ekS - Instytut Agrofizyki im. Bohdana Dobrzańskiego PAN w Lublinie ...

where: n w is the free water refractive index at 20 °C, n 20 is the soil refractive index
at 20 °C, ∆n(T ) is its temperature dependent correction, f a , f s are volumetric
fractions of soil air and solid phases, respectively.
The soil water content, θ = θ 20 , in Eq. (42) is determined gravimetrically in
laboratory conditions or using TDR soil water content measurement method
appliued for the soil at 20°C, ie in TDR meters calibration conditions. The values
of the refractive index, n T , for soils are determined at T temperature can be
corrected to the values at 20 °C, n 20 , according to the Eq. (42), which can be
rewritten as:
n
( d(
) 1)
20= n −
20
nw
T −
T
θ (43)
As the dielectric soil water content measurement is based on the calibration
formula θ = f (n), the TDR calculated soil water content depend on its
temperature:
T
( ) = θ( n ∆n(
))
θ = θ +
(44)
n T
20
T
For linear shape of the calibration formula the following is true:
dθ
dθ
θ
T
= θ ( n20 + ∆n( T )) = θ
20
+ ∆n
= θ
20
+ θ
20
⋅ nw
( d ( T ) − 1) (45)
dn dn
Rewriting the Eq. (45) we can receive the TDR determined soil water content,
θ 20 , corrected to its value at 20°C:
θT
θ
20
=
(46)
dθ
1 +
n w
( d ( T ) −1) ⋅ dn
The above corrections based on the temperature change affecting the soil free
water and applied to the soil water content values determined by TDR
measurement method will be verified below using experimental data.
7.3. Materials and methods
The influence of temperature on the refractive index, n, of the EM waves was
tested on the saturated peat soil and three mineral soils with texture presented in
Table 7: sand, silt and clay.
56