37
water potential. Measurements of the soil water retention curve
require considerably less detail and are generally published for a
wide variety of soil types. Van Genuschten (1980) presented a method
by which the hydraulic conductivity could be calculated for unsaturated
soils using the soil water retention curve and the saturated hydraulic
conductivity. The Van Genuschten approach involves estimation of an
equation for the soil water retention curve of the form
9 = ( 10 r 1) m (2-39)
Ts r 1 + (a)n
where
9 = dimensionless water content
0 = volumetric water content [m3m-3]
r = residual volumetric water content [m3m-3]
es = volumetric water content at saturation [m3m-3]
= soil water potential [ m ]
m,a = regression coefficients
n = (1 m)
The coefficients m and a are determined by nonlinear least squares
regression of the soil water retention curve. That function is then
substituted into equations for the hydraulic conductivity presented by
Mualem (1976). The resulting expression for the diffusivity is
D(B) = (1 -m) k (0.5 I/m) [(1-e1/m)-m+ (1_01/m)m_2] (2-40)
am(Os-Or)
The Van Genuschten method yields a continuous function for the
hydraulic diffusivity which is highly desirable for numerical