Hysteresis of soil water retention
The SWCC can be obtained either by using wetting or drying processes in the laboratory. Interestingly, the wetting SWCC is different from the drying curve due to hysteresis. Typically, the drying retention curve lies above the wetting curve as shown in the Fig. 3.11. Therefore, more water is retained by the soil during the drying process when compared to the wetting process under the same suction value. The hysteresis is not distinct near the residual water content region and very distinct in the saturation region.
The following and well-known mechanisms provide the insight into soil water hysteresis.
Contact angle hysteresis
According to this mechanism, there may be more than one stable contact angle can exist for a given soil solid-water system, i.e. called contact angle hysteresis. It can be illustrated by considering a drop of water on a horizontal clay surface as shown in Fig. 3.12a. The water drop attains an stable contact angle, θc , under the present equilibrium condition. If the clay surface is gradually tilted to either side, the contact angle at the advancing edge of the drop will increase ( θa ) as shown in the Fig. 3.12b. On the other hand, a smaller contact angle ( θt ) develops at the trailing edge (frying front). At the equilibrium, the tangential component of the surface tension will compensate the tangential component of the drop weight, until the limiting condition is achieved. Therefore, many stable contact angles can exist for a given range from θa to θt .