compacts more easily.
• Use conservation tillage or no-till methods to preserve soil structure and macropore networks. Leaving crop residue on the surface also helps.
• Maintain surface cover (mulch, plant litter, cover crops) to absorb raindrop energy and prevent aggregate breakdown.
• Incorporate organic matter (compost, manure, plant roots) to build stable aggregates and increase porosity over time. Organic matter acts as a binding agent for soil particles, improving both structure and water-holding capacity.