Soil-Water Characteristic Curves for Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 201

Table 5.2 Empirical Equations Used to Best Fit SWCC Data

Reference Equation Description

1

Gardner (1958b) d = a g = fitting parameter which is a

function of air-entry value of the
soil
n g =

a g ψ n g 1 +

where

fitting parameter which is a
function of rate of water
extraction from soil once air-entry
value of soil has been exceeded

w (ψ)
w s

d =

Brooks and Corey (1964) w (ψ) = w s or n = 1for ψ ≤ ψ aev ψ aev = air-entry value of soil

ψ
ψ aev

− λ bc λ bc = pore size distribution index

w r = residual water content located

through trial-and-error process
that yields straight line on semi
log plot of degree of saturation
versus suction

n = ψ > ψ aev

where

w(ψ) − w r

n =

w s − w r

1

Brutsaert (1967) n =

+ ψ/a b n b

a b = fitting parameter which is a

function of air-entry value of soil
n b =

1

fitting parameter which is a
function of rate of water
extraction from soil once air-entry
value has been exceeded

where

w (ψ) − w r

n =

w s − w r

2 erfc a l −

1 b l

Laliberte (1969) n = a l ,b l ,c l = parameters assumed to

be unique functions of pore-size
distribution index λ

c l + (ψ/ψ aev )

where

w (ψ) − w r

n =

w s − w r

Campbell (1974) w = w s ,ψ < ψ aev ψ aev = air-entry value of soil

w s ψ
ψ aev

− 1 /b c b c = fitting parameter

w = ,ψ ≥ ψ aev

1

van Genuchten (1980) n = a v g ,a v m ,a v b = fitting parameters

primarily related to inverse of
air-entry value (units equal to 1/
kPa)
n v g ,n v m ,n v b =

(a v g ψ) n v g ] m v g

[1 +

where

w (ψ) − w r

n =

fitting parameters
primarily related to rate of water
extraction from soil once air-entry
value has been exceeded
m v g ,m v m ,m v b =

w s − w r

1

van Genuchten (1980)-
Mualem (1976)

n =

[1 + (a v m ψ) n v m ] m v m

where

fitting parameters
that are primarily related to
residual water content conditions

1
n v m

m v m = 1 −

( continued overleaf )