Soil Water Flow - Transport in the Atmosphere-Vegetation-Soil Continuum - page 171

where ε is the effective permittivity of the medium around the wave guide and c is
the velocity of light in vacuum (3 × 10 8 m s -1 ). Thus, ε and θ can be determined by
measuring v of an electromagnetic pulse travelling along a sensor embedded in soil
(Dirksen, 1999 ).

Question 4.24: A common length L of the TDR sensor is 0.10 m. How long is the travel
time of an electromagnetic wave from one end of the sensor to the other and return if the
sensor is in water? And how long is the travel time if the sensor is in air?

The relation θ ( ε ) was measured by Topp et al. ( 1980 ) for a number of soils:

( ) ×

2 3 −

θ =− + 530 292 ε ε − 55 . + 0 043 . ε 10

(4.48)

Equation ( 4.48 ) has given satisfactory results for many soils and contributed greatly
to the rapid introduction and development of TDR for automated water content mea-
surements. For very accurate results and for soils with very low bulk density, high
clay or organic content and speciic mineralogical properties, soil-speciic calibration
might be necessary (Roth et al., 1992 ; Dirksen and Dasberg, 1993 ).

4.12 Measurement of Hydraulic Conductivity
The hydraulic conductivity varies over many orders of magnitude, not only between
different soils but also for the same soil as function of its water content. As a result, k ( θ )
functions are hard to measure accurately. Many direct and indirect methods have been
proposed as listed in Table 4.3 (Klute, 1986 ; Dirksen, 1999 ; Hopmans et al., 2002 ).
There is no single “universal” method that is suitable for all soils and circum-
stances. Criteria for selecting a suitable method include theoretical basis, control of

Table 4.3 Laboratory measurement methods of k ( θ )

Method Range (cm) Reference

Suction cell -100 < h < 0 Klute and Dirksen ( 1986 )

Crust method -100 < h < 0 Bouma et al. ( 1983 )

Drip iniltrometer -100 < h < 0 Dirksen ( 1991 )

Evaporation method -800 < h < 0 Wendroth et al. ( 1993 )

Pressure cell -1000 < h < 0 Van Dam et al. ( 1994 )

Sorptivity method -1000 < h < 0 Dirksen ( 1979 )

Hot air method -10 000 < h < -100 Van Grinsven et al. ( 1985 )

Centrifuge method -1000 < h < 0 Nimmo et al. ( 1987 )

Spray method -250 < h < 0 Dirksen and Matula ( 1994 )

Tension disc iniltrometer -100 < h < 0 Simunek et al. (1998)