Interactive Soil Water Retention Curves

Average soil water retention parameters by USDA textural class

SOIL WATER RETENTION MODEL (Campbell)

The soil water retention curve is described using the Campbell (1974) model:

θ(ψ) = θs                          , |ψ| ≤ |ψe|
θ(ψ) = θs (|ψ| / |ψe|)^(-1/b)      , |ψ| > |ψe|

where |ψ| is the absolute matric potential (kPa), θ is volumetric water
content (m³ m⁻³), θs is the saturated water content, ψe is the air-entry
matric potential (kPa), and b is the pore-size distribution index.

The Campbell model represents soil as a bundle of capillary pores and
explicitly defines an air-entry point separating saturated and unsaturated
conditions.

Reference:
Campbell, G. S. (1974). A simple method for determining unsaturated
conductivity from moisture retention data. Soil Science, 117, 311–314.

SOIL WATER RETENTION MODEL (van Genuchten)

The soil water retention curve is described using the van Genuchten (1980) model:

θ(ψ) = θr + (θs − θr) [1 + (α |ψ|)^n]^(-m) ,
with m = 1 − 1/n

where |ψ| is the absolute matric potential (kPa), θ is volumetric water
content (m³ m⁻³), θr is the residual water content, θs is the saturated
water content, α is the inverse air-entry parameter (kPa⁻¹), and n is a
shape parameter.

Reference:
van Genuchten, M. T. (1980). A closed-form equation for predicting the
hydraulic conductivity of unsaturated soils. Soil Science Society of America
Journal, 44, 892–898.

VARIABLE DESCRIPTIONS

ψ (matric potential, kPa)
  Soil water tension expressed as matric potential. Positive values represent
  increasing difficulty for roots to extract water.

θ (volumetric water content)
  Fraction of soil volume occupied by water (m³ m⁻³).

θs (theta_s)
  Saturated volumetric water content (cm³ of water per 1 cm³ of soil; same as cm³ cm⁻³ or m³ m⁻³).
  Represents the upper limit of water storage when all soil pores are filled with water.

θr (theta_r)
  Residual volumetric water content (cm³ cm⁻³). Lower asymptote of the retention
  curve where water is strongly bound and largely unavailable to plants. Some 
  researchers interpret the residual water as the point at which liquid water stops flowing in the soil.
  From here on water vapor tends to dominate.

α (alpha, 1/kPa)
  Inverse air-entry parameter of the van Genuchten model. Controls the matric
  potential at which drainage begins.

n
  Shape parameter of the van Genuchten model controlling the steepness of the
  retention curve. Higher values correspond to sharper transitions.

ψe (psi_e, kPa)
  Air-entry matric potential of the Campbell model. Represents the absolute
  matric potential at which air first enters the largest soil pores and drainage
  begins. For |ψ| ≤ |ψe| the soil is assumed to remain saturated. Smaller values
  (closer to zero) indicate coarser soils with earlier drainage.

b
  Pore-size distribution index of the Campbell model (unitless). Controls the
  rate at which volumetric water content decreases with increasing matric
  potential beyond the air-entry point. Larger values indicate a wider pore-size
  distribution and a more gradual decline in water content with tension.

θ(10 kPa)
  Water content at field capacity, commonly used as the upper bound of plant-
  available water. Some older literature used 33 kPa of tension for this point,
  but new evidence shows that 10 kPa is a better approximation compared to field observations.

θ(1500 kPa)
  Water content at the permanent wilting point, commonly used as the lower bound
  of plant-available water. This tension can change for different crops, although the 
  resulting volumetric water content barely changes, so most researchers and practitioners use
  1500 kPa as the default for simplicity and consistency.

PAWC (plant available water capacity)
  Difference between θ(10 kPa) and θ(1500 kPa). Represents the total extractable
  water assuming equal work across θ. One drawback of this range is that it assumes that every
  unit of water taken up by the plants requires the same amount of work or effort, but in reality
  it becomes increasingly more difficult (see the "work" estimate below for a more realistic
  representation of the cumulative effort to extract water from one soil moisture level to another).

FAW (fraction of available water)
  Fraction of PAWC remaining in the root zone:
    FAW = (θ − θ1500) / (θ10 − θ1500)

PAWC / θs (%)
  Fraction of saturated water content that is plant-available. Indicates how
  much of the total soil water storage can be accessed by plants.

θ_stress
  Volumetric water content corresponding to the selected FAW threshold for the
  onset of plant water stress.

ψ_stress (kPa)
  Matric potential corresponding to θ_stress, obtained by inverting the van
  Genuchten retention curve.

Inflection point (log10 ψ)
  Matric potential at which the curvature of the retention curve changes sign
  when plotted against log10(ψ). Often associated with pore-size
  distribution transitions.

Work (J m⁻³)
  Cumulative energy required to extract water between two matric potentials:
    W = ∫ ψ dθ
  Represents the physical effort required by plants to remove water from the soil.

Created by Andres Patrignani, Soil Water Processes
Department of Agronomy, Kansas State University
Contact: andrespatrignani@ksu.edu