Infiltration and Soil Water

Infiltration is the entry of water into the soil, and soil-water dynamics in the unsaturated zone partition precipitation between runoff, storage, evapotranspiration, and recharge to groundwater.

Definition

Infiltration is the movement of water from the surface into the soil, and soil water is the water held in the unsaturated zone; their dynamics, governed by capillary and gravitational forces, determine how precipitation is partitioned at and below the land surface.

Scope

This topic covers the process and rate of infiltration, the physics of water movement in the unsaturated (vadose) zone, the soil-water characteristic relationships, and the role of soil water in linking surface and subsurface hydrology. It focuses on the unsaturated soil column; saturated groundwater flow is treated in the groundwater area.

Core questions

• What controls the rate at which water infiltrates the soil?
• How does water move through the unsaturated zone?
• How is soil water held against gravity, and how is it characterized?
• How does soil water connect surface runoff, evapotranspiration, and recharge?

Key concepts

• Infiltration rate and capacity
• Unsaturated (vadose) zone
• Matric potential and capillarity
• Soil-water characteristic curve
• Richards equation
• Field capacity and wilting point

Key theories

Richards equation

Richards combined Darcy's law for unsaturated media with continuity to give a partial differential equation governing water flow in the vadose zone, the physical basis for modeling infiltration and soil-water movement.

Infiltration theory

Analytical and conceptual infiltration models, such as Philip's solution and the Green-Ampt approach, describe how infiltration capacity declines with time as the soil wets up, predicting when rainfall begins to generate runoff.

Mechanisms

Water entering the soil is held by capillary forces in the pores and driven downward by gravity; the infiltration rate is high initially and declines toward the saturated conductivity as the soil wets and the matric gradient weakens. The unsaturated conductivity and the relationship between water content and matric potential, captured by the soil-water characteristic curve, govern how fast and how far water moves.

Clinical relevance

Infiltration and soil-water behavior control the onset and amount of surface runoff and thus flooding, the water available to plants and the timing of irrigation, the recharge of aquifers, and the transport of agricultural chemicals toward groundwater.

History

Building on Buckingham's extension of Darcy's law to unsaturated soils, Richards in 1931 formulated the governing equation for vadose-zone flow. Mid-20th-century work by Philip and by Green and Ampt produced practical infiltration theories that connected soil physics to catchment runoff generation.

Key figures

• Lorenzo A. Richards
• John R. Philip
• S. Lawrence Dingman

Seminal works

• richards1931
• philip1957
• dingman2015

Frequently asked questions

Why does infiltration slow down during a storm?

As the soil wets up, its pores fill and the capillary gradient pulling water in weakens, so the infiltration rate falls from a high initial value toward the soil's saturated hydraulic conductivity; once rainfall exceeds this rate, runoff begins.

What is the vadose zone?

The vadose, or unsaturated, zone is the soil and rock between the land surface and the water table, where pores hold both air and water; water moves through it under combined capillary and gravitational forces before reaching the saturated groundwater below.