Soil Water Content

Soil water content generally is defined as the ratio of the mass of soil water to the mass of dried soil, or as the volume of water per unit volume of soil. In both cases, accuracy in calculating the water content depends on a clear and rigorous definition of the dry soil condition.

Because the interest of practical applications relies largely upon the determination of the magnitude of relative time changes in water content in a certain point, the condition of dry soil refers by tradition to a standard condition obtained by evaporating the water from a soil sample placed in an oven at 100-110°C until variations in sample weight are no longer noticed. The choice of these temperatures is somewhat arbitrary and does not result from scientific outcomes. Rather, within the above range of temperatures the evaporation of free water from the sample is guaranteed and the standard condition can be attained easily using commercial ovens.

It is useful to define the water content of soil on a volumetric basis 6 (m3/m3) as the dimensionless ratio of water volume Vw (m3) to total soil volume Vt (m3). Especially when subjecting a soil sample to chemical analyses, the soil water content is expressed usually on a mass basis as where Mw is the mass of water (kg) and Ms is the mass of dry soil particles (kg).

If pb = Ms/ Vt denotes the oven-dry bulk density (kg/m3) and pw = Mw/ Vw is the density of liquid water (kg m-3), the volumetric soil water content 6 and the gravimetric

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