Hodnett & Tomasella (2002), tropical-soil regressions for four van Genuchten parameters.¶
Source¶
Hodnett, M. G., & Tomasella, J. (2002). Marked differences between van Genuchten soil water-retention parameters for temperate and tropical soils: A new water-retention pedo-transfer functions developed for tropical soils. Geoderma, 108(3-4), 155-180.
DOI: 10.1016/S0016-7061(02)00105-2
Scope¶
Territory: Tropical soils between approximately 25 degrees N and 25 degrees S.
Dataset: The IGBP-DIS tropical-soil dataset contains 771 retained horizons from 249 profiles in 22 countries, split into 492 calibration curves and 279 validation curves.
Functions¶
calc_ptf_hodnett2002¶
Estimate four van Genuchten water-retention parameters for tropical soils.
Status: implemented
Prediction target: Residual and saturated volumetric water content and the alpha and n shape parameters of the van Genuchten water-retention model.
Models: \(h(\theta)\) — van Genuchten model with m = 1 - 1 / n
Inputs¶
| Name | Type | Unit | Domain | Description |
|---|---|---|---|---|
sand | number | % | 0.5 <= value <= 99.0 | Sand content in the 0.05-2 mm USDA particle-size fraction, expressed as a percentage. |
silt | number | % | 0.4 <= value <= 77.4 | Silt content in the 0.002-0.05 mm USDA particle-size fraction, expressed as a percentage. |
clay | number | % | 0.0 <= value <= 95.4 | Clay content in the less-than-0.002 mm USDA particle-size fraction, expressed as a percentage. |
organic_carbon | number | % | 0.0 <= value <= 30.8 | Soil organic carbon content expressed as a percentage. |
bulk_density | number | Mg/m^3 | 0.28 <= value <= 1.88 | Soil bulk density. |
cation_exchange_capacity | number | cmol/kg | 0.0 <= value <= 93.7 | Soil cation exchange capacity. |
ph | number | dimensionless | 3.60 <= value <= 9.60 | Soil pH. |
Outputs¶
| Name | Unit | Domain | Description |
|---|---|---|---|
alpha | kPa^-1 | value > 0 | Shape parameter of the van Genuchten water-retention model. |
n | dimensionless | value > 0 | Shape parameter controlling water-retention curve steepness. |
theta_s | m^3/m^3 | — | Saturated volumetric soil water content. |
theta_r | m^3/m^3 | — | Residual volumetric soil water content. |
Note
The regressions use percentages of sand, silt, clay, and organic carbon, bulk density in Mg/m^3, CEC in cmol/kg, and pH.
Note
Table 8 reports the reoptimised Step 3 coefficients used here and states that its displayed values are multiplied by 100; each polynomial is therefore divided by 100 before alpha and n are back-transformed.
Note
The fitted ln(alpha) and ln(n) values are back-transformed with the natural exponential, as specified by the paper's use of natural logarithms.
Warning
The paper reports n = 497 when introducing the continuous PTF, but its dataset split gives 492 calibration curves and 279 validation curves, which account for all 771 retained horizons. The extraction treats 497 as a typographical error and records 492 as the calibration-set size.
Warning
The PTF did not reproduce water-retention curves accurately for soils with bulk density below 0.8 Mg/m^3, most of which were Andosols.
Warning
The PTF did not reproduce very low alpha values well because high-alpha soils dominated the calibration data; mineralogy and structure were not directly represented.