Skip to content
Terra Humana Foundation Logo
Terra Humana
ENDEIT
Water and slope

Infiltration

Terra Humana editorial teamLast updated: 2026-10-127 min read

Mulched soil where the rain soaks in, next to a crusted patch with a puddle

In brief

The entry of rain into the soil. How fast it happens depends on soil structure, cover, prior moisture and slope, and it decides between soil water and runoff.

What is infiltration?

Infiltration is the moment in which rain becomes soil water. Water arriving at the surface either enters the soil or it does not; what does not enter collects in hollows, evaporates or runs off down the slope. Infiltration is therefore the hinge on which a site's whole water balance turns. It decides whether a winter storm recharges the soil and the springs, or whether it leaves as a muddy flood.

Soil physicists measure it as a rate, usually in millimetres per hour. At the beginning of a rain shower a dry, open soil takes in water quickly, because its pores are empty and draw water in. As they fill, the rate falls until it levels off at a fairly constant value, the basic or steady infiltration rate. An FAO training manual gives guide values of about 10 to 20 millimetres per hour for loam and 1 to 5 millimetres per hour for clay, with sandy soils considerably faster. These are guide values for comparison, not a property fixed for all time: the same loam can take in many times more or many times less, depending on its structure.

Where rain becomes soil waterLearning panel on infiltration, the moment in which rain becomes soil water. It is decided in the first few centimetres of soil. Two soils in the same rain: on bare soil the drops break up the crumbs, fine particles seal the pores, a crust forms, and the water runs off and carries soil away. On covered, crumbly soil, mulch catches the drops, and continuous channels such as earthworm burrows and old root channels carry the water deep down. A dry, open soil takes in water quickly at the start of a rain, then the rate falls until it levels off at the steady infiltration rate. If rain falls faster than that, the excess runs off. FAO guide values: sand considerably faster, loam about 10 to 20 millimetres per hour, clay 1 to 5 millimetres per hour. Soil structure shifts these values strongly, in both directions. Hindering: crust from raindrop impact, compaction from machines and trampling, water repellency after long drought. Helping: plant cover and mulch against raindrop impact, roots and humus that build continuous channels, and terraces, swales and plantings along the contour that hold the water. TERRAWIKI · LEARNING PANEL Where rain becomes soil water It is decided in the first few centimetres of soil. 1TWO SOILS IN THE SAME RAIN crustrunoffmulchworm burrowroot channel Bare and crusted drops break up the crumbs,fine particles seal the pores,water runs off andcarries soil away Covered and crumbly mulch catches the drops,continuous channels carrythe water deep down 2WHEN RAIN RUNS OFF infiltration ratetime since rain began heavy rain steady infiltration rate excessruns off dry, open soildrinks quickly FAO GUIDE VALUES Sandconsiderably fasterLoam10 to 20 mm per hourClay1 to 5 mm per hour Soil structure shifts these values strongly,in both directions. 3WHAT HINDERS, WHAT HELPS HINDERS Crustfrom raindrop impact Compactionfrom machines and feet Water repellencyafter long droughtHELPS Cover and mulchagainst raindrop impact Roots and humusbuild continuous channels The contourterraces and swales hold Basis: FAO, Irrigation Methods (1988); Horton (1933) · TerraWiki: Infiltration

When rain becomes runoff

The American hydrologist Robert Horton described in the 1930s what happens when rain falls faster than the soil can absorb it: the excess runs off over the surface. This infiltration-excess runoff is typical of dry regions, crusted soils and short, intense storms. A second type occurs when the soil is already full: then even gentle rain runs off, because there is no room left. On Mediterranean slopes, the first type dominates. The rain often comes in heavy bursts, and it meets ground that has been dry and hard for months.

Every millimetre that runs off is lost twice. It is missing in the soil when the summer comes, and as it runs it gathers speed and carries soil away. Improving infiltration is therefore the most direct form of erosion control and one of the cheapest ways to store water.

What helps and what hinders

Water enters the soil through its pores, above all through the large, continuous ones: earthworm burrows, old root channels, cracks between soil crumbs. Everything that keeps these pathways open helps infiltration, and everything that closes them hinders it.

The most frequent obstacle forms at the surface. Raindrops striking bare soil break up its crumbs, and the fine particles wash into the pores and seal them. When the surface dries, a crust forms that can cut infiltration drastically. Compaction from machines, vehicles or trampling squeezes the larger pores together below the surface. A third obstacle is particularly important in the Mediterranean: after long drought many soils become water repellent, because dried organic coatings on the particles make water bead up instead of soaking in.

The remedies follow from this. Plant cover and mulch protect the surface from the impact of the drops. Roots and soil life, nourished by organic matter and humus, build a stable crumb structure with continuous channels. Avoiding compaction preserves what is there. And on slopes, the shape of the land helps: terraces, swales and plantings along the contour hold the water in place long enough for even a slow soil to absorb it. That is the shared principle behind rainwater harvesting and keyline design.

Measuring infiltration

Infiltration can be measured without a laboratory. The USDA's Soil Quality Test Kit Guide describes a simple ring test: a metal ring 15 centimetres in diameter is driven a few centimetres into the ground, and 444 millilitres of water, equivalent to 2.5 centimetres of rain, are poured in. The time is recorded until no water stands in the ring any more and the surface only just glistens. On dry soil the first pour often only wets the ground, so a second pour gives the more meaningful value.

