ASSESSING A CALIBRATION PROCEDURE TO ESTIMATE SOIL WATER CONTENT WITH SENTEK DIVINER 2000 CAPACITANCE PROBE
Giovanni Rallo1, Giuseppe Giordano1, Giuseppe Provenzano1
(1)
Department of Agro-Environmental Systems (SAGA) - Università degli Studi di Palermo, Viale delle Scienze 12, 90128 Palermo, Italy.
The capacitance sensor marketed as Diviner 2000 (Sentek Pty. Ltd., South
Australia) determines the soil water content by measuring the frequency
change induced by the changing permittivity of the soil.
According to Paltineanu and Star (1997) in fact, the precision of the calibration
equation, obtained with in-situ measurements depends on the errors related to
the sampling of the soil volume investigated by the sensor, that must be done
accurately.
OBJECTIVES
The main objective of the research is to propose a practical calibration
procedure for FDR sensors using a mini-lysimeter containing undisturbed soil,
allowing to take into account the possible variations of the bulk density with the
soil water content.
Experiments were carried out on three soils, two of which collected in
irrigated areas of Sicily (Castelvetrano, Lat. 37.6433° Long. 12.8470°
and Partinico, Lat. 38.0128° Long. 13.0694°) and the third in the
natural reserve of Monte Conca (Lat. 37.4969°, Long. 13.7140°).
Disturbed soil samples were used for the preliminary texture analysis
and for the evaluation of skeleton content and saturated soil electrical
conductivity, by using standard procedures.
In order to develop the site-specific calibration equations minilysimeters, with diameter and height equal to 25 cm, were collected
with undisturbed soil, around the access tube previously inserted in
the soil. In all the sites, sampling was carried out after intensive
rainfall, when soil water content was near to saturation.
The same mini-lysimeters were also filled with sieved soil (0,5 mm)
and compacted to a bulk density similar to the value observed in the
field under saturated conditions.
For both undisturbed and sieved soil samples, Sentek Diviner 2000
probe was daily used in laboratory, to measure the scaled frequency,
dependent on the SWC. The samples were contemporarily weighted
and the subsidence of the soil surface measured with a caliper,
having a precision of 0.1 mm, on eight different points. At the same
time, lateral contraction of the soil was determined with analysis of
images captured on the soil surface.
Corresponding to:
Giovanni Rallo, PhD, Department of Agro-Environmental Systems, Viale delle Scienze Ed. 4 – 90128, Palermo (Italy).
E-mail: rallo.giovanni@gmail.com; Tel: +39(0)91 7028150; Fax: +39(0)91 484035.
0,9
y = -2,3581x 3 + 3,1062x 2 - 0,2202x + 0,5473
R2 = 0,6812
0,9
y = 0,0174x + 0,6055
R2 = 0,2286
0,8
0,7
0,6
0,5
0,8
0,7
0,6
Undisturbed Soil
0,35
0,30
0,25
0,20
0,15
0,10
0,05
PT Sample
0,00
0,3
0,4
0,5
0,1
0,2
0,3
PT Sample
0,4
0,5
0,7
0,6
0,5
MC Sample
0,4
0,0
0,1
0,2
0,3
0,4
Gravimetric water content, U [g g -1]
0,5
0,9
1,0
Sieved Soil
0,40
Undisturbed Soil
0,35
0,30
0,25
0,20
0,15
0,10
0,05
MC Sample
0,4
0,5
0,6
0,7
0,8
0,9
1,0
Scaled Frequency [-]
0,4
0,0
0,1
0,2
0,3
CT Sample
0,4
0,5
1,0
0,8
0,8
0,45
0,3
Gravimetric water content, U [g g ]
y = 1,4075x + 0,1889x + 0,5214
R2 = 0,9449
0,7
0,00
-1
2
0,6
Scaled Frequency [-]
0,5
0,0
0,9
0,40
For undisturbed soils, figures show
the
Shrinkage
Characteristic
Curves, represented as the
relationships between specific
volume of the soil samples (n) and
gravimetric water content (U). Of
course,
no
contraction
was
measured on PT sample, because
of the very low clay content (7.3%).
Values of soil bulk density, variable in the range 1.23-1.88 g/cm3 and
1.49-1.82 g/cm3, as respectively observed for MC and CT samples,
were used to determine q from measured U.
The calibration equation used to convert the probe output into
moisture content is:
q=a SFb + c
where q is the volumetric water content, SF is the scaled frequency
measured by the sensor and a, b, c, are fitting parameters.
Sieved Soil
Volumetric Water Content, q [cm 3 cm -3]
[dS m ]
0.65 10-3
2.55
0,11
1,0
0,4
In order to simplify the proposed methodology, the feasibility of using sieved soil
samples for determining the sensor calibration curve is also investigated.
Measuring the subsidence of the soil surface
Sampling the soil
[%]
0.60
0.75
0.60
1,0
Gravimetric water content, U [g g ]
Materials and Methods
-
Main physical parameters of the investigated soils
-1
The comparison with the default equation suggested by the manufacturer,
obtained for soils of different characteristics using the standard procedure, is
finally shown.
EC (1:5)
Volumetric Water Content, q [cm 3 cm -3]
The procedure to calibrate the sensor suggested by the manufacturer is time
consuming and measurements can be affected by the moisture gradients in the
investigated soil volume.
