Water retention curve is the relationship between the water content

Water content

Water content or moisture content is the quantity of water contained in a material, such as soil , rock, ceramics, fruit, or wood. Water content is used in a wide range of scientific and technical areas, and is expressed as a ratio, which can range from 0 to the value of the materials' porosity at...

, θ, and the soil water potential

Water potential

Water potential is the potential energy of water per unit volume relative to pure water in reference conditions. Water potential quantifies the tendency of water to move from one area to another due to osmosis, gravity, mechanical pressure, or matrix effects such as surface tension...

, ψ. This curve is characteristic for different types of soil, and is also called the soil moisture characteristic.

It is used to predict the soil water storage, water supply to the plants (field capacity

Field capacity

Field capacity is the amount of soil moisture or water content held in soil after excess water has drained away and the rate of downward movement has materially decreased, which usually takes place within 2–3 days after a rain or irrigation in pervious soils of uniform structure and texture...

) and soil aggregate stability. Due to the hysteretic effect of water filling and draining the pores, different wetting and drying curves may be distinguished.

The general features of a water retention curve can be seen in the figure, in which the volume water content, θ, is plotted against the matric potential, . At potentials close to zero, a soil is close to saturation, and water is held in the soil primarily by capillary forces. As θ decreases, binding of the water becomes stronger, and at small potentials (more negative, approaching wilting point) water is strongly bound in the smallest of pores, at contact points between grains and as films bound by adsorptive forces around particles.

Sandy soils will involve mainly capillary binding, and will therefore release most of the water at higher potentials, while clayey soils, with adhesive and osmotic binding, will release water at lower (more negative) potentials. At any given potential, peaty soils will usually display much higher moisture contents than clayey soils, which would be expected to hold more water than sandy soils. The water holding capacity of any soil is due to the porosity and the nature of the bonding in the soil.

Shape parameters

The shape of water retention curves can be characterized by several models, one of them known as the van Genuchten model:
where is the water retention curve [L³L⁻³]; is suction pressure ([L⁻¹] or cm of water); saturated water content [L³L⁻³]; residual water content [L³L⁻³]; is related to the inverse of the air entry suction, ([L⁻¹], or cm⁻¹); and, is a measure of the pore-size distribution, (dimensionless).

Based on this parametrization a prediction model for the shape of the unsaturated hydraulic conductivity - saturation - pressure relationship was developed.

History

In 1907, Edgar Buckingham created the first water retention curve. It was measured and made for six soils varying in texture from sand to clay. The data came from experiments made on soil columns 48 inch tall, where a constant water level maintained about 2 inches above the bottom through periodic addition of water from a side tube. The upper ends were closed to prevent evaporation.