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Determination of Hydraulic Conductivity Function (HCF)

Hydraulic Conductivity Function (HCF) gives the relationship between the variations in hydraulic conductivity and the amount of moisture present within a soil. As the moisture content of soil decreases, the hydraulic conductivity decreases. And vice versa.
A Multi-Step Outflow Technique is a laboratory technique used to determine the hydraulic conductivity function of unsaturated soil. This technique involves obtaining hydraulic conductivity variation from volumetric water content or hydraulic head of soil. The multi-step outflow technique is a popular technique that is widely used for the determination of the Soil-Water Characteristic Curve and Hydraulic Conductivity Function at the same time. However, It is not useful for establishing the entire Hydraulic Conductivity Function of clays.
SWCC and HCF Models
The most used Soil-Water Characteristic Curve (SWCC) models are Brooks and Corey's model, Van Genuchten's model, and Fredlund Xing's model. The Brooks and Corey's method doesn't provide a smooth function. However, the Van Genuchten model can provide a smooth Soil-Water Characteristic Curve (SWCC). The drawback of the van Genuchten's model is that the volumetric water content can never go to 0 at any given suction. The Fredlund and Xing's model can generate a smooth SWCC and have its water content forced to 0 at very high suction values Hydraulic Conductivity Function (HFC) models are empirical, macroscopic, or statistical.

Fitting of SWCC & HCF Modelling
The four stages of modeling a flow through unsaturated soils are: Obtain soil-water retention data Utilise retention models to obtain smooth SWCC by using optimization techniques, Use statistical models to obtain a smooth HCF function, Use the SWCC and HCF to model the flow through unsaturated soils The optimisation strategies that can be used to obtain the Soil-Water Characteristic Curve from the measured water retention data are: Gradient-based optimization, Genetic algorithm, Ant Colony optimisation, Particle swarm optimisation, Bee algorithms. Pedo-Transfer Functions (PTF)
Pedo-Transfer Functions is called the Arya Paris model. This model exists for the determination of the soil-water characteristic curve. It utilises grain-size distribution data to predict the soil-water characteristic curve. The steps in using the Arya Paris Method to estimate the Soil-Water Characteristics curve from particle size data are: Obtain the void ratio of the soil, Determine the pore volume of the soil, Determine the volumetric water content, Obtain the pore radius of the soil, Calculate the suction head