Partinico, PT
Monte Conca, MC
Castelvetrano, CT
C
[%]
7.3
54.3
37,5
Skeleton
Sieved Soil
According to these preliminary
results,
undisturbed
soil
samples must be preferred to
those prepared in laboratory;
moreover accurate SWC values
can be obtained after a sitespecific calibration of the
sensor, rather than using the
default relationship proposed by
the manufacturer.
The comparison between the experimental q(SF) determined on
undisturbed samples with the manufacturer’s default equation,
obtained by considering measured soil bulk density, evidences
systematic differences in SWC estimation whereas, for sieved soils,
the differences increase only for SF higher than about 0,7.
0,6
0,6
default
PT
0,5
default
RMSE
8.0
7.1
8.6
MC
CT
Volumetric Water Content, q [cm 3 cm -3]
For swelling/shrinking soils the change of the soil bulk volume with the SWC
causes modification in the pore geometry and affects the bulk density/SWC
relationship (Allbrook, 1992). Errors in SWC could reach 20-30% if the soil
shrinkage characteristic curve is not considered (Fares et al., 2004).
where Fa and Fw are the sensor frequencies in air and water, respectively.
USDA
L
S
Textural
Class
[%] [%]
6.7 85.9
S-L
36.0 9.7
C
17,4 45,1
S-C
Specific volume n [cm 3 g-1]
However, due to the high variability of K with soil minerals and dry plant
tissues, it is necessary to proceed to a site-specific calibration of the sensor
(Baumhardt et al., 2000), even to consider the effects of soil temperature, bulk
density and soil salinity.
Sample
Specific volume n [cm 3 g-1]
The main advantage of these devices is that they can be automated to take
continuous measurements of SWC in the same site.
(Fa  Fs )
SF 
(Fa  Fw )
[cm 3 g-1]
Over the past decades Frequency Domain Reflectometry (FDR) probes,
measuring the dielectric permittivity of the soil (K), have been improved, due to
the high potentiality of capacitance based sensors to measure SWC in the
field.
RESULTS AND DISCUSSION
The sensor frequency in soil, Fs, is scaled, SF, using the equation:
Specific volume
In irrigated systems, soil water content (SWC) is a key factor determining plant
growth. Irrigation scheduling criteria are often related to measurements of
SWC or soil matric potential. Ground truth data of SWC at different depths are
essential, in particular when regulated deficit irrigation is applied and the crop
is subject to periods of moisture stress that should have minimal effects on
yields.
Significant differences between
0,40
Undisturbed Soil
q(SF) obtained for the different
0,35
soils
as
well
as
between
0,30
0,25
undisturbed and sieved samples
0,20
were observed. Despite differences
0,15
in q for undisturbed and sieved
0,10
0,05
samples were generally lower than
CT Sample
0,00
5%, the different behavior could be
0,3
0,4
0,5
0,6
0,7
0,8
0,9
1,0
Scaled Frequency [-]
due to the effects of the diverse
pore distribution.
Moreover, for MC and CT soils, characterized by high clay contents,
there could also be an effect of the soil contraction, not well taken
into account when considering sieved soils.
0,45
0,45
Volumetric Water Content, q [cm 3 cm -3]
INTRODUCTION
Volumetric Water Content, q [cm 3 cm -3]
EGU2012-8727
0,4
0,3
Undisturbed Soil
0,2
0,1
PT
0,5
MC
CT
RMSE
4.9
4.6
6.1
0,4
0,3
Sieved Soil
0,2
0,1
0,0
0,0
0,0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
Scaled Frequency [-]
0,8
0,9
1,0
0,0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
1,0
Scaled Frequency [-]
CONCLUSIONS
A practical procedure to calibrate FDR sensors by using mini-lysimeters containing
undisturbed or sieved soils has been proposed, in order to simplify the tedious and timeconsuming methodology suggested by the manufacturer. Contextual measurements of
subsidence of the soil surface and lateral contraction of the samples allows, for clay soils, to
take into account the possible variations of the soil bulk density with water content.
The recognized differences between q(SF) relationships obtained from undisturbed and sieved
soils with the default calibration equation proposed by the manufacturer, suggest that a sitespecific calibration of the sensor is crucial to determine accurate SWC values.
REFERENCES
Allbrook, R.F. 1992. Shrinkage of some New Zealand soils and its implications for
soil physics. Austr. J. Soil Res., 31:111-118.
 Baumhardt, R.L., Lascano, R.J., Evett , S.R. 2000. Soil material, Temperature, and
salinity effects on calibration of Multisensor Capacitance Probes. Soil Sci. Soc. Am. J.
64(6):1940-1946.
Undisturbed soils before and after shrinkage
 Fares, P.B., Dalton, M., El-Kadi, A.I., Parsons, L.R. 2004. Dual Field Calibration of
Capacitance and Neutron Soil Water Sensors in a Shrinking–Swelling Clay Soil.
Vadose Zone J. 3: 1390-1399.
Groves, S.J., Rose, S.C. 2004. Calibration equations for Diviner 2000 capacitance
measurements of volumetric soil water content of six soils. Soil Use and
Management, 20, 96–97.
Paltineanu, I.C., Starr, J. L. 1997. Real-time soil water dynamics using multisensor
capacitance probes: laboratory calibration. Soil Sci. Soc. Am. J. 61(6): 1576-1585.
Scobie M. 2006. Sensitivity of capacitance probes to soil cracks. Thesis for the
Bachelor in Engineering (Environmental). University of Southern Queensland. pp.1-67