Why is suction plotted on a log scale?

Soil water spans a huge range of matric potentials, from near-saturation to thousands of kPa at wilting. A log axis shows both the wet shoulder and the dry tail on one graph.

What do α and n mean physically?

α roughly sets the air-entry suction scale: larger α means the soil drains at lower suction. n controls the steepness of the pore-size distribution; larger n gives a sharper drainage curve typical of coarser soils.

Why can θ still be high near wilting point?

Fine-textured soils may retain substantial water at high suction, but it is tightly held in small pores and films, so plants cannot extract it easily and hydraulic conductivity is very low.

Hysteresis between wetting and drying, salinity/osmotic potential, soil structure, preferential flow, root uptake, and layered profiles are not modeled.

Soil Water Retention Curve

Soil water retention curves relate volumetric water content θ to matric suction |ψ|. This simulator uses the van Genuchten form S_e = [1 + (α|ψ|)^n]^{-m}, with m = 1 − 1/n and θ = θ_r + S_e(θ_s − θ_r), where θ_r is residual water content, θ_s is saturated water content, α sets an air-entry suction scale, and n controls pore-size distribution. Hydraulic conductivity is estimated with the Mualem-van Genuchten expression K = K_s S_e^{1/2}[1 − (1 − S_e^{1/m})^m]^2, showing why flow capacity collapses rapidly as soil dries. The page marks conventional field capacity (about 33 kPa suction), permanent wilting point (about 1500 kPa), and plant-available water θ_FC − θ_WP. Presets for sand, loam, and clay illustrate how coarse soils drain quickly while fine soils retain more tightly bound water.

Who it's for: Students in soil physics, hydrology, agronomy, environmental science, and groundwater courses learning unsaturated-flow material functions.

Key terms

Soil water retention
van Genuchten model
Matric suction
Effective saturation
Hydraulic conductivity
Field capacity
Wilting point
Plant available water

Live graphs

How it works

Soil water retention curve: van Genuchten θ(ψ), Mualem hydraulic conductivity K(θ), field capacity, wilting point, and plant-available water.

Key equations

S_e = [1 + (α|ψ|)^n]^{-m}, m = 1 − 1/n

θ = θ_r + S_e(θ_s−θ_r), K(S_e)=K_s S_e^{1/2}[1−(1−S_e^{1/m})^m]^2

Frequently asked questions

Why is suction plotted on a log scale?: Soil water spans a huge range of matric potentials, from near-saturation to thousands of kPa at wilting. A log axis shows both the wet shoulder and the dry tail on one graph.
What do α and n mean physically?: α roughly sets the air-entry suction scale: larger α means the soil drains at lower suction. n controls the steepness of the pore-size distribution; larger n gives a sharper drainage curve typical of coarser soils.
Why can θ still be high near wilting point?: Fine-textured soils may retain substantial water at high suction, but it is tightly held in small pores and films, so plants cannot extract it easily and hydraulic conductivity is very low.
What is left out?: Hysteresis between wetting and drying, salinity/osmotic potential, soil structure, preferential flow, root uptake, and layered profiles are not modeled.

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Soil Water Retention Curve

Live graphs

How it works

Key equations

Frequently asked questions

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