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The capillary fringe is the area above the water table where the water is held in tension so that the connected pore space is completely saturated, although variation in pore size means that there is variability in the height of capillary rise, and thus, the thickness of the capillary fringe can include a zone where the largest pores are not saturated (a number of workers only consider the zone where all pores are saturated as the capillary fringe). The capillary fringe can be several meters thick in clays. Above the capillary fringe, there is a zone of variable water saturation called the vadose zone where water is held in tension but the pores are not fully saturated.

Can water wick up into the vadose zone from the capillary fringe to any significant distance (say more than a cm or so) and to any significant amount? If a plant has roots in the vadose zone, that do not reach the saturated capillary fringe, perhaps 30 cm to a meter or more above the fringe, could they obtain a significant amount of water from the saturated groundwater?

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The vadose zone can be on the order of meters, depending on the soil type (higher with finer soils). The maximum height of capillary rise ($h_c$) occurs where capillary suction head is equal to the elevation head above the water table.

In a capillary tube with diameter $d$, and where water has surface tension $T_s$, a zero contact angle, and unit weight $\gamma$, then the maximum capillary rise is $h_c=\frac{4T_s}{d\gamma}$ (Lu and Likos 2004).

Some empirical studies compiled in Lu and Likos (2004) Table 4.1 have found maximum capillary rises in various soils as high as 360 cm (95% silt/clay from Lane and Washburn (1946)). That being said, in such soils the plant would not be able to get the water at a large rate, since the hydraulic conductivity is very low.

Reference: Lu, Ning and Likos, William J. (2004). Unsaturated Soil Mechanics.

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  • $\begingroup$ Does your answer address rise into the unsaturated zone? When talking about capillary tubes, my understanding is that the equation considers how high the tube can be saturated. I am trying to understand the dynamics of moisture above the saturated fringe. This is both to consider the water availability and isotopic composition in semi-arid vegetation (where the water source may be a mixture of local precipitation and groundwater) and for the water dynamics of horticulture wicking-beds. Thank you for the answer. $\endgroup$
    – haresfur
    Commented May 21, 2022 at 0:57
  • $\begingroup$ That should include the unsaturated zone. If I remember correctly (don't have the book on hand at the moment), some of the smaller capillary tubes will pull water well above the saturated fringe. I'm not sure about the more detailed mechanics. $\endgroup$
    – damp_civil
    Commented May 21, 2022 at 21:05

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