Professionals work with double-ring infiltrometers, in which an outer ring keeps the water in the inner ring from spreading sideways, or with tension infiltrometers that measure the flow through the soil matrix under controlled conditions. For practical work on a piece of land, absolute precision matters less than comparison: under the old olive trees and on the bare path, in a moist channel and on the dry main area, before and after a few years of ground cover. Measured repeatedly at the same spots, the simple ring shows whether the soil is learning to drink again.

How Terra Humana understands it

On dry land the problem is often not that too little rain falls, but that too little of it gets into the soil. Infiltration is where that is decided, in the first few centimetres of the ground. That is why we look at the surface before we look at the water tank. A soil that drinks quickly needs fewer pipes later, and every improvement in its structure and cover makes the following winter's rain worth more.

The valley of Sa Medada near Jerzu, Sardinia, with olive grove and macchia

Sa Medada

What this looks like at Sa Medada

At Sa Medada the dry season runs from May to September, so the rain of the cool season falls on ground that has been dry for months. The LiDAR wetness map shows a comparatively dry main area and moister channels along the edges. Swales are planned, not yet built. Project Eden begins with pioneer plants, ground cover and holding water, so that more of the winter rain enters the soil instead of running off.

More about Project Eden

Frequently asked

What is infiltration?

Infiltration is the process by which water at the surface enters the soil. It is measured as an infiltration rate, usually in millimetres per hour. At the start of a rain shower a dry, open soil takes in water quickly; as the pores fill, the rate falls until it settles at a fairly constant value, called the basic or steady infiltration rate. Whatever rain falls faster than the soil can take it either collects in hollows or runs off over the surface. Infiltration therefore decides how much of the rain becomes soil water and groundwater, and how much becomes runoff and erosion.

How fast does water soak into soil?

That depends above all on the soil. An FAO training manual gives guide values for the basic infiltration rate of about 10 to 20 millimetres per hour for loam and 1 to 5 millimetres per hour for clay, while sandy soils take in water considerably faster. Structure can shift these values strongly in either direction: a crumbly loam full of earthworm channels can absorb far more than the guide value, a compacted or crusted one far less. A heavy Mediterranean downpour can easily deliver more water per hour than a crusted clay can absorb.

How do you measure infiltration in the field?

The simplest method is the ring test described in the USDA's Soil Quality Test Kit Guide. A metal ring 15 centimetres in diameter is driven a few centimetres into the soil, and 444 millilitres of water, equivalent to 2.5 centimetres of rain, are poured in. The time until the water has disappeared from the surface is measured. Because the first pour often only moistens dry soil, a second pour gives a more meaningful value. Professionals use double-ring or tension infiltrometers. For comparing places on the same land, the simple ring is entirely sufficient.

What improves infiltration?

Everything that keeps the surface open and the pores connected. Plant cover and mulch absorb the energy of raindrops and prevent the surface from sealing into a crust. Roots and soil animals, especially earthworms, create continuous channels through which water can flow deep into the soil. Humus and a stable crumb structure keep the pore space open. Avoiding compaction from vehicles and trampling preserves it. On slopes, terraces, swales and contour plantings help by holding water in place long enough for it to soak in.

Why does dry soil sometimes repel water?

Because many soils become water repellent when they dry out completely. Organic compounds from plant residues, roots and fungi coat the soil particles and, when dry, behave like a thin wax layer. The first rain then beads up on the surface and runs off instead of soaking in, even though the soil underneath is thirsty. The effect is common under Mediterranean shrubs and pines, after long dry periods and after fires. It usually weakens again once the soil has been moist for some time, which is why the first autumn rains are the most critical.

Aerial view of a slope with planting rows along the contoursNext in this themeKeyline DesignA land-planning method that reads a slope's shape to move rainwater from moist valleys toward drier ridges, storing it in the soil.

Related terms

Crumbly, living soil next to a dense, smooth clod of compacted earthSoil StructureThe way soil particles are arranged into crumbs and the pores between them. It decides how a soil breathes, takes in water and lets roots through.A lifted layer of straw and wood-chip mulch with moist dark soil underneath, next to a dry, cracked pathMulchA layer of organic or mineral material on the soil surface that cuts evaporation, keeps the ground cooler, stops erosion and feeds soil life.Water drops beading on parched soil instead of soaking inSoil Water RepellencyA soil surface that repels water after fire or severe drying. Rain beads up and runs off instead of soaking in.Deep tractor ruts in a wet field where rainwater stands instead of soaking inSoil CompactionThe loss of pore space through pressure, from machines, hooves or feet. Compacted soil lets less water and air in and blocks roots.Two swales along the contour after rain: still water in the level trenches, mulched berms with plants belowRainwater Harvesting & SwalesHolding rain where it falls: soaking it into ditches and basins, storing it in dams and cisterns, spreading it along the shape of the slope.Aerial view of a slope with planting rows along the contoursKeyline DesignA land-planning method that reads a slope's shape to move rainwater from moist valleys toward drier ridges, storing it in the soil.

Mentioned in

See this in practice

Project Eden →

Sources

  • FAO (Brouwer et al.), Irrigation Water Management Training Manual No. 5: Irrigation Methods, Annex 2, Infiltration rate and infiltration test (1988)
  • USDA ARS and NRCS, Soil Quality Test Kit Guide (1999)
  • D. Hillel, Introduction to Environmental Soil Physics, Elsevier Academic Press (2004)
  • R. E. Horton, The role of infiltration in the hydrologic cycle, Transactions of the American Geophysical Union 14 (1933)
  • Doerr, Shakesby and Walsh, Soil water repellency: its causes, characteristics and hydro-geomorphological significance, Earth-Science Reviews 51, pp. 33-65 (2000)

Last updated: 2026-10-12