Sorptivity

John Philip (1957) described sorptivity as "a measure of the capacity of the medium to absorb or desorb liquid by capillarity". Calculation of the true sorptivity required numerical iterative procedures dependent on soil water content and diffusivity.
John Philip (1969) showed that sorptivity can be determined from horizontal infiltration where water flow is mostly controlled by capillary absorption:

$I\; =\; S\; sqrt\{t\}$

where "S" is sorptivity, "I" is the cumulative infiltration at time, "t".