Soil-water characteristic curve refers to

Soil water characteristic curve, also called soil characteristic curve or soil pF curve, expresses the relationship curve between soil water content and soil suction.

Generally, soil water content Q is expressed as volume percentage, and soil suction S is expressed as atmospheric pressure. Due to the action of soil air and the different solid-liquid contact angles in the process of soil water absorption and release, the measured soil moisture characteristic curve is not a single-valued function curve.