Van Genuchten Soil Water Retention Curve Calculator
Explore how different soils hold and release water. The van Genuchten model is the standard for describing soil water retention and hydraulic conductivity.
Parameters
Derived: m = 1 - 1/n
The Van Genuchten Model
The van Genuchten (1980) model describes the relationship between soil water content (θ) and matric potential (ψ):
Combined with the Mualem (1976) pore-size distribution model, the unsaturated hydraulic conductivity is:
Parameter Meanings
θs (Saturation): Maximum water content when all pores are filled. Higher for clay (more small pores).
θr (Residual): Water that cannot be removed by drainage or plant uptake. Bound to soil particles.
α (Alpha): Inverse of air-entry pressure. Large α = water drains easily (sand). Small α = water held tightly (clay).
n (Shape): Pore size distribution. Large n = uniform pores (sharp drainage). Small n = varied pores (gradual drainage).
Ksat: Saturated hydraulic conductivity. How fast water moves through saturated soil.
Soil Texture Effects
Sandy soils: Large pores, high α and n, water drains quickly. Low water holding capacity but high conductivity.
Clay soils: Small pores, low α and n, water held tightly. High water holding capacity but low conductivity.
Loamy soils: Mix of pore sizes, intermediate properties. Often best for plant growth.
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(5), 892-898.
Van Genuchten Parameters by Soil Type
Reference table of van Genuchten model parameters for common USDA soil texture classes. Values from the USDA ROSETTA database (Schaap et al., 2001).
| Soil Type | θs | θr | α (1/m) | n | Ksat (m/day) | Field Capacity | Wilting Point |
|---|---|---|---|---|---|---|---|
| Sand | 0.43 | 0.045 | 14.5 | 2.68 | 7.13 | ~10% | ~5% |
| Loamy Sand | 0.41 | 0.057 | 12.4 | 2.28 | 3.50 | ~12% | ~6% |
| Sandy Loam | 0.41 | 0.065 | 7.5 | 1.89 | 1.06 | ~18% | ~8% |
| Loam | 0.43 | 0.078 | 3.6 | 1.56 | 0.25 | ~27% | ~12% |
| Silt Loam | 0.45 | 0.067 | 2.0 | 1.41 | 0.11 | ~33% | ~13% |
| Clay Loam | 0.41 | 0.095 | 1.9 | 1.31 | 0.062 | ~34% | ~19% |
| Clay | 0.38 | 0.068 | 0.8 | 1.09 | 0.048 | ~36% | ~21% |
Field capacity estimated at -33 kPa matric potential. Wilting point at -1500 kPa. Values are representative averages; actual parameters vary with organic matter content, bulk density, and soil structure.
Frequently Asked Questions
What are the van Genuchten parameters for clay?
Typical van Genuchten parameters for clay soil are: saturated water content (θs) = 0.38, residual water content (θr) = 0.068, alpha (α) = 0.8 1/m, shape parameter (n) = 1.09, and saturated hydraulic conductivity (Ksat) = 0.048 m/day. Clay has the lowest alpha and n values of any soil type, meaning it holds water very tightly and drains very slowly. The low n value also means clay has a gradual drainage curve rather than the sharp transition seen in sandy soils.
What is the difference between field capacity and wilting point?
Field capacity is the amount of water remaining in the soil after gravity drainage has essentially stopped, typically defined at -33 kPa matric potential. Wilting point is the moisture level at which plants can no longer extract water from the soil, defined at -1500 kPa. The difference between field capacity and wilting point is called plant available water (PAW) - this is the water reservoir that plants can actually use. Sandy soils have low PAW (5-10%) because they drain quickly, while loam and silt loam soils have the highest PAW (15-20%).
How do I find van Genuchten parameters for my soil?
There are several approaches: (1) Use the USDA soil texture presets in this calculator as a starting point - select your soil type from the dropdown. (2) Use the USDA ROSETTA model, which estimates van Genuchten parameters from basic soil properties like sand/silt/clay percentages and bulk density. (3) For precise values, laboratory measurement of the soil water retention curve using pressure plate apparatus, then fit the van Genuchten equation to the measured data. The USDA Web Soil Survey (websoilsurvey.nrcs.usda.gov) provides soil texture data for any location in the US.
What does the alpha parameter mean physically?
The alpha (α) parameter is the inverse of the air-entry pressure, measured in 1/m. It controls where the retention curve transitions from saturated to unsaturated conditions. A high alpha (10-15, typical of sand) means water begins draining at very low suction pressures - the soil "lets go" of water easily. A low alpha (0.5-2, typical of clay) means the soil holds onto water tightly and requires high suction to begin drainage. Physically, alpha is related to the dominant pore size: large pores (sand) drain first at low suction, while small pores (clay) require more energy to empty.
What is the Mualem model used for hydraulic conductivity?
The Mualem (1976) model predicts unsaturated hydraulic conductivity K(θ) from the soil water retention curve. Combined with the van Genuchten retention model, it gives the van Genuchten-Mualem equation: K(Se) = Ksat × Se0.5 × [1 - (1 - Se1/m)m]2, where Se is effective saturation. This relationship is essential for modeling water movement through soil because hydraulic conductivity drops dramatically as soil dries - by several orders of magnitude between saturation and wilting point.