Answer:

Explanation:

The amount of liquid retained in a porous medium is not uniquely defined by the value of the matric potential but is ALSO dependent on the ‘history’ of wetting and drying. This phenomenon, known as hysteresis, is closely related to various aspects of pore geometry, capillarity, and surface wettability. The macroscopic manifestation of hysteresis in soil water retention (or soil water characteristic) is rooted in several microscale mechanisms, including: (1) DIFFERENCES in liquid–solid contact angles for advancing and receding water menisci (Figure 14a), which is accentuated during DRAINAGE and wetting at different rates; (2) the ‘ink bottle’ effect resulting from nonuniformity in shape and sizes of interconnected pores, as illustrated in Figure 14b, whereby drainage of the irregular pores is governed by the smaller pore radius r, and wetting is dependent on the larger radius R. Additional effects stem from pore angularity; (3) differences in air-entrapment mechanisms; and (4) swelling and shrinking of the soil under wetting and drying, respectively. From early observations to the present, the role of individual factors remains unclear, and hysteresis is a subject of ongoing research.

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