Moisture release curve

The relationship between the subsurface moisture content (volumetric wetness) and the negative pressure potential (Pa) is called the moisture release curve (Fig. 6.7); it is also known as the moisture retention curve and moisture characteristic curve. The simplest expression of this empirical relationship is,

1 b 0p (volume) = a0w where 0p(Volume)~1 is the negative pressure potential, and a and b are the parameters that can be determined for a particular soil from the intercept and slope of a straight line fitted to the plot of log &p(Volume)-1 against log 0w (Visser, 1969). This curve is used to estimate one moisture variable from the other and in the estimation of the unsaturated hydraulic conductivity curve (Section 5.2).

Over the range of pressure potential 0 to —20kPa, a sand tension table (Fig. 6.8a) can be used to derive the curve. With this device, undisturbed soil cores are placed on the sand surface and allowed to equilibrate with the negative pressure potential created in the device. These cores are then weighed as part of a gravimetric measurement of soil moisture content (Section 6.1.2).

Water content (gg 1)

Fig. 6.7 A moisture release curve, where x is the mass wetness (0m, gg— 1) and y is the pressure potential (0pvolume) in —MPa). (Reproduced with permission of Decagon Devices Inc.)

Water content (gg 1)

Fig. 6.7 A moisture release curve, where x is the mass wetness (0m, gg— 1) and y is the pressure potential (0pvolume) in —MPa). (Reproduced with permission of Decagon Devices Inc.)

Fig. 6.8 Devices for measuring the moisture release curve: (a) a sand tension table; and (b) a pressure plate apparatus (reproduced with permission of Soil Moisture Equipment Corporation, Santa Barbara, USA).

Over the range of pressure potential -20 to -150 000 kPa, a pressure plate device (Fig 6.8b) can be used to derive the curve. The undisturbed samples are placed on a porous plate within a pressure vessel. A pump is used to set the pressure potential, and once in equilibrium with the pressure plate device, samples are weighed and finally dried as part of gravimetric analysis.

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