130
Water Flow in Soil
the pore space drained (roughly field capacity) the conductivity has decreased, typically, by a factor of a thousand. When three-fourths of the
pore space has drained (roughly permanent wilting point) the conductivity
is only one-millionth of its value at saturation.
A simple equation that gives a good approximation of the hydraulic
conductivity is (Campbell, 1985):
where ly, is called the air entry water potential and K, is the saturated
conductivity of the soil. The parameter b is the exponent of the moisture release equation, similar to Eq. (4.4):
where 8 is the volumetric water content, and 8, is the saturation water
content. The values of b, K,, and ye depend on soil physical characteristics such as texture.
Saturated conductivity is large for coarse textured soils and small for
h e textured, while the inverse is true for ye. Since ye and K, both depend
on the size of the largest pores in the soil, they are not independent, and,
in fact are related by the equation: K,y," = C, a constant.Table 9.1 gives
values of K,, ye, b and other hydraulic properties for a range of soil
texture. This table, with Eqs. (9.2) and (9.3), allows one to estimate the
TABLE 9.1. Hydraulic properties of soils as a function of soil texture
(recomputed from Rawls et al. 1992).
Texture
sand
loamy sand
sandy loam
loam
silt loam
sandy clay loam
clay loam
silty clay loam
sandy clay
silty clay
clay
Siit Clay -ye b
Jflrg
Water Flow in Soil
the pore space drained (roughly field capacity) the conductivity has decreased, typically, by a factor of a thousand. When three-fourths of the
pore space has drained (roughly permanent wilting point) the conductivity
is only one-millionth of its value at saturation.
A simple equation that gives a good approximation of the hydraulic
conductivity is (Campbell, 1985):
where ly, is called the air entry water potential and K, is the saturated
conductivity of the soil. The parameter b is the exponent of the moisture release equation, similar to Eq. (4.4):
where 8 is the volumetric water content, and 8, is the saturation water
content. The values of b, K,, and ye depend on soil physical characteristics such as texture.
Saturated conductivity is large for coarse textured soils and small for
h e textured, while the inverse is true for ye. Since ye and K, both depend
on the size of the largest pores in the soil, they are not independent, and,
in fact are related by the equation: K,y," = C, a constant.Table 9.1 gives
values of K,, ye, b and other hydraulic properties for a range of soil
texture. This table, with Eqs. (9.2) and (9.3), allows one to estimate the
TABLE 9.1. Hydraulic properties of soils as a function of soil texture
(recomputed from Rawls et al. 1992).
Texture
sand
loamy sand
sandy loam
loam
silt loam
sandy clay loam
clay loam
silty clay loam
sandy clay
silty clay
clay
Siit Clay -ye b
Jflrg