where Vs (L3) is the total volume of soil, F, (L3) is the volume of air, Vw (L3) is the volume of water, and pm (ML"3) is the particle density (normally about 2.65 g/cm3).

The volumetric water content, or soil moisture, 0 (L3L~3) is the ratio of water volume to soil volume:

K ",/ Pw where (M) is the weight of water, Wd (M) is the weight of dry soil, and pK (M/L3) is the densiw of water. Soil moisture can vary in both time and space, w ith a

theoretical range from 0 to <p, but for natural soils the range is significantly reduced due to isolated pore space and tightly held or "adsorbed" water (Fig. 3).7 If a soil is saturated, then allowed to drain until the remaining water held by surface tension is in equilibrium with gravitational forces, it is at field capacity, 0f . Vegetation can remove water from the soil until the permanent wilting point, 9W, is reached. Therefore, the available water content for plant use, 6a — — 9W. Typical ranges of porosity, field capacity, and wilting point for different soils are given in Fig. 4.8

In unsaturated soils, water is held in the soi! against gravity by surface tension (Fig. 3). This tension, suction, or matric potential, tj/ (L), increases as the radii of curvature of the meniscus or water content decreases (Fig. 5),9 Matric potential is expressed in reference to atmospheric pressure, so for saturated soil ^ = 0 and for unsaturated soil ij/ < 0,

The hydraulic conductivity, K (L/T), is a measure of the ability of the soil to transmit water that varies n on linearly over a large range depending on both soi!

Figure 4 Water holding properties of various soils.8
Figure 5 Effect of texture on water retention characteristics.9

properties and water content (Fig. 6).10 Many laboratory and field hydraulic conductivity measurement methods exist for use with various soils; see Bouwer and Jackson" or Green et al.12 for details.

Soil water content can significantly impact infiltration by (1) increasing the hydraulic conductivity, which increases infiltration, and (2) reducing the surface tension that draws moisture into the soil, which reduces infiltration. The net effect of these impacts depends on the water content itself, the water input rate, and duration and the distribution of hydraulic conductivity.

The water retention characteristic describes a soil's ability to store and release water and is defined by the relationship between soil moisture and the matric potential (Fig. 5). This is a power function relationship that has been described by Brooks and Corey13 and Van Genuchten,14 among others. The water tension characteristic is usually measured in air pressure chambers where the water content of a soil sample can be monitored over a wide pressure range.15

The water retention relationship may actually change between drying and wetting due to the entrapment of air in soil pores (Fig. 7).16 For practical applications, this effect, called hysteresis, is usually neglected.17

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