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The soil samples including the porous plate are initially water saturated.
Subsequently, we measure the volumes of outfl ow water collected in the calibrated
tube, or burette, as we incrementally increase the air pressure in the pressure cham-
ber. We start with a very small overpressure, e.g., 2 kPa above the atmospheric pres-
sure in the laboratory, and register the volume of outfl ow. Next, we increase the
overpressure to 5 kPa and again register the volume of water collected in the burette.
We continue extracting water from the soil using progressively larger, nonlinear
steps of overpressure up to a magnitude of about 2 MPa.
The incremental volumes of water measured in the outfl ow burette are converted
into values of soil water content that were in equilibrium with each applied over-
pressure. These pairs of data plotted in a graph yield the main drainage curve shown
in Fig. 8.11 . The shape of this curve, also known as a water retention curve, is
roughly similar to that of the curve in Fig. 8.7 derived for the capillary rise model of
a bundle of various-sized capillary tubes. The impact of hysteresis introduced in
Sect. 8.3 can also be seen in Fig. 8.11 by observing that when water wets an initially
dry soil, its water content is less than that of a soil draining from an initially
water-saturated content. The experienced soil physicist measures soil water
retention functions to derive many important soil properties, e.g., the size of soil
pores and their distribution in the soil, the extent of soil compression, the relative
ease that soil conducts water, and the relative diffi culty for plants to suck soil water
into their roots.
Fig. 8.11 Soil water retention curve. Hysteresis is demonstrated by its two main branches
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