Soil Water Retention Characteristics

The relationship between volumetric soil water content 0 and pressure potential h (expressed here as an equivalent height of water) is called the water retention function. The relationship 0 (h) is strongly and chiefly affected by soil texture and structure. With reference to drying conditions, Fig. 5.8 reports typical shapes of the water retention curve for a sandy and a clayey soil. Starting from an equilibrium condition at saturation (h = 0), a slight reduction in h may not cause reductions in 0 until pressure potential in the soil reaches a critical value hE (air-entry potential head), which depends mainly on the pore-size distribution, especially that of larger soil pores. Thus, for hE < h < 0, the soil should not necessarily be under unsaturated conditions. However, the presence of an air-entry potential is particularly evident only for coarse-textured soils. Beyond this critical value, decreases in h will result in a more or less rapid decrease in 0. When water content is reduced so that soil conditions are very dry (residual water-content conditions), a slight reduction in soil water content may cause the pressure potential to decrease even by orders of magnitude.

The hysteretic nature of soil water retention characteristic 0 (h) under nonmonotonic flow conditions has been demonstrated both theoretically and experimentally. In practice, the 0 values are related to the h values in different ways, depending on the drying or wetting scenarios to which the porous medium is subjected and specifically even on the (0 - h) value when the time derivative d0/dt changes its sign. One could say that soil has "memory" of the drying and wetting histories that precede the setting up of a new equilibrium condition and this phenomenon of hysteresis is more evident when approaching fully saturated conditions. Figure 5.9 shows the excursion of water content and pressure potential (main wetting, main drying, and scanning soil water

clayey soil

jr sandy soil

Figure 5.8. Schematic water retention characteristics for a sandy and a clayey soil during drainage.

Figure 5.9. Measured soil water retention characteristics for a sandy soil exhibiting hysteresis. Source: From [10], with permission.

characteristics) measured in the laboratory on a sandy soil exhibiting hysteresis. The main wetting curve is represented by the solid line, with the open dots being experimental values measured as water saturation increases. The close dots are retention data points measured under drying conditions. The highest dashed line is the main drying curve, whereas the others correspond to a few scanning curves. Note that, in general, the main drying curve starts at a value equal to the total soil porosity, whereas the main wetting curve may reach, at h = 0, an effective water content that is less than the total pore space of the soil because of entrapped air. Major causes of the hysteretic behavior of the water retention characteristics are the following: different water-solid contact angles during wetting and drying cycles, as well as high variability in both size and shape of soil pores (ink-bottle effects); the amount of air entrapped in the pore space; phenomena of swelling or shrinking of the individual particles [2, 3].

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