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. EGU2012-8727 INTRODUCTION Significant differences between q(SF) obtained for the different soils as well as between undisturbed and sieved samples were observed. Despite differences in q for undisturbed and sieved samples were generally lower than 5%, the different behavior could be due to the effects of the diverse pore distribution. RESULTS AND DISCUSSION 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. 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. The main advantage of these devices is that they can be automated to take continuous measurements of SWC in the same site. 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. 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). 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. The sensor frequency in soil, Fs, is scaled, SF, using the equation: where Fa and Fw are the sensor frequencies in air and water, respectively. 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. 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. Main physical parameters of the investigated soils 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. 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. 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. In order to simplify the proposed methodology, the feasibility of using sieved soil samples for determining the sensor calibration curve is also investigated. The comparison with the default equation suggested by the manufacturer, obtained for soils of different characteristics using the standard procedure, is finally shown. 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 site-specific calibration of the sensor, rather than using the default relationship proposed by the manufacturer. 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%). Materials and Methods Sampling the soil 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 mini-lysimeters, 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. CONCLUSIONS 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. 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 time-consuming 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 site-specific calibration of the sensor is crucial to determine accurate SWC values. Measuring the subsidence of the soil surface 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. 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 Undisturbed soils before and after shrinkage 